The 22 February 2011 Magnitude 6.3 Christchurch Earthquake

Volume 82, Number 6

November/December 2011

The Seismological Society of America (SSA) is a scientific society devoted to the advancement of earthquake science. Founded in 1906 in San Francisco, the Society now has members throughout the world representing a variety of technical interests: seismologists and other geophysicists, geologists, engineers, insurers, and policymakers in preparedness and safety.

SEISMOLOGICAL SOCIETY OF AMERICA ITS OBJECT i Promote research in seismology, the scientific investigation of earthquakes and related phenomena. ii Promote public safety by all practical means. iii Enlist the interest of engineers, architects, contractors, insurers, and property owners in the obligation to protect the community against disasters due to earthquakes and earthquake fires by showing that it is reasonably practicable and economical to build for security. iv Inform the public by appropriate publications, lectures, and other means to an understanding of the fact that earthquakes are dangerous chiefly because we do not take adequate precautions against their effects, whereas it is possible to insure ourselves against damage by proper studies of their geographic distribution, historical sequence, activities, and effects on buildings.

The Seismological Society of America 201 Plaza Professional Bldg. • El Cerrito, CA 94530 +1-510-525-5474; Fax +1-510-525-7204 http://www.seismosoc.org

Special Focused Issue: The 22 February 2011 Magnitude 6.2 Christchurch Earthquake Guest Editor: Erol Kalkan

Volume 82, Number 6 November/December 2011 E indicates that online material is available on the SSA Web site, http://www.seismosoc.org.

THE 22 FEBRUARY 2011 MAGNITUDE 6.2 CHRISTCHURCH EARTHQUAKE

News and Notes

761

Preface to the Focused Issue on the 22 February 2011 Magnitude 6.2 Christchurch Earthquake 765 Erol Kalkan

DC Currents

763

Numerical Study on the Role of Basin Geometry and Kinematic Seismic Source in 3D Ground Motion Simulation of the 22 February 2011 MW 6.2 Christchurch Earthquake 767 Roberto Guidotti, Marco Stupazzini, Chiara Smerzini, Roberto Paolucci, and Paolo Ramieri Kinematic Source Model of the 22 February 2011 Mw 6.2 Christchurch Earthquake Using Strong Motion Data Caroline Holden

783

Fault Location and Slip Distribution of the 22 February 2011 MW 6.2 Christchurch, New Zealand, Earthquake from Geodetic Data 789 E John Beavan, Eric Fielding, Mahdi Motagh, Sergey Samsonov, and Nic Donnelly Coulomb Stress Change Sensitivity due to Variability in Mainshock Source Models and Receiving Fault Parameters: A Case Study of the 2010–2011 Christchurch, New Zealand, Earthquakes 800 Zhongwen Zhan, Bikai Jin, Shengji Wei, and Robert W. Graves InSAR and Optical Constraints on Fault Slip during the 2010–2011 New Zealand Earthquake Sequence 815 E William D. Barnhart, Michael J. Willis, Rowena B. Lohman, and Andrew K. Melkonian Stress Control of an Evolving Strike-Slip Fault System during the 2010–2011 824 Canterbury, New Zealand, Earthquake Sequence Richard Sibson, Francesca Ghisetti, and John Ristau Large Apparent Stresses from the Canterbury Earthquakes of 2010 and 2011 833 B. Fry and M. C. Gerstenberger Fine-scale Relocation of Aftershocks of the 22 February Mw 6.2 Christchurch Earthquake Using Double-difference Tomography 839 E Stephen Bannister, Bill Fry, Martin Reyners, John Ristau, and Haijiang Zhang The Character of Accelerations in the Mw 6.2 Christchurch Earthquake B. Fry, R. Benites and A. Kaiser Near-source Strong Ground Motions Observed in the 22 February 2011 Christchurch Earthquake Brendon A. Bradley and Misko Cubrinovski

846

853

Transitions 764 New Publications

965

SSA Annual Meeting Announcement

966

Coming in BSSA

968

Meeting Calendar

991

MEMBERSHIP APPLICATION

993

SUBSCRIPTION INFORMATION 995

Ground Motion Attenuation during M 7.1 Darfield and M 6.2 Christchurch, New Zealand, Earthquakes and Performance of Global Predictive Models 866 E Margaret Segou and Erol Kalkan Strong Ground Motions and Damage Conditions Associated with Seismic Stations in the February 2011 Christchurch, New Zealand, Earthquake Hiroaki Iizuka, Yuki Sakai, and Kazuki Koketsu

875

Ground Motions versus Geotechnical and Structural Damage in the February 2011 Christchurch Earthquake Eleni Smyrou, Panagiota Tasiopoulou, İhsan Engin Bal, and George Gazetas

882

Soil Liquefaction Effects in the Central Business District during the February 2011 Christchurch Earthquake 893 Misko Cubrinovski, Jonathan D. Bray, Merrick Taylor, Simona Giorgini, Brendon Bradley, Liam Wotherspoon, and Joshua Zupan Comparison of Liquefaction Features Observed during the 2010 and 2011 Canterbury Earthquakes 905 R. P. Orense, T. Kiyota, S. Yamada, M. Cubrinovski, Y. Hosono, M. Okamura, and S. Yasuda Ambient Noise Measurements following the 2011 Christchurch Earthquake: Relationships with Previous Microzonation Studies, Liquefaction, and Nonlinearity 919 Marco Mucciarelli Use of DCP and SASW Tests to Evaluate Liquefaction Potential: Predictions vs. Observations during the Recent New Zealand Earthquakes 927 Russell A. Green, Clint Wood, Brady Cox, Misko Cubrinovski, Liam Wotherspoon, Brendon Bradley, Thomas Algie, John Allen, Aaron Bradshaw, and Glenn Rix Performance of Levees (Stopbanks) during the 4 September 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, Earthquakes 939 Russell A. Green, John Allen, Liam Wotherspoon, Misko Cubrinovski, Brendon Bradley, Aaron Bradshaw, Brady Cox, and Thomas Algie Performance of Bridges during the 2010 Darfield and 2011 Christchurch Earthquakes 950 Liam Wotherspoon, Aaron Bradshaw, Russell Green, Clinton Wood, Alessandro Palermo, Misko Cubrinovski, and Brendon Bradley EASTERN

SECTION

RESEARCH LETTERS

Reassessment of Stable Continental Regions of Southeast Asia Russell L. Wheeler Moment Magnitude (MW) Conversion Relations for Use in Hazard Assessment in Eastern Canada Allison L. Bent

971

984

Volume 82, Number 6, November/December 2011

Editor in Chief Jonathan M. Lees Eastern Section Editor Martin C. Chapman Associate Editors Jennifer S. Haase Susan E. Hough Erol Kalkan EduQuakes Editor Alan Kafka Electronic Seismologist Editor John N. Louie Historical Seismologist Editor John Ebel Electronic Supplements Editor Kim Olsen Managing Editor Mary George Typesetter Rodney Sauer Copy Editor Laura Caruso Editorial Assistant Melissa Houle Subscriptions to Seismological Research Letters (SRL): The subscription rate for new institutions and other nonmembers in North America is $150 ($160 outside North America). Members of the Seismological Society of America receive SRL as a perquisite of membership. Individuals may apply for membership using the form printed near the back of this issue.

Single copies: Many back issues of SRL are available from SSA Headquarters.

Seismological Research Letters (ISSN 08950695) is published bimonthly in January, March, May, July, September, and November by the Seismological Society of America, 201 Plaza Professional Building, El Cerrito, California 94530. Periodicals postage paid at El Cerrito, California and at additional mailing offices.

Seismological Research Letters was formerly published as Earthquake Notes (ISSN 00128287) from 1929–1986.

Postmaster: Send address changes to Seismological Research Letters (SRL), 201 Plaza Professional Building, El Cerrito, California 94530-4003.

Communications regarding publications, apart from submission of manuscripts, should be addressed to the Seismological Society of America, 201 Plaza Professional Building, El Cerrito, California 94530. © 2011 by the Seismological Society of America. Printed in the U.S.A. by The Sheridan Press, Hanover, Pennsylvania.

The Seismological Society of America 201 Plaza Professional Building El Cerrito, California 94530 +1-510-525-5474; Fax +1-510-525-7204 http://www.seismosoc.org

Seismological Research Letters—Submissions Seismological Research Letters (SRL) is a journal containing articles and items of broad appeal on topics in seismology and earthquake engineering. Articles submit-ted to SRL should be informational in nature and should be of current interest to a cross-section of SSA membership. Articles expressing some particular view about seismology or seismological research also will be accepted. Articles that contain original research results should be submitted to the Bulletin of the Seismological Society of America (BSSA). News and notes, special reports on particular earthquakes, seismic network summaries, information on computer hardware or software pertinent to seismology, seismological equipment information, book reviews, and letters to the editor also are solicited for publication in SRL. Consult the SRL Information for Authors at http://www.seismosoc.org/publications/srl/srl-authorsinfo.php for details about making submissions. In general, articles should not exceed 20 pages of double-spaced text (excluding figures) unless approved by the editor. Electronic supplements can be considered for SRL; the electronic supplement policy is posted at http://www.seismosoc.org/publications/ esupps.php. The SRL Editor in Chief is Jonathan M. Lees, srled@seismosoc.org.

Upload submissions via SRL’s electronic submission system at http://srl. edmgr.com. Direct questions about the system to the managing editor at srl@seismosoc.org.

Submissions to the Eastern Section of the SSA (ES-SSA) Section of SRL The ES-SSA Section of SRL is devoted to the seismology of continental interiors.

Articles pertaining to eastern North American earthquakes, intraplate seismotectonics, and earthquake engineering are particularly encouraged. Upload submissions to the Eastern Section via SRL’s electronic submission system at http:// srl.edmgr.com. The ES-SSA editor is Martin C. Chapman, Dept. of Geological Sciences, Virginia Polytechnic & State Univ., 4044 Derring Hall, Blacksburg, VA 24061; telephone +1-540-231-5036; fax +1-540-231-3386; e‑mail mcc@vt.edu.

Appropriate review articles and tutorials are encouraged, as well as news and notes pertaining to the Eastern Section of SSA. Referees are sought on all ES-SSA papers exceeding four published pages in length. Page charges for articles in the ES-SSA Section are $25 for each printed page. The editor may allow exceptions to the page charges under certain circumstances.

On the Cover On 22 February 2011 a magnitude 6.2 earthquake struck southeast of Christchurch, New Zealand; this issue of SRL focuses on that event, the deadliest and most disastrous in New Zealand since 1931. Front: More than 100,000 buildings were damaged in the Christchurch earthquake, from high-rise office buildings to timber-frame homes like this one, located in the city’s heavily impacted central business district. The structure has been tagged with a red card, indicating that it is set for demolition (photo by İ.E. Bal; see more at Smyrou et al., 882–892).

Back: The Christchurch earthquake provided an unprecedented dataset for testing the effectiveness of 3D numerical modeling tools. Shown here are the spatial distributions of peak ground velocity values for two simulations described in Guidotti et al., 767–782.

Authorization to photocopy items for internal or personal use, or for the internal or personal use of specific clients, is granted by the Seismological Society of America provided that the appropriate fee of $3 per copy is paid directly to Copyright Clearance Center, ISSN 0895-0695, 222 Rosewood Drive, Danvers, MA 01923, USA; telephone 978-7508400. Prior to photocopying items for educational classroom use, please contact Copyright Clearance Center at the above address. Consent for reproduction as described above does not extend to other types of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. For permission to reprint material, please read “Permission to Reproduce Material from SRL” on page 994.

News and Notes (continued) Nominations Open Now for SSA Awards SSA members are invited to submit nominations for the following four SSA awards by 15 February 2011.

Electronic submission is encouraged, though nominations may be submitted in hard copy.

Please note that the principal nominator should integrate the nomination letters and send one nomination package to ensure that all letters of endorsement reach the decision makers on time. Previous recipients of all awards are listed on the SSA Web site, www.seismosoc.org. No current member of the SSA Board of Directors shall be eligible for award nomination.

Electronic submissions should be e‑mailed in .TXT, .PDF or .DOC files to awards@seismosoc.org.

Electronic submissions are encouraged, but hard copies may be mailed or FAX’d to: Secretary, Seismological Society of America c/o Susan Newman 201 Plaza Professional Building El Cerrito, California 94530 U.S.A. Fax: +1-510-525-7204 You will receive a confirmation of receipt of your nomination. Names of the award winners will be announced at the SSA Annual Meeting Luncheon in San Diego, California, 17 April 2012. The awards will be presented at the 2013 SSA Annual Meeting Luncheon in Salt Lake City, Utah.

Harry Fielding Reid Medal The Harry Fielding Reid Medal of the Seismological Society of America, formerly known simply as “The SSA Medal,” is the Society’s highest honor. It is awarded for outstanding contributions in seismology or earthquake engineering. A Reid Medal nomination package should include letters of nomination from at least two but no more than five Society members. Each nominating letter may have more than one signatory, but each signatory should sign only one letter. A single curriculum vitae and bibliography of the nominee may be included. To simplify communications with the Secretary, nominators of a particular nominee should select among themselves a chief

nominator for correspondence purposes. Submitting one integrated package insures that all endorsement letters will be included in the packet to the Board of Directors.

For more information about the Reid Medal nomination process, contact Bob Engdahl, chairman of the Reid Medal Subcommittee, at engdahl@colorado.edu.

Charles F. Richter Early Career Award The Charles F. Richter Early Career Award honors outstanding contributions to the goals of the Society by a member early in her or his career. A nominee must satisfy the following criteria: 1) Regular or Honorary Member of the Society in good standing, 2) the most recent academic degree must have been awarded no more than six years prior to 18 April of the year that she or he is selected for the award, and 3) not more than 40 years old on 18 April of the year that she or he is selected for the award.

Any member of the SSA who is not on the Richter Award Subcommittee may nominate a candidate for the Richter Award. A single nomination package must be submitted to the Secretary of the Society at the above address no later than 15 February of each year. The package should contain 1) a letter of nomination no more than 2 pages long summarizing the nominee’s significant accomplishments, 2) a curriculum vitae including bibliography, 3) two to four supporting letters no more than two pages long, at least two letters of which must come from individuals not currently employed at the nominee’s current institution or the institution from which the nominee received her or his most recent degree, and 4) an eligible birth date and date of degree. Questions may be directed to Charlotte Rowe, chairman of the Richter Award Subcommittee, char@lanl.gov.

Frank Press Public Service Award The Frank Press Public Service Award honors any individual, combination of individuals, or any organization that has served the profession of seismology or the advancement of public safety or public information relating to seismology. The Press Award

Seismological Research Letters Volume 82, Number 6 November/December 2011 761

News and Notes (continued) nomination package should include a letter of nomination no longer than two pages that summarizes the nominee’s significant accomplishments, and two to four supporting letters, each no longer than two pages, at least one written by a member of the Society. If the nominee is an individual, the nomination may include a curriculum vitae or biography.

For more information about the Press Public Service Award nomination process, contact David Wald, chairman of the Frank Press Public Service Award Subcommittee, at wald@usgs.gov.

SSA Distinguished Service Award The SSA Distinguished Service Award honors a person who has provided outstanding service to the Society. This award may be given to any person, and any Society member may make the nomination. Distinguished Service Award nominations should be submitted in a letter to the Secretary at the address above. For more information about the Distinguished Service Award nomination process, contact Lisa Grant Ludwig, chairman of the SSA Honors Committee, at lgrant@uci.edu.

Nominations Open Now for the Bruce Bolt Medal The Bruce Bolt Medal is awarded jointly by COSMOS, EERI, and SSA to recognize individuals worldwide whose accomplishments involve the promotion and use of strong-motion earthquake data and whose leadership in the transfer of scientific and engineering knowledge into practice or policy has led to improved seismic safety. Members of EERI, SSA, and COSMOS are encouraged to submit nomination packages for this distinguished award. Nominations will be reviewed in confidence by a six-person Joint Nomination Panel formed by two representatives from each of the three sponsoring organizations. The recommended nominee will be considered in confidence by each organization’s

board for their approval and joint selection of the medalist.

The following criteria are used to evaluate the recipient: 1. Promotion of strong-motion instrumentation or advancing strong-motion data processing or data utilization; 2. Technical contributions in seismic engineering or engineering seismology; and 3. Leadership in the transfer of knowledge into practice or policy that has led to improved seismic safety.

The nomination letter, which should be no longer than two pages, must address the ways in which the candidate meets all three of the criteria. Along with the letter, the nomination package must include a substantial summary of the professional history of the candidate including employment, significant publications, and activities and accomplishments relevant to the Bolt Medal criteria. The current contact information for the candidate must also be supplied. Up to three supporting letters (each no longer than two pages) may be included in the nomination package. The closing date for submitting nominations is December 31, 2011.

Incomplete nomination packages will not be considered by the Joint Nomination Panel.

Nomination packages for the Bruce Bolt Medal should be sent to the Bolt Medal Nomination Panel, in care of William (Woody) Savage at woodysavage@ gmail.com. Questions regarding the Bolt Medal criteria or the nomination process for candidates may be directed to Woody at the above e‑mail address.

While electronic submissions are preferred, but hard copy may be mailed to the Bolt Medal Nomination Panel c/o Woody Savage, 1930 Village Center Circle #3-292, Las Vegas, NV 89134; please notify Woody by e‑mail that you are mailing a nomination.

Please send items for “News and Notes” to SRL Editor Jonathan M. Lees in care of the SRL managing editor at srl@ seismosoc.org.

762 Seismological Research Letters Volume 82, Number 6 November/December 2011

DC Currents (continued) SSA MEMBERS VISIT WASHINGTON SSA members went to Washington the week of September 20. They participated in a variety of events aimed at communicating the importance of the geosciences and seismology to policymakers.

Geo-Science Congressional Visits Day SSA co-sponsored and participated in the 2011 GeoScience Congressional Visits Day (Geo-CVD) on 20–21 September. This year’s Geo-CVD attracted more than 50 participants and included a half-day workshop that addressed how the appropriations process works, what to expect from Congressional visits, budget overview presentations for key science agencies, and more.

Participants then walked the halls of Congress and visited more than 100 offices of representatives, senators, and committees. SSA participants were able to highlight earthquake programs and their funding requirements as well as the upcoming re-authorization of the National Earthquake Hazards Reduction Program (NEHRP).

Geo-CVD, an annual event, also was co-sponsored by the American Geoscience Institute (AGI), the American Geophysical Union (AGU), the Association for the Sciences

of Limnology and Oceanography (ASLO), the Geological Society of America (GSA), and the University Corporation for Atmospheric Research (UCAR). AGI’s report on GeoCVD is at http://www.agiweb.org/gap/events/geocvd11/

index.html

USGS Coalition Reception On 21 September SSA helped arrange and participated in an annual reception on Capitol Hill that is sponsored by the USGS Coalition, an alliance of more than 70 organizations united by a commitment to the continued vitality of the United States Geological Survey (USGS) and its ability to provide critical data and services to the nation. U.S. Sen. Jeff Bingaman (D-N.M.) received the U.S. Geological Survey Coalition’s Leadership Award. The award recognized Bingaman for his long-term support of the USGS and for his leadership on legislation and policies on natural resources, public lands, and energy for current and future generations.

AGI Leadership Forum On 20 September the American Geoscience Institute sponsored a Leadership Forum on the Importance of the Geosciences. SSA Immediate Past President Rick Aster represented SSA and spoke at the forum, as did SSA member Wayne Pennington, the incoming president of AGI. Information on the forum is at http://www.agiweb.org/ events/LF2011/index.html

Government Relations Visits On 22 and 23 September, SSA Government Relations Committee Chairman Stu Nishenko joined SSA Executive Director Susan Newman and Federal Liaison Elizabeth Duffy for targeted visits to federal agencies and congressional offices to voice support for re-authorization of the National Earthquake Hazards Program and funding for NEHRP agencies. ▲▲ Several SSA members participated in Geoscience Congressional Visits Day (Geo-CVD) orientation; pictured are Wayne Pennington, Lisa Walsh, Chuck Langston, Bob Anderson, Linda Rowan, and John Vidale.

▲▲ Chuck Langston, University of Memphis, visits with U.S. Rep. Steve Cohen (D-Tenn.)

▲▲ Rick Aster, SSA’s immediate past president, represented the Seismological Society at AGI’s Leadership Forum on the Importance of the Geosciences. Here Aster speaks with Cathy Manduca of the National Association of Geoscience Teachers.

Wayne Pennington, AGI’s president-elect, is pictured behind them.

Seismological Research Letters Volume 82, Number 6 November/December 2011 763

Transitions

Edwin Victor Apel III recently moved to Risk Management Solutions from AMEC Geomatrix.

and Mining Engineering and Sciences at Michigan Technological University.

Jonathan Bray, a professor of geotechnical engineering at University of California Berkeley, has been selected as the 2012 Joyner Lecturer.

Stephane Rondenay moved recently from the Massachusetts Institute of Technology to University of Bergen.

Ethan D. Brown recently moved to RDRTec from Science Applications International Corp.

Andres Chavarria recently moved from Vialogy to SR2020 Borehole Seismic Services.

Vedran Lekic recently moved to the University of Maryland from Brown University.

Wayne D. Pennington was inducted as the new president American Geosciences Institute (AGI) at the recent Geological Society of America Annual Meeting in Minneapolis, Minnesota. He is chairman of the Department of Geological

Takahiko Uchide recently moved from Scripps Institution of Oceanography at University of California San Diego to the Disaster Prevention Research Institute at Kyoto University in Japan.

Peter Yanev recently founded Yanev Associates, a consulting firm that specializes in earthquake risk management and engineering.

Your contributions of items for “Transitions” are most welcome. Please tell us about your awards, promotions, job changes, reorganizations, and retirements. Send announcements to SRL Editor Jonathan M. Lees in care of the SRL managing editor at srl@seismosoc.org.

764 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.764

Preface to the Focused Issue on the 22 February 2011 Magnitude 6.2 Christchurch Earthquake Erol Kalkan

Erol Kalkan

U.S. Geological Survey, Menlo Park

The 22 February 2011 magnitude 6.2 Christchurch earthquake, centered southeast of Christchurch, was part of the aftershock sequence that has been occurring since the September 2010 magnitude 7.1 quake near Darfield, 40 km west of the city. The Christchurch earthquake killed more than 180 people, damaged or destroyed more than 100,000 buildings, and is New Zealand’s most deadly disaster since the earthquake that struck the Napier and Hastings area on 3 February 1931.

This special focused issue of Seismological Research Letters, which I had the fortune to edit, contains a selected set of 19 original technical papers. These papers cover different aspects of the 2011 Christchurch earthquake from seismological, geodetic, geological, and engineering perspectives.

The first eight papers focus on earthquake source modeling, fault stress variation, and aftershock sequence. The paper by Guidotti et al. presents three-dimensional numerical simulations of the Christchurch earthquake by comparing different fault and interface models. Using data from a dense network of strong motion instruments, Holden et al. presents the inversion scheme for constraining the source kinematics of the Christchurch event. The constrained geodetic source model is presented next

by Beavan et al. using a large amount of ground-displacement data. The following paper by Zhan et al. concentrates on how applicable the static Coulomb stress triggering mechanism is to the 2011 Christchurch aftershock, and it examines the sensitivity of the stress changes to mainshock slip distribution and aftershock fault orientation. Along the same line, Barnhart et al. performs inversions of optical imagery data for spatial distribution of fault slip that occurred during the Darfield and Christchurch earthquakes, and assesses the potential contribution of the static Coulomb stress change during the Darfield event to the eventual rupture of the Christchurch event. The next paper, by Sibson et al., evaluates how the complex earthquake sequence of the region likely has arisen through reactivation under the contemporary tectonic stress field of a mixture of comparatively newly formed and older inherited fault structures. The paper by Fry and Gerstenberger presents apparent stresses of the three largest regional earthquakes, and compares them to global and regional data to improve future seismic hazard estimates due to similar high-stress events. In order to better understand the regional complex fault system, Bannister et al. provides relocation analysis of aftershocks that have occurred since the February earthquake through May 2011. doi: 10.1785/gssrl.82.6.765

The next three papers concentrate on recorded strong ground motions and their engineering implications. Fry et al. investigates characteristics of recorded horizontal and vertical waveforms and their correlation with the observed nonlinear site response. The following paper, by Bradley and Cubrinovski, provides a preliminary assessment of the near-source ground motions recorded in the Christchurch region by examining their spatial distribution including source, path, and site effects. The next paper of this series is by Segou and Kalkan, which evaluates the performance of global ground-motion prediction models using the strong motion data obtained from the Darfield and Christchurch earthquakes in order to improve future seismic hazard assessment and building code provisions for the Canterbury region.

The next set of eight papers focus on observed structural and geotechnical damages associated with strong ground shaking during both the Darfield and Christchurch earthquakes.

The paper by Iizuka et al. investigates the damage around the seismic stations to determine the relationship between structural damage and strong motions during the Christchurch earthquake. Similarly, Smyrou et al. evaluates the strong ground motions of this event in an effort to broadly explain and quantify the observed structural and geotechnical damages.

The next paper, by Zupan et al., summarizes the key field observations made following the Christchurch earthquake regarding the effects of soil liquefaction on building performance in the central business district. Along the same line, Orense et al. compares the Darfield and Christchurch earthquakes according to the results of the reconnaissance works with emphasis on the geotechnical implications of liquefaction-observed damage in the affected areas. Using the ambient noise measurements following the Christchurch earthquake, Mucciarelli investigates the relationships with previous microzonation studies, liquefaction, and soil nonlinear response. Green, Wood et al. compare the observed versus predicted liquefaction occurrence during the Darfield and Christchurch earthquakes using DCP and SASW tests; and Green, Allen et al., summarizes the performance of the levees along the Waimakariri and Kaiapoi rivers during these two events. The final paper of this special focused issue is by Wotherspoon et al. and presents a summary of field observations, and subsequent analyses on the damage to some of the bridges in the Canterbury region as a result of the Christchurch earthquake.

Seismological Research Letters Volume 82, Number 6 November/December 2011 765

The 19 papers presented here meet our goals of covering a wide spectrum of topics related to the strong earthquake sequence and their impacts on the Canterbury region of New Zealand in order to inform and advance our understanding of seismic source mechanism, nonlinear site response, ground motion attenuation, and infrastructure performance, as well as to point out new avenues of investigation for future studies.

have been possible without the dedicated work of the many reviewers who provided valuable feedback to authors in a timely way. Their contributions to this issue cannot be overstated.

Lastly, I would like to thank SRL Editor-in-Chief Jonathan Lees and the SRL production staff for their outstanding organization and meticulous attention to detail during various stages of this effort.

ACKNOWLEDGMENTS We at SRL thank all the authors for their valuable contributions. The publication of this focused issue of SRL would not

766 Seismological Research Letters Volume 82, Number 6 November/December 2011

U.S. Geological Survey 345 Middlefield Road Menlo Park, California 94025 U.S.A.

ekalkan@usgs.gov

Numerical Study on the Role of Basin Geometry and Kinematic Seismic Source in 3D Ground Motion Simulation of the 22 February 2011 MW 6.2 Christchurch Earthquake Roberto Guidotti, Marco Stupazzini, Chiara Smerzini, Roberto Paolucci, and Paolo Ramieri

Roberto Guidotti,1 Marco Stupazzini, 2 Chiara Smerzini,1 Roberto Paolucci,1 and Paolo Ramieri3

INTRODUCTION Almost six months after the M W 7.1 Darfield (Canterbury) earthquake, on 22 February 2011 at 12:51 p.m. (local time), an M W 6.2 earthquake struck the city and suburbs of Christchurch—the largest city on the South Island of New Zealand, with about 400,000 inhabitants. The Christchurch earthquake can be considered one of the greatest natural disasters recorded in New Zealand. The death toll was more than 180, with around 2,000 people injured, and structures already weakened by the Darfield event and its aftershocks were badly affected (Cubrinovski and Green 2010; Tonkin and Taylor Ltd. 2010; Kam et al. 2011). The earthquake was generated by an oblique thrust fault located between the Australian and Pacific plates, within about 6 km of the city center. It is worth recalling that prior to the Darfield event there was no surface evidence of the fault that generated the Christchurch earthquake on February 2011, nor of the Greendale fault, recognized as responsible for the September 2010 earthquake (Quigley et al.

2010). During the last decade a set of seismic surveys across the Canterbury Plains had been carried out (Green et al. 2010), but they did not reveal any convincing evidence of the Greendale fault and there was no clear indication that a major earthquake was imminent in this particular region. Beyond the effects and the consequences of the seismic event, the attention of the scientific community was drawn to two aspects that had a primary role in the Christchurch earthquake: 1) the extremely severe, strong ground shaking observed, especially on the vertical component; and 2) the widespread liquefaction phenomena across the city (Cubrinovski and Green 2010; Green et al.

2011, page 927 of this issue).

1. Department of Structural Engineering, Politecnico di Milano, Milan, Italy 2. Munich RE, Munich, Germany 3. Consorzio Interuniversitario Lombardo per l’Elaborazione Automatica (CILEA), Segrate, Milan, Italy doi: 10.1785/gssrl .82.6.767

Between September 2010 and June 2011 the Canterbury area experienced three major earthquakes with M W ≥ 6.0 and a large number of aftershocks (Gledhill et al. 2011; Bannister et al. 2011, page 839 of this issue). The Christchurch earthquake was recorded by several digital stations of the permanent network operated by the Institute of Geological and Nuclear Sciences (GNS; data available at the GeoNET Data Centre: http://www.geonet.org.nz/). Peak ground motion accelerations in the epicentral region of the earthquake range up to 1.261 g on the horizontal component and up to 1.629 g on the vertical component. Table 1 shows a list of the accelerometric stations located within a 40-km-radius from the epicenter with the corresponding values of peak ground acceleration (PGA) and peak ground velocity (PGV) on both the horizontal and vertical components (data from the Center for Engineering Strong Motion Data, CESMD: http://www.strongmotioncenter.org/; bandpass filter transition bands are 0.1–0.25 Hz and 24.50–25.50 Hz). The ground accelerations recorded within the city of Christchurch are among the largest ever recorded for a New Zealand earthquake, with exceptionally high vertical ground acceleration (Bradley and Cubrinovski 2011, page 853 of this issue). The unusual severity of the ground shaking can be explained as a combination of four major effects: 1) the proximity of the causative fault to the city, 2) the directivity of ground motion toward the urban area, 3) the strong amplification effects of the soft alluvial sediments beneath the city, and 4) the hanging wedge effect, causing a significant increase of ground shaking on the hanging wall.

The availability of this unprecedented dataset of nearfault strong ground motion, combined with the peculiar geological configuration of the Christchurch area, makes the Christchurch earthquake a relevant benchmark to test the effectiveness of 3D numerical tools for the prediction of the variability of strong ground motion in near-fault conditions.

To this end, numerical simulation of seismic wave propagation within the Canterbury Plains, extending from the northwestern portion of the Southern Alps mountain range to the

Seismological Research Letters Volume 82, Number 6 November/December 2011 767

TABLE 1 Peak ground acceleration (PGA) and peak ground velocity (PGV) values recorded for the stations within a 40-km radius from the epicenter. Stations located on rock are typed in italic (data from CESMD, Center for Engineering Strong Motion Data: http://www.strongmotioncenter.org/; bandpass filter transition bands are 0.1–0.25 Hz and 24.50–25.50 Hz). Re denotes the epicentral distance.

Stations Name

Id

Re [km]

Heathcote Valley Primary School Lyttelton Port Company Pages Road Pumping Station Christchurch Cathedral College Christchurch Cashmere High School Christchurch Resthaven Christchurch Hospital Christchurch Botanic Gardens Shirley Library Hulverstone Drive Pumping Station Riccarton High School Christchurch Papanui High School Styx Mill Transfer Station McQueens Valley Christchurch Canterbury Aero Club Lincoln Crop and Food Research Templeton School Kaiapoi North School Rolleston School Swannanoa School Selwyn Lake Road Ashley School Cust School

HVSC LPCC PRPC CCCC CMHS REHS CHHC CBGS SHLC HPSC RHSC PPHS SMTC MQZ CACS LINC TPLC KPOC ROLC SWNC SLRC ASHS CSTC

1 4 6 6 6 8 8 9 9 9 12 12 14 15 18 19 19 23 26 29 33 35 39

Lyttelton-Akaroa volcanic region, was performed through the software package GeoELSE (http://geoelse.stru.polimi. it). Based on the Spectral Element formulation proposed by Faccioli et al. (1997), GeoELSE is designed to perform linear and nonlinear dynamic wave propagation analyses in heterogeneous media, exploiting in 3D its implementation in parallel computer architectures. Examples of application of GeoELSE to seismic wave propagation studies in complex geological configurations can be found in Stupazzini et al. (2009) and Smerzini et al. (2011). Different 3D numerical models were constructed for the Christchurch earthquake, to check the dependence of the results on: 1) the kinematic source model, based on the information retrieved from recent seismic source inversion studies, and 2) the shape of the alluvial-bedrock interface within the Canterbury Plains. To check the accuracy of the numerical models, the synthetic results are compared against the strong ground motion records. The misfit between simulated and recorded waveforms is evaluated in a quantitative way in a format suitable for engineering applications, making use of the criteria proposed by Anderson (2004). To give insights into the variability of surface earthquake ground

PGA EW [g]

PGA NS [g]

PGA UP [g]

PGV EW [cm/s]

PGV NS [cm/s]

PGV UP [cm/s]

1.230 0.919 0.665 0.415 0.369 0.719 0.357 0.536 0.324 0.232 0.297 0.187 0.182 0.147 0.182 0.081 0.091 0.211 0.164 0.251 0.086 0.088 0.075

1.261 0.771 0.589 0.375 0.403 0.372 0.337 0.432 0.319 0.155 0.254 0.243 0.144 0.098 0.180 0.164 0.099 0.201 0.163 0.143 0.088 0.076 0.078

1.466 0.413 1.629 0.692 0.796 0.529 0.511 0.271 0.500 0.858 0.188 0.195 0.176 0.072 0.185 0.084 0.136 0.057 0.072 0.056 0.049 0.037 0.042

79.96 37.27 82.26 64.75 41.69 87.30 65.17 63.95 52.73 36.21 29.95 36.73 28.55 7.10 15.85 7.59 10.36 14.19 6.30 14.69 7.25 7.28 6.46

89.12 40.97 74.83 43.58 46.79 45.54 56.58 43.12 51.74 26.75 23.63 28.24 20.49 5.44 13.88 16.17 10.09 19.39 7.77 13.94 8.32 5.44 6.71

38.51 16.43 49.48 21.53 15.17 21.29 20.87 13.42 21.73 33.93 12.11 16.82 12.31 4.05 11.84 5.41 8.16 5.71 4.28 4.73 2.92 2.30 3.09

motion, due to the interaction between near-fault conditions and strong geological variations, ground-shaking maps and snapshots of the velocity wavefield are shown.

GEOLOGY OF THE CHRISTCHURCH REGION The area under study extends from the Southern Alps to the Lyttelton-Akaroa volcanic region and includes the city of Christchurch and parts of the Canterbury Plains and the Banks Peninsula. Information about the geology of this area comes from the 1:250,000 geological map of Christchurch, produced by GNS (Forsyth et al. 2008). Based on the geological map and on the available cross-sections, it was possible to infer a preliminary stratigraphy for the alluvial cover filling the Canterbury Plains in the area of interest (Figure 1). The basement rock of the whole region is the Torlesse composite terrane, a deformed package of Carboniferous to Cretaceous sedimentary rocks. Late Miocene volcanism forms the two major overlapping volcanoes Lyttelton and Akaroa on Banks Peninsula, today largely eroded. Erosion of the shallow landmass and glacial-interglacial climatic fluctuations led to the

768 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 1. A) Area of the model under study, including part of the Canterbury Plains and the Lyttelton-Akaroa volcanic region, along with the strong ground motion stations. B) Geological A-A’ cross-section.

widespread decomposition of unconsolidated Quaternary sediments that constitute the Canterbury Plains. The alluvial sequence is formed by coal, clay, limestone, and sand (Forsyth et al. 2008).

It is possible to consider the deep crustal model as basin sediments overlying a layer of Torlesse graywacke down to around 5 km, overlying, in turn, a layer of Haast Schist down to around 20 km depth. The lower crust is interpreted as mafic (diorite, diabase, and gabbro) Mesozoin ocean crust (Reyners and Cowan 1993; Godfrey et al. 2001, 2002; Melhuish et al.

2005). This deep crustal layer subdivision is confirmed by the high-resolution seismic wide-angle data, collected within the framework of the South Island Geophysical Transect (SIGHT) experiment (Kleffmann et al 1998; Mortimer et al. 2002; Long et al. 2003; Scherwarth et al. 2003).

3D NUMERICAL SIMULATIONS OF THE SEISMIC RESPONSE OF THE CANTERBURY PLAINS 3D numerical simulations consist of the following features: 1) kinematic description of the close-by seismic source, 2) horizontally layered deep geological model, 3) a simplified but realistic description for the Cretaceous-Cenozoic alluvial Canterbury Plains, and 4) a linear visco-elastic soil behavior. Note that in these preliminary analyses we considered a relatively rough model for the soil behavior by assuming a linear-visco elastic constitutive law, with a quality factor Q proportional to frequency (further details about the implementa-

tion of the visco-elastic soil behavior model can be found in Stupazzini et al. 2009). Different 3D numerical models were built for the Christchurch earthquake to achieve the best fit with the ground motion observations, combining: 1) two different kinematic seismic fault solutions, based on recent seismic source inversion studies, and 2) two simplified models for the shape that defines the interface between the alluvial soft soil sediments and the rigid volcano materials. The 3D model of the region of the South Island of New Zealand covers an area of approximately 60 x 60 x 20 km around the city of Christchurch, including the information available in the geological map and the 2D cross-sections, shown in Figure 1 and described in the previous section.

Two different models were constructed to approximate the complex geological configuration of the Canterbury Plains.

The models, referred to hereafter as “step-like” and “smooth,” basically differ in the transition between the alluvial soft sediments and the rigid volcano materials, as sketched in Figure 2.

In the “step-like” model (Figure 2A), a rough approximation of the alluvial-bedrock shape is adopted. More specifically, the thickness of the alluvial basin is assumed constant across the whole area under study and equal to 1.5 km. In the “smooth” model, the shape of the interface between the soft soil and the volcanic materials is improved, with constraints inferred from the topography of the volcano (Figure 2B). For both models, the alluvial basin consists of three different layers with VS ranging from 300 m/s in the top 300 m to 1,500 m/s at the interface with the volcanic materials (top three layers of Table

Seismological Research Letters Volume 82, Number 6 November/December 2011 769

▲▲ Figure 2. A) Sketch of the “step-like” and B) “smooth” model, referred to as an approximation of the transition between the soft sediments of the Canterbury Plains and the rigid volcano materials. Note that the alluvial basin has a maximum thickness of 1.5 km and consists of three horizontal layers with Vs ranging from 300 m/s to 1,500 m/s.

TABLE 2 Soil profile adopted in the numerical simulations. The top three layers constitute the Canterbury Plains.

Layer

Depth [m]

1 2 3 4 5 6

0–300 300–750 750–1500 0–5000 1500–5000 5000–20000

Thickness [m] 300 450 750 5000 3500 15000

2). In the absence of direct measurements for the considered area, VS values adopted in Table 2 are in reasonable agreement with available geotechnical data and engineering geological models, although in a different region of New Zealand (Boon et al. 2011; Semmens et al. 2011). The volcano region (layer 4 in Table 2), with VS = 3,175 m/s, extends down to a maximum depth of 5 km. As regards the background geology, a horizontally layered crustal model was assumed, as summarized in Table 2 (layers 5 and 6 in Table 2). The frequency proportional quality factor Q values given in Table 2 correspond to a frequency f equal to 0.67 Hz, according to the following equation: Q ( f ) = Q0

f , (1) f0

where Q 0 = πf 0/γ, γ is an attenuation parameter, and f 0 is a reference value representative of the frequency range to be propagated, herein equal to 2 Hz (Stupazzini et al. 2009).

We considered two different preliminary static fault solutions, proposed by the Istituto Nazionale di Geofisica

Vp [m/s]

Vs [m/s]

ρ [kg/m 3]

Q

600 1870 2800 5500 5000 6000

300 1000 1500 3175 2890 3465

1700 2000 2300 2600 2700 2700

70 100 100 200 200 250

e Vulcanologia (INGV; http://www.sigris.it/) and by GNS (ht tp ://www.gns.cr i.nz/Home/News-and-Event s/MediaReleases/Most-damaging-quake-since-1931/Canterbury-quake/ Hidden-fault). The source parameters adopted for the two mod-

els are summarized in Table 3, while the slip distribution across the fault plane, along with the hypocenter location, is presented in Figure 3 for both the INGV and GNS seismic source inversions. The source model proposed by INGV was identified by the ASI-SIGRIS system, exploiting COSMO-SkyMed images (Atzori and Salvi 2011). The “GNS model,” published as a press release, is a static model derived from pre-earthquake and postearthquake geodetic data using both InSAR and GPS data. An updated version of the model is given by Beavan et al. (2011, page 789 of this issue) and a true kinematic source model based on inversions of strong-motion data has been developed

by Holden (2011, page 783 of this issue). While the two finite fault solutions have similar sizes and slip patterns, they differ quite significantly in strike angle and location of the hypocenter. It is important to emphasize that the “kinematic source models” used in this work have been obtained by turning these static models into kinematic models by assuming a rupture

770 Seismological Research Letters Volume 82, Number 6 November/December 2011

TABLE 3 Kinematic source parameters adopted for the simulation of the 22 February 2011 Christchurch earthquake. VR denotes the velocity rupture and τ the rise time Hypocenter °N INGV GNS

°E

Z

–43.58°N 172.68°E –10.3 km –43.56°N 172.70°E –6.47 km

LxW [km]

Strike [°]

Dip [°]

Rake [°]

Depth of upper points [km]

VR [km/s]

τ [s]

14 x 10 18 x 9

45 58

67 68

145 145

1.5 1

2.4 2.4

0.9 0.9

▲▲ Figure 3. Slip distribution according to the A) INGV and the B) GNS fault solutions. The superimposed star denotes the hypocenter location.

velocity, rise time, and slip origin, and simplifying the “GNS model” shown on the GNS Web site by making the rake constant, equal to 145°. A value V R equal to 2,400 m/s is assumed as rupture velocity. The slip source time function is given by an approximate Heaviside function, as follows:

(

)

1 t − 2τ ⎤ , (2) M 0 ( t ) = ⎡ 1+ erf 2.0 ⎢ 2⎣ τ / 2 ⎥⎦ where erf( ) is the error function and τ = 0.9 s is the rise time, assumed to be constant across the fault plane. Figure 4 shows a plot of moment rate function and its spectrum.

The 3D spatial discretization by spectral elements of the area requires the design of a large-scale unstructured mesh of hexahedral elements. The computational domain is subdivided into small chunks; each of them is meshed starting from the alluvial basin down to the bedrock. The mesh was constructed making use of the software CUBIT (available at http://cubit. sandia.gov/), according to the technique already described by Casarotti et al. (2007). Note that four different numerical models were constructed to attain the different hypotheses regarding, on one side, the seismic source (INGV vs. GNS) and, on the other side, the alluvial-volcano interface (see above). Both seismic source models, either INGV or GNS, have been tested with the: 1) “step-like” and 2) “smooth” approximation for the alluvial-volcano interface. The 3D hexahedral spectral element mesh adopted for the numerical simulations by GeoELSE con-

sists of about 476,000 and 496,000 elements for the INGV and GNS model, respectively. The size of the elements ranges, in both cases, from a minimum of about 150 m (at the top of the alluvial basin) up to 1,500 m at bedrock. The mesh is, hence, designed to propagate up to about 2 Hz, for spectral degree equal to 4 (see Figure 5). The numerical simulations were performed on the Lagrange cluster located at CILEA. The main characteristics and the performances of the analyses are summarized in Table 4.

NUMERICAL RESULTS IN THE NEAR-SOURCE REGION This section aims to show the main results obtained through the numerical simulations by GeoELSE. We compare the 3D synthetic seismograms with the observed waveforms at a set of stations located in the near-source region of the earthquake. In order to point out the role of the 3D geometry of the Canterbury Plains and of the kinematic seismic source, we proceed in the following way. At first, we address the issue regarding the effect of the model assumed for the alluvialbedrock transition, “step-like” vs. “smooth,” considering the fault solution proposed by INGV; afterward, we evaluate the dependence of the results on the fault model, INGV vs. GNS, considering only the “smooth” alluvial-bedrock transition.

Figures 6 and 7 show the comparison between 3D numerical simulations with strong ground-motion observations, in

Seismological Research Letters Volume 82, Number 6 November/December 2011 771

▲▲ Figure 4. A) Adopted moment rate function and B) its spectrum.

▲▲ Figure 5. A) 3D geometry of the area under study, with depth contours of the contact between the alluvial soft sediments and the rigid volcano materials (depth in meters). B) Zoom of the corresponding hexahedral spectral elements mesh.

TABLE 4 3D numerical models size and computational time. Data of CPU time refer to the Lagrange cluster located at CILEA. Model INGV – “Step-like” INGV – “Smooth” GNS – “Smooth”

SD

Number of Spectral Elements

Number of LGL Nodes

Number of cores

Simulation time (h)

4 4 4

475,992 475,992 495,385

~31.6 × 10 6 ~32 × 10 6 ~33.3 × 10 6

64 64 128

~75 ~80 ~107

772 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 6. Comparison of 3D numerical simulations by GeoELSE with strong ground motion observations, in terms of three-component velocity time histories, obtained at stations: A) HVSC, at outcrop rock; B) REHS, on soft sediments in the CBD; C) SHLC, on soft sediments at Re = 9 km; D) SLRC, on soft sediments at Re = 33 km, in the southwestern portion of the model. Observed and simulated data are bandpass filtered between 0.1 and 2.0 Hz

Seismological Research Letters Volume 82, Number 6 November/December 2011 773

▲▲ Figure 7. As in Figure 6 but in terms of velocity amplitude Fourier spectra.

774 Seismological Research Letters Volume 82, Number 6 November/December 2011

terms of velocity time histories and corresponding Fourier amplitude spectra, respectively. The comparison is presented for four representative stations: HVSC, which lies on outcropping rock; REHS, located on alluvial soil in the Central Business District (CBD); SHLC, situated on alluvial soil at epicentral distance Re = 9 km, close to the CBD; and SLRC, lying on alluvial soil at Re = 33 km, southwest of the epicenter. Both observed and simulated waveforms have been processed with a bandpass acausal Butterworth filter between 0.1 and 2 Hz. A quantitative estimation of the overall quality of the numerical analyses can be inferred evaluating the misfit parameters proposed by Anderson (2004). Stating that a single parameter is incomplete to assess the correspondence between simulated and observed time-histories, Anderson (2004) introduced a set of 10 parameters, each one evaluated for a specific frequency band of interest: Arias duration (AD), energy duration (ED), Arias intensity (AI), energy integral (EI), peak acceleration (PA), peak velocity (PV), peak displacement (PD), response spectra (RS), Fourier spectra (FS), and cross-correlation (CC). A score between 0 and 10, with 0 indicating no agreement and 10 perfect agreement, is calculated for each of these parameters, yielding an overall goodness of fit.

Figure 8 depicts the goodness of fit parameters computed in the frequency band 0.25–0.50 Hz for the three models under study, “step-like” INGV, “smooth” INGV, and “smooth” GNS, and for three components of motion (EW, NS, and UD).

The scores of the aforementioned parameters are shown for the 23 stations summarized in Table 1.

Dependence of Results on the Geometry of the Canterbury Plains In this section we address the issue regarding the role of the 3D geometry of the Canterbury Plains, i.e., the “step-like” vs. “smooth” model, on the simulated waveforms. Referring to Figures 6 and 7, we note that, in spite of the rough approximations behind the “step-like” model, the overall agreement is fairly satisfactory. For most of the considered stations, the “step-like” model reproduces the first arrivals and the PGVs with reasonable accuracy. The agreement between synthetics and observations is satisfactory for the stations located in the central-western portion of the Canterbury Plains, while it deteriorates for the station located on the southern volcanic region.

Nonetheless, such model tends to overestimate PGV values measured in the eastern area of Christchurch, where major liquefaction effects were observed. This effect may be due to the rough representation of the interface between the volcanic material and the soft soil of the basin, leading to an excessively large concentration of energy toward the city of Christchurch as a consequence of the high impedance contrast between the volcano region and the surrounding soft sediments.

The introduction of the “smooth” model yields significant improvements of the simulated waveforms, in particular for the reproduction of the coda-waves inside the alluvial plain.

The smooth interface between volcanic rock and alluvial soil leads to a better agreement in terms of PGV; nonetheless, the model still tends to overestimate the peak values measured in

the CBD and in the eastern-coastal area of Christchurch and to underestimate the peak values recorded far from the epicenter, especially in the southwestern region of the model.

Figure 8 allows us to have a quantitative criterion to assess the performance of the different numerical simulations. While the “step-like” model shows at least six stations with good scores for the whole set of parameters (Figure 8A), the INGV “smooth” model, with at least nine stations with good average scores (Figure 8B), yields actual improvements of the numerical analyses. As a general remark, for both simulations integral measures of Arias and energy duration (AD and ED) and peak ground acceleration, velocity, and displacement values (PA, PV, and PD) present a good fit for all the considered stations, while a poor score is achieved for intensity measures (AI and EI), spectral amplitudes, and cross-correlation (RS, FS, and CC).

Effect of the Kinematic Seismic Source After having illustrated the results obtained for the “step-like” and “smooth” model, we now turn to evaluating the effect of different kinematic seismic sources. To this end, we will show a comparison of the numerical results obtained for the INGV and GNS fault solutions (see Figure 3), relying on the “smooth” model, which turns out to produce satisfactory results as discussed in the previous section.

The comparison in Figures 6 and 7 shows that the GNS fault model leads to a better agreement between recorded and simulated ground motion velocities at the four stations under consideration. For this kinematic source model, a good agreement is found both at stations located on alluvial soil a few kilometers from the epicenter and at those stations located several kilometers farther away in the southwestern portion of the model. In spite of the rough assumptions behind the GNS “smooth” model, numerical simulations are able to reproduce with reasonable accuracy the PGVs within the Canterbury Plains. Nonetheless, the agreement between synthetics and observed values is still quite poor for the station located in the volcanic region. This is most likely due to the simplified model assumed for the topography of the Banks Peninsula, which is approximated as a smooth surface and does not capture the complex geometry of bays and coves that may play an important role in seismic wave propagation phenomena. Furthermore, a homogeneous soil profile is assumed for the volcano region, so that erosion and weathering phenomena of the surface rock layers are not taken into account.

The analysis of the Anderson misfit criteria (Figure 8C) confirms the quality of the numerical simulations, showing good average scores for almost all the stations under consideration. As mentioned previously, the figure highlights a good agreement in terms of PGVs (parameter PV) for many stations inside the computation domain. Figure 9 shows a comprehensive comparison between recorded and simulated (GNS “smooth” model) velocity time histories at the entire set of accelerometric stations inside the computational model, ordered by epicentral distance. A good agreement is found in terms of arrival times, peak ground values, and attenuation with distance, in spite of the rough assumptions concerning the characteriza-

Seismological Research Letters Volume 82, Number 6 November/December 2011 775

▲▲ Figure 8. Misfit parameters (Anderson 2004) of simulated ground motion for the three components of motion, evaluated at the 23 stations under consideration (see Table 1 and Figure 1) in the frequency band between 0.25 and 0.50 Hz. The results are shown for the A) INGV “step-like” , B) INGV “smooth” (center panel), and C) GNS “smooth” model (bottom panel).

776 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 9. Comparison between recorded and simulated (GNS “smooth”) velocity time histories (in cm/s), on the EW, NS, and UD component for the whole set of stations at Re < 40 km. The label on the left vertical axis reports the peak value of the corresponding time history.

tion of soil mechanical properties. To give a broad picture of seismic wave propagation effects, Figure 10 depicts the spatial distribution of PGV values obtained through GeoELSE along with the observed values (superimposed filled dots) for both the INGV and GNS “smooth” models. Furthermore, Figure 11 shows some representative snapshots of the simulated fault normal velocity wavefield, in which the seismic wave propagation field with the high contrast between rock and alluvial soil and the directivity toward the city of Christchurch is clearly distinguishable. In particular, looking at Figure 10 it is possible to notice that the GNS fault model better reproduces the variability of PGV values on the whole modeled area. From Figures 10 and 11 it is apparent that the INGV source model produces strong “up-dip” directivity effects in the central-eastern part of Christchurch, in reasonable agreement with the spatial distribution of observed damage and liquefaction phenomena. The wave propagation pattern obtained with the GNS fault model produces noticeable directivity off the sides of the fault due to the relatively shallow hypocenter (around 6.5 km) and because the rake is oblique (145°). This leads to larger ground motion amplitudes in the southwestern portion of the city.

Comparison with Observed Standard Spectral Ratios As a concluding check of the quality of 3D numerical simulations, in this section we compare simulations with observations in terms of standard spectral ratios (SSRs). Most of the 23 stations included in the numerical model are located on alluvial soil, while three of them lie on outcropping volcanic rock, namely HVSC, LPCC, and MQZ. For our purposes, we con-

sidered the four stations located in the Christchurch CBD, on soft alluvial sediments, namely CCCC, REHS, CHHC, and CBGS. Station LPCC, located on rock around 15 km southeast from the CBD, is considered as reference rock station.

The SSRs computed as the ratio of the Fourier spectrum of the recordings at CCCC, REHS, CHHC, and CBGS (geometric mean of the horizontal components), over that at LPCC reference station, are shown in Figure 12, for 4 September 2010 Darfield earthquake, with the epicenter located around 40 km west of the considered set of stations. We refer to the Darfield earthquake because we believe that this event is more reliable than the Christchurch one in evaluating the SSR because of the strong nonlinear soil behavior verified in the latter and the short distance (less than 10 km) between the epicenter and the considered stations. It is worth noting that stable resonance peaks are found at around 0.3, 0.6, 1.3, and, more consistently, at around 1.7 Hz. The recorded SSRs are compared, on the one hand, with the 1D analytical transfer function obtained for a system of four layers (layers 1, 2, 3, and 5 in Table 2) over a halfspace (layer 6 in Table 2), and, on the other hand, with the SSRs obtained through 3D numerical simulation (GNS “smooth” model). Compared with the 1D amplification function, the 3D SSRs show a better agreement with the records, pointing out resonance frequencies at about 0.4, 0.7, 1.3, and 1.7 Hz.

CONCLUDING REMARKS The main aim of this paper was to perform 3D numerical simulations of the M W 6.2 Christchurch earthquake on 22 February

Seismological Research Letters Volume 82, Number 6 November/December 2011 777

▲▲ Figure 10. Spatial variability of peak ground velocity (geometric mean of horizontal components) as estimated by 3D numerical simulations with the A) INGV “smooth” model and B) GNS “smooth” model. The recorded PGV values (filled dots) are superimposed for comparison purposes.

778 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 11. Snapshots (t = 7, 9, 11, and 13 s, from top to bottom) of the simulated fault normal velocity wavefield with the A) INGV “smooth” model and B) GNS “smooth” model.

Seismological Research Letters Volume 82, Number 6 November/December 2011 779

▲▲ Figure 12. Standard spectral ratios (SSRs) of the four stations of CBD: A) CCCC, B) REHS, C), and D) CBGS located on soft alluvial sediments, over the rock reference station LPCC. The SSRs obtained from strong ground motion recordings during the Darfield earthquake are compared with the 1D analytical transfer function and with the SSRs computed with the 3D numerical simulations (GNS “smooth” model).

2011, the most devastating and deadliest event of the seismic sequence that struck the Canterbury Plains, and particularly the city of Christchurch, between September 2010 and June 2011, and to compare the numerical results with strong ground motion observations.

The numerical simulations of seismic wave propagation within the Canterbury Plains, where widespread damage was recognized during the post-earthquake reconnaissance surveys, were performed by means of the Spectral Element code GeoELSE (http://geoelse.stru.polimi.it). Based on the available geological and seismological data, 3D numerical simulations of the Christchurch earthquake were carried out, combining the following features: 1) two different kinematic finite fault models, provided by INGV and GNS seismic source inversion studies, and 2) two simplified models for the description

of the interface between the stiff volcanic rock of the Banks Peninsula and the soft materials within the Canterbury Plains, referred to as the “step-like” and the “smooth” model. As a preliminary assumption a linear visco-elastic soil behavior was assumed. The comparison of the results obtained through 3D numerical simulations with the strong ground motion records in the epicentral area of the earthquake (Re < 40 km) shows a good agreement both in time and frequency domain, especially for the “smooth” model with the GNS kinematic extended fault model. It is worth remarking that the simplified assumption of linear visco-elastic soil behavior cannot adequately describe the amplification phenomena and the shift of fundamental frequency, clearly recorded in many stations located on the alluvial soil of the Canterbury Plains. Although the GNS “smooth” model is found to produce the best agreement with

780 Seismological Research Letters Volume 82, Number 6 November/December 2011

the observed waveforms, it should be noted that accounting for a more complex constitutive model could improve significantly the results of the INGV smooth model.

3D numerical simulations allow us to reproduce the most significant features of surface earthquake ground motion in the near-fault region. Ground motion shaking maps, in terms of PGV, and snapshots of simulated velocity wavefield are discussed, giving insights into seismic wave propagation effects in realistic geological structures and under near-fault conditions.

In spite of the simplified assumptions behind the numerical model, 3D numerical simulations represent a relevant tool to predict realistic earthquake ground motion in complex tectonic and geological environments, and for different seismic source scenarios that may play a major role in seismic hazard assessment studies.

ACKNOWLEDGMENTS The authors acknowledge Simone Atzori of INGV for kindly providing the data about the seismic source inversion studies. John Beavan and Caroline Holden of GNS are greatly acknowledged for their useful suggestions and remarks about the GNS source inversion adopted in this work. Also gratefully acknowledged are Pilar Villamor, Andrew King, Rafael Benites of GNS, Misko Cubrinovski, Brendon Bradley, John Berril of the Canterbury University, and Hugh Cowan of the Earthquake Commission of New Zealand (EQC). We are also grateful to Anselm Smolka, Martin Käser, and Alexander Allmann (Munich RE) for their fruitful comments. We deeply thank the research center CRS4 (http://www.crs4.it/) and in particular Fabio Maggio and Luca Massidda, for the essential cooperation in the development of GeoELSE. Finally, a particular thanks to Robert Graves, U.S. Geological Survey, for the detailed, constructive criticism he devoted to our paper.

REFERENCES Anderson, J. G. (2004). Quantitative measure of the goodness-of-fit of synthetic seismograms. In Proceedings of the 13th World Conference on Earthquake Engineering, Vancouver, B.C., Canada. Paper no.

243. Oakland, CA: Earthquake Engineering Research Institute.

Atzori, S., and S. Salvi (2011). Preliminary source of the destructive Christchurch earthquake identified by the SIGRIS system. SIGRIS Activities for the Christchurch (New Zealand) Earthquake; http:// www.sigris.it/.

Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).

Fine-scale relocation of aftershocks of the 22 February Mw 6.2 Christchurch earthquake using double-difference tomography.

Seismological Research Letters 82, 839–845.

Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters 82, 789–799.

Boon, D., N. D. Perrin, G. D. Dellow, R. Van Dissen, and B. Lukovic (2011). NZS 1170.5:2004 Site Subsoil Classification of Lower Hutt. Proceedings of the Ninth Pacific Conference on Earthquake Engineering, Building an Earthquake-Resilient Society. 14–16 April 2011, Auckland, New Zealand, paper no. 13. Auckland, New Zealand: 9PCEE.

Bradley, B. A., and M. Cubrinovski (2011). Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake. Seismological Research Letters 82, 853–865.

Casarotti, E., M. Stupazzini, S. Lee, D. Komatitsch, A. Piersanti, and J. Tromp (2007). CUBIT and seismic wave propagation based upon the spectral-element method: An advanced unstructured mesher for complex 3D geological media. In Proceedings of the 16th International Meshing Roundtable, ed. M. L. Brewer and D. Marcum, 579–597. New York: Springer.

Cubrinovski, M., and R. A. Green, eds. (2010). Geotechnical reconnaissance of the 2010 Darfield (Canterbury) earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43, 243–320.

Faccioli, E., F. Maggio, R. Paolucci, and A. Quarteroni (1997). 2D and 3D elastic wave propagation by a pseudospectral domain decomposition method. Journal of Seismology 1 (3), 237–251.

Forsyth, P. J., D. J. A. Barrell, and R. Jongens (2008). Geology of the Christchurch Area. Institute of Geological and Nuclear Sciences.

1:250 000 geological map 16, 1 sheet + 67 pp. Lower Hutt, New Zealand: GNS Science.

Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011). The Darfield (Canterbury, New Zealand) M W 7.1 earthquake of September 2010: A preliminary seismological report. Seismological Research Letters 82, 379–386.

Godfrey, N. J., F. Davey, T. A. Stern, and D. Okaya (2001). Crustal structure and thermal anomalies of the Dunedin region, South Island, New Zealand. Journal of Geophysical Research 106 (B12), 30,835– 30,848.

Godfrey, N. J., N. I. Christensen, and D. Okaya (2002). The effect of crustal anisotropy on reflector depth and velocity determination from wide-angle seismic data: A synthetic example based on South Island, New Zealand. Tectonophysics 355, 145–161.

Green, A. G., F. M. Campbell, A. E. Kaiser, C. Dorn, S. Carpentier, J. A. Doetsch, H. Horstmeyer, D. Nobes, J. Campbell, M. Finnemore, R. Jongens, F. Ghisetti, A. R. Gorman, R. M. Langridge, and A. F. McClymont (2010). Seismic reflection images of active faults on New Zealand’s South Island. In Fourth International Conference on Environmental and Engineering Geophysics, Chengdu, China, June 2010.

Green, R. A., C. Wood, B. Cox, M. Cubrinovski, L. Wotherspoon, B. Bradley, T. Algie, J. Allen, A. Bradshaw, and G. Rix (2011).

Use of DCP and SASW tests to evaluate liquefaction potential: Predictions vs. observations during the recent New Zealand earthquakes. Seismological Research Letters 82, 927–938.

Holden, C. (2011). Kinematic source model of the 22 February 2011 Mw 6.2 Christchurch earthquake using strong motion data.

Seismological Research Letters 82, 783–788.

Kam, W. Y., U. Akguzel, and S. Pampanin (2011). 4 Weeks On: Preliminary Reconnaissance Report from the Christchurch 22 Feb 2011 6.3 M W Earthquake; http://db.nzsee.org.nz:8080/en/web/ chch 2011/structural/.

Kleffmann, S., F. Davey, A. Melhuish, D. Okaya, T. Stern, and the SIGHT Team (1998). Crustal structure in the central South Island, New Zealand, from the Lake Pukaki seismic experiment. New Zealand Journal of Geology and Geophysics 41, 39–49.

Long, D. T., S. C. Cox, S. Bannister, M. C. Gerstenberger, and D. Okaya (2003). Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data. New Zealand Journal of Geology & Geophysics 46, 21–39.

Melhuish, A., S. Holbrook, F. Davey, D. Okaya, and T. Stern (2005).

Crustal and upper mantle seismic structure of the Australian plate, South Island, New Zealand. Tectonophysics 395, 113–135.

Mortimer, N., F. J. Davey, A. Melhuish, J. Yu, and N. J. Godfrey (2002).

Geological interpretation of a deep seismic reflection profile across the Eastern Province and Median Batholith, New Zealand: Crustal architecture of an extended Phanerozoic convergent orogen. New Zealand Journal of Geology & Geophysics 45, 349–363.

Seismological Research Letters Volume 82, Number 6 November/December 2011 781

Quigley, M., R. Van Dissen, P. Villamor, N. Litchfield, D. Barrell, K. Furlong, T. Stahl, B. Duffy, E. Bilderback, D. Noble, D. Townsend, J. Begg, R. Jongens, W. Ries, J. Claridge, A. Klahn, H. Mackenzie, A. Smith, S. Hornblow, R. Nicol, S. Cox, R. Langridge, and K. Pedley (2010). Surface rupture of the Greendale fault during the M W 7.1 Darfield (Canterbury) earthquake, New Zealand: Initial findings. Bulletin of the New Zealand Society for Earthquake Engineering 43, 236–242.

Reyners, M., and H. Cowan (1993). The transition from subduction to continental collision: Crustal structure in the North Canterbury region, New Zealand. Geophysical Journal International 115, 1,124–1,136.

Scherwath, M., T. Stern, F. Davey, D. Okaya, W. S. Holbrook, R. Davies, and S. Kleffmann (2003). Lithospheric structure across oblique continental collision in New Zealand from wide-angle P wave modeling. Journal of Geophysical Research 108 (B12), 2,566; doi:10.1029/2002JB002286.

Semmens, S., N. D. Perrin, G. Dellow, and R. Van Dissen (2011).

NZS 1170.5:2004 Site subsoil classification of Wellington City. Proceedings of the Ninth Pacific Conference on Earthquake Engineering, Building an Earthquake-Resilient Society, 14–16 April 2011, Auckland, New Zealand, paper no. 7. Auckland, New Zealand, 9PCEE.

Smerzini, C., R. Paolucci, and M. Stupazzini (2011). Comparison of 3D, 2D and 1D numerical approaches to predict long period earthquake ground motion in the Gubbio plain, central Italy. Bulletin of Earthquake Engineering (June) 1–23; doi:10.1007/s10518-0119289-8.

Stupazzini, M., R. Paolucci, and H. Igel (2009). Near-fault earthquake ground motion simulation in the Grenoble Valley by a high performance spectral element code. Bulletin of the Seismological Society of America 99, 286–301.

Tonkin and Taylor Ltd. (2010). Darfield Earthquake 4 September 2010, Geotechnical Land Damage Assessment & Reinstatement Report.

Earthquake Commission. Stage 1 Report for the New Zealand Earthquake Commission. Christchurch, New Zealand: Tonkin & Taylor Ltd.

782 Seismological Research Letters Volume 82, Number 6 November/December 2011

Department of Structural Engineering Politecnico di Milano Piazza Leonardo da Vinci, 32 Milano 20133 Italy guidotti@stru.polimi.it

(R. G.)

Kinematic Source Model of the 22 February 2011 Mw 6.2 Christchurch Earthquake Using Strong Motion Data Caroline Holden

Caroline Holden GNS Science

INTRODUCTION The Canterbury earthquake sequence began in September 2010 with the Mw 7.1 (source: GeoNet catalog, http://geonet. org.nz/canterbury-quakes/) Darfield earthquake that ruptured the previously unknown 40-km-long Greendale fault 30 km west of Christchurch (Gledhill et al. 2011). Extreme ground accelerations as high as 1.8 g near the epicenter were recorded.

The event caused intense liquefaction in the eastern suburbs of Christchurch as well as closer to downtown, near the course of the Avon River. The Darfield earthquake was followed by a major aftershock on 22 February local time (21 February UTC) of magnitude Mw 6.2 (source: GeoNet), but Me 6.7 (source: USGS, http://earthquake.usgs.gov/earthquakes/ eqinthenews/2011/usb0001igm/neic_b0001igm_e.php). This earthquake was centred only a few kilometers south of the Christchurch city center. Extremely high accelerations (as high as 2.2 g) were also recorded near the epicenter (Kaiser, Benites et al. 2011). In addition to the extreme liquefaction seen after the Darfield earthquake, this event also caused landslides, large rockfalls, widespread damage to earthquake-risk buildings in Christchurch, and, most tragically, about 180 casualties. Another large aftershock of Mw 6.0 (source: GeoNet), but with Me 6.7 (source: USGS), subsequently occurred on 13 June local time (12 June UTC) just a few kilometers south of the February event, causing further damage, landslides, rockfalls, and liquefaction.

Following the Darfield earthquake, the GeoNet network (New Zealand National Hazard Monitoring Network) and its regional component the CanNet network (Berrill et al. 2011) was supplemented by the deployment of 13 additional strong motion instruments regionally (and another nine following the February earthquake). We used this dense network of strong motion instruments to constrain the source kinematics of the February event. We present the inversion scheme and discuss its limitations. These results are preliminary, since more thorough data processing is needed; however, they already provide a key model that will help in understanding the sequence of large aftershocks that has developed near Christchurch. This work is strongly dependent on other studies by Beavan et al. 2011, page 789 of this issue; Fry et al. 2011; Bannister et al. 2011, page doi: 10.1785/gssrl.82.6.783

839 of this issue; Sibson et al. 2011, page 824 of this issue; and Kaiser, Benites et al. (2011).

THE STRONG MOTION DATASET At the time of the February earthquake there were 14 strong motion GeoNet sites, from both the national and the regional Canterbury network CanNet, within 20 km of the epicenter (Figure 1). However, there were strong site effects at stations PRPC, SHLC, and HPSC, each of which sits on very soft ground and suffered intense liquefaction from the earthquake; therefore those three were excluded, leaving 11 stations to be included in the inversion scheme. The source-station distance ranges from 2 to 20 km.

All of the recordings used in this study suffered from site effects to some degree. Stations on rock sites are found only on the hills of Banks Peninsula (south of Christchurch) where strong topographic effects are the likely cause of an intense damage pattern over the hills of Banks Peninsula as described

by Hancox et al. (2011). Stations on the plains suffered from very soft shallow layers inducing non-linear amplifications and extreme phenomena such as liquefaction and trampoline effects (Fry et al. 2011). Unfortunately, ground conditions within Christchurch are highly variable and will require further studies for stations in this region to be included in the modeling (Kaiser, Holden et al. 2011).

For our inversion study, the acceleration data has been integrated into velocity and filtered using a Butterworth bandpass filter from 0.1 to 1.0 Hz. Since we are interested in the polarity and amplitude of the first onset we used a causal filter.

We applied the same filter to observed and synthetic data. The data from the CanNet stations were rotated from their original orientation to north-south and east-west components.

INVERSION SCHEME We inverted three-component data for 11 well-distributed strong motion stations within 20 km of the epicenter. We used a fixed fault plane geometry of strike 59 and dip 67 as defined

by Beavan et al. 2011 (page 789 of this issue) since processed InSAR data clearly shows deformation fringes resulting from

Seismological Research Letters Volume 82, Number 6 November/December 2011 783

slip on a 59-degree striking fault plane. Their solution is also very similar to the regional moment tensor solution of strike/ dip/rake: 55/66/129 (Sibson et al. 2011, page 824 of this issue). We constrained the fault plane location using the relocated hypocenter of Bannister et al. 2011 (page 839 of this issue) of latitude –43.571, longitude 172.703, and 6 km depth.

The geometry of the fault plane is fixed, but the rupture is allowed to start anywhere on the fault plane. We didn’t constrain the rupture to start at the hypocenter since we are inverting for the time history of the low-frequency source of energy release, which can have a different origin time location than the hypocenter. In order to account for delays in the onset of the main slip patch, the slip distribution is independent of the starting time. The modeled fault plane dimensions are 20 × 20 km discretized into 400 1 × 1 km2 subfaults. We invert for an elliptical distribution of slip on the fault plane (Di Carli et al.

2010) described by nine parameters: the size (two semi-axis lengths) and location (along strike and along dip) of the ellipse, location of rupture starting time (along strike and along dip), rake, maximum slip, and rupture velocity.

Synthetic seismograms were computed using the discrete wavenumber approach of Bouchon (1981), using the 1D velocity model from Reyners and Cowan (1993).

To search for a minimum waveform misfit, we used the non-linear Neighborhood Algorithm (Sambridge 1999). The misfit function is a least-squares scheme applied to the first 10 seconds of the recordings. The 10-second window is justified by the short rupture time of this moderate-size earthquake and the proximity of the stations (less than 20 km). The following search parameters are empirically based on many years of experience using the neighborhood algorithm for kinematic inversion purposes. In this study the parameters are tuned to be more exploitative than explorative (Sambridge 1999). The inversion scheme first computes 400 models then runs 600 iterations. For each iteration, the algorithm computes 12 models and selects the four models with the lowest misfit to define the parameter search for the next iteration.

RESULTS: SOURCE MODELS A total of 7,600 models were computed during the inversion.

Our final slip distribution is shown in Figure 2. The inversion procedure has converged clearly (Figure 3) to a final misfit value of 45.1458. Each individual parameter has also converged sharply, and the differences between the final values and values of the last 1,000 computed models are negligible as seen on Figure 4. The misfit value for model 6530 (iteration 511) is 45.1476.

The waveform fit is overall very good (Figure 5). All three onset, amplitude, and polarities criteria are well-matched. The match is even better for vertical components, showing fewer complexities than the horizontal components, where we also fit the first wave envelopes of the synthetic and observed seismograms.

The final slip distribution parameters are shown on Figure 1 and Figure 2 and described in Table 1. The rupture is charac-

terized by a high rupture velocity of 2.8 km/s and a maximum slip value of 4.2 m. The maximum slip is located at about 4 km depth; there is still up to 1.5 m slip at 500 m depth. The rupture area is 12 by 18 km2 . The total rupture duration is less than 4 seconds. The rake angle is 135 degrees. The direction of the rupture is nearly vertical, oriented toward Christchurch, hence contributing to the extreme ground shaking experienced in Christchurch city.

DISCUSSION This proposed kinematic model shows consistent location of a slip patch right below the Avon River estuary, depth range (maximum slip around 4 km depth), and moment of 3.46 × 1018 Nm to the model obtained by Beavan et al. 2011 (Mo 3.13 × 1018 Nm) (page 789 of this issue). However, in their one-fault model, Beavan et al. obtained a smaller maximum slip amplitude (2.5 m), although this value can increase by changing the smoothing parameter in their inversion (Beavan, personal communication 2011). Our rake angle of 135 degrees shows a larger reverse component than their value of 154 degrees; however, the rake angle of their main fault decreases if they introduce a secondary strike-slip source. Our solution is very close to the regional moment tensor solution of strike/dip/ rake 55/66/129 (Mo 2.49 × 1018Nm) (Sibson et al. 2011, page 824 of this issue).

The proposed kinematic model is characterized by high rupture parameter values for an earthquake of its size such as high rupture velocity (2.8 km/s) and very large slip (maximum 4.2 m), suggesting that this was a high stress drop event. The high rupture velocity is also noted in Fry et al. 2011, page 833 of this issue; based on data filtered up to 5 Hz, their model requires a rupture velocity of 3.2 km/s in order to reproduce the very high accelerations observed near source. Finally, the large difference between the energy magnitude of 6.7 (USGS) for this earthquake versus the Mw magnitude of 6.2 supports the possibility of this event being a high stress drop event (Fry and Gerstenberger 2011, page 833 of this issue).

Similar to the Darfield Mw 7.1 event, geodetic studies and seismic data observations of the February earthquake suggest that this is a segmented event. First, geodetic studies (Beavan et al. 2011, page 789 of this issue) suggest that a second strike-slip source (strike/dip/rake 80/90/180) of magnitude Mw 5.9 was involved in the rupture process. However they are not able to assign a chronology to the two events. Then, the focal mechanism solution using first motions analysis of the raw data (B. Fry, personal communication 2011) indicates an almost pure right-lateral strike-slip mechanism (strike/dip/ rake 95/75/180), strongly suggesting that the February earthquake started on a small strike-slip patch and then ruptured a larger oblique-reverse slip patch. Finally, the waveform data used in the present kinematic inversion (Figure 5) show a dominant peak, even larger on the horizontal components, coming a few seconds after the main rupture and prior to the 3-second resonant site effect for the soft site stations (the 3-second site effect was observed also in the Darfield earthquake by Cousins

784 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 1. Final slip distribution on a plane oriented 59 o strike and 67o dip. Only the non-zero slip patches are shown here. The red dots are relocated aftershocks from Bannister et al. (2011, this issue) up until 29 May 2011. The light gray region represents the area of Christchurch city. The stations named xxxC belong to the regional CanNet strong motion network of GeoNet; others are GeoNet national strong motion stations. Stations used for the inversions are in bold straight letters. The slip distribution is characterized here by a patch of maximum 4.2 m slip occurring north and up-dip of the relocated hypocenter (yellow star). Banks Peninsula volcano extends from just south of HVSC and beyond MQZ.

▲▲ Figure 2. Slip and rake history for the fault plane oriented strike 59 o, dip 67o. Slip is shown in color, and the rake is represented by black vectors for each grid cell. Distances are in kilometers and rupture time iso-contours are in seconds. The yellow star is the relocated hypocenter from Bannister et al.

(2011, this issue).

▲▲ Figure 3. Convergence of the inversion scheme after 600 iterations. Each iteration computes a waveform misfit value for 12 models; a total of 7,600 models have been computed.

Seismological Research Letters Volume 82, Number 6 November/December 2011 785

▲▲ Figure 4. Convergence of inverted parameters for the fault plane described above after 7,600 models: A) rupture initiation location along strike (km), B) rupture initiation location along dip (km), C) ellipse starting point along strike (km), D) ellipse starting point along dip (km), E) rake angle (degree), F) rupture velocity (km/s), G) and H) semi-axis lengths of the ellipse (km), I) maximum slip (m). The y-axis is the parameter space range sampled during the inversion. The x-axis is the number of models run. The figure shows excellent convergence of the parameters to their final values detailed in Table 1.

and McVerry 2010). For central Christchurch stations this signal is even larger than the signal modeled from the main patch.

This strongly suggests the presence of a large strike-slip source shortly following the main slip patch. Therefore preliminary seismic observations indicate that at least three subevents were involved in the overall rupture process. This is a subject of ongoing studies.

SUMMARY AND FURTHER STUDIES This model is based on a comprehensive kinematic inversion scheme: high-frequency velocity seismograms and well distributed very-near-source stations. The results are consistent with other source models of the February earthquake and observed characteristics of a very energetic event. Simple waveform

TABLE 1 Source Parameters for the Final Source Model Max Slip (m) Rake (degrees) Half-length of main axis (km) Half-length of secondary axis (km) Depth (km) @ max slip Min. depth (km) Max Depth (km) Vr (km/s) M 0 (×1018 Nm) Mw

786 Seismological Research Letters Volume 82, Number 6 November/December 2011

4.2 135 8.86 6.0 4.2 0.5 9.7 2.80 3.46 6.3

▲▲ Figure 5. Observed (black line) and synthetic (dashed line) velocity seismograms computed for the slip model described in the paper; the duration is 30 seconds. Values above the traces are the maximum observed absolute velocities (m/s). The model fits well the onset polarities and amplitudes, especially for the vertical components. The top four recordings are from stations located on rock sites or very shallow soft sites (CMHS); the other ones are located on very soft sites in the plains. A resonant period of about 3 seconds is noticeable on “soft site” horizontal recordings. A sharp single peak, not modeled by our solution, is also noticeable just prior to the 3-second period signal (at 10 seconds on CCCC) on all horizontal components. The amplitude of the peak is actually twice as large for stations further away from the modeled fault plane.

Seismological Research Letters Volume 82, Number 6 November/December 2011 787

observations and other source studies that suggest the occurrence of at least one secondary event make this earthquake very segmented for its magnitude. Such rupture segmentation has already been observed regionally for the Darfield Mw 7.1 earthquake, as well as in preliminary studies of another large aftershock, the Mw 6.0 June earthquake (Beavan, personal communication 2011). Earthquakes in Canterbury are also particularly energetic (larger ones all present a high Me/Mw ratio (Fry and Gerstenberger 2011, page 833 of this issue). The region is characterized by the presence of a very dehydrated and brittle structure, the Hikurangi plateau (Reyners and Cowan 1993). This structure pushes the regional brittle-ductile transition deeper (35 km), fostering strain release following a large event through the generation of aftershocks rather than aseismic slip. The Christchurch area is also marked by the presence of the intraplate volcanism formation of the now extinct Banks Peninsula volcano, about 11 My ago (Timm et al. 2009). The intrusion of the volcano has not only highly segmented faults in the region near Christchurch, but may have also brought closer to the surface the very brittle and dehydrated Hikurangi plateau. This segmented and energetic fault system may explain an event like the February earthquake.

Finally, we hope to bypass issues arising from variable site conditions by first obtaining a better defined local velocity model from aftershock studies (Reyners et al. 2011; Bannister et al. 2011, page 839 of this issue), and second by using the large database of well-recorded aftershocks as empirical Green’s functions. This will allow us to increase the frequency bandwidth of the waveforms and hence define the slip history in more details.

ACKNOWLEDGMENTS The author would like to acknowledge the anonymous reviewer for significantly improving this manuscript. This study made use of SAC and GMT software.

REFERENCES Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).

Fine-scale relocation of aftershocks of the 22 February Mw 6.2 Christchurch earthquake using double-difference tomography.

Seismological Research Letters 82, 839–845.

Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters 82, 789–799.

Berrill, J., H. Avery, M. B. Dewe, A. Chanerley, N. Alexander, C. Dyer, C. Holden, and B. Fry (2011). The Canterbury Accelerograph Network (CanNet) and some results from the September 2010, M 7.1 Darfield earthquake. In Proceedings, Ninth Pacific Conference on Earthquake Engineering, NZSEE, Auckland, New Zealand paper no. 181.

Bouchon, M. (1981). A simple method to calculate Green’s functions for elastic layered media. Bulletin of the Seismological Society of America 71, 959–971.

Cousins, J., and G. McVerry (2010). Overview of strong motion data from the Darfield earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 222–227.

Di Carli, S., C. François-Holden, S. Peyrat, and R. Madariaga (2010).

Dynamic inversion of the 2000 Tottori earthquake based on elliptical subfault approximations. Journal of Geophysical Research 115, B12328; doi:10.1029/2009JB006358.

Fry, B., R. Benites, M. Reyners, C. Holden, A. Kaiser, S. Bannister, M. Gerstenberger, C. Williams, J. Ristau, and J. Beavan (2011). Very strong shaking in the New Zealand earthquakes. Submitted to Eos. Fry, B., and M. Gerstenberger (2011). Large apparent stresses from the Canterbury earthquakes of 2010 and 2011. Seismological Research Letters 82, 833–838.

Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011).

The Darfield (Canterbury, New Zealand) Mw 7.1 earthquake of September 2010: A preliminary seismological report. Seismological Research Letters 82 (3), 378–386; doi:10.1785/gssrl.82.6.378.

Hancox, G., C. Massey, and N. Perrin (2011). Landslides and related ground damage caused by the Mw 6.3 Christchurch earthquake of 22 February 2011. Geomechanics News (New Zealand) 81 (June 2011), 53–67.

Kaiser, A. E., R. A. Benites, A. I. Chung, A. J. Haines, E. Cochran, and B. Fry (2011). Estimating seismic site response in Christchurch city (New Zealand) from dense low-cost aftershock arrays. Extended Abstract of the Fourth IASPEI/IAEE International Symposium on the Effects of Surface Geology on Seismic Motion, August 23–26, Santa Barbara, California.

Kaiser, A., C. Holden, J. Beavan, D. Beetham, R. Benites, A. Celentano, D. Collett et al. (2011). The February 2011 Christchurch earthquake: A preliminary report. Submitted to New Zealand Journal of Geology and Geophysics.

Reyners, M. E., and H. Cowan (1993). The transition from subduction to continental collision: Crustal structure in the north Canterbury region, New Zealand. Geophysical Journal International 115 (3), 1,124–1,136.

Reyners, M., D. Eberhart-Phillips, and S. Bannister (2011). Tracking repeated subduction of the Hikurangi Plateau beneath New Zealand. Earth and Planetary Science Letters; doi:10.1016/j.epsl.

Sambridge, M. (1999). Geophysical inversion with a neighbourhood algorithm—I. Searching a parameter space. Geophysical Journal International 138, 479–494.

Sibson, R., F. Ghisetti, and J. Ristau (2011). Stress control of an evolving strike-slip fault system during the 2010–2011 Canterbury, New Zealand, earthquake sequence. Seismological Research Letters 82, 824–832.

Timm, C., K. Hoernle, P. Bogaard, I. Bindeman, and S. Weaver (2009).

Geochemical evolution of intraplate volcanism at Banks Peninsula, New Zealand: Interaction between asthenospheric and lithospheric melts. Journal of Petrology 50 (6), 989–1,023.

788 Seismological Research Letters Volume 82, Number 6 November/December 2011

GNS Science 1 Fairway Drive Lower Hutt Avalon 5010 New Zealand c.holden@gns.cri.nz

E

Fault Location and Slip Distribution of the 22 February 2011 MW 6.2 Christchurch, New Zealand, Earthquake from Geodetic Data John Beavan, Eric Fielding, Mahdi Motagh, Sergey Samsonov, and Nic Donnelly

John Beavan,1 Eric Fielding, 2 Mahdi Motagh, 3 Sergey Samsonov,4 and Nic Donnelly5 Online material: Additional figures showing interferograms and fault models; data tables

INTRODUCTION The 22 February (local time) M W ~6.2 Christchurch earthquake occurred within the aftershock region of the 4 September 2010 M W 7.1 Darfield (Canterbury) earthquake (Gledhill et al. 2011). Both the Darfield and Christchurch earthquakes occurred on previously unknown faults in a region of historically low seismicity, but within the zone of plate boundary deformation between the Pacific and Australian plates. The Darfield earthquake caused surface rupture up to 5 m (Quigley et al. 2010, forthcoming), but none has been observed associated with the Christchurch earthquake. Geodetic data indicate that strain has been slowly accumulating within the region (Wallace et al. 2007; Beavan et al. 2002), and the presence of active subsurface faults was known or suspected (e.g., Pettinga et al. 2001). Earthquakes of magnitude up to 7.2 in this region had been allowed for in the national seismic hazard model (Stirling et al. 2002), but the observed high apparent stresses (Fry and Gerstenberger 2011, page 833 of this issue) and high ground accelerations (Fry et al. 2011, page 846 of this issue) had not been anticipated, particularly those experienced in the Christchurch event. These and other factors (Fry and Gerstenberger 2011, page 833 of this issue; Fry et al.

2011, page 846 of this issue; Holden 2011, page 783 of this issue), plus the close proximity of the February earthquake to 1. GNS Science, Lower Hutt, New Zealand 2. Jet Propulsion Laboratory/Caltech, Pasadena, California, U.S.A. 3. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany; also at Department of Geomatics and Surveying Engineering, University of Tehran, Tehran, Iran 4. European Center for Geodynamics and Seismology, Walferdange, Luxembourg; now at Canada Centre for Remote Sensing, Ottawa, Canada 5. Land Information New Zealand, Wellington, New Zealand doi: 10.1785/gssrl.82.6.789

Christchurch city center, were responsible for the major damage caused by the earthquake (e.g., Kaiser et al. 2011).

A large amount of geodetic ground-displacement data is available to constrain the source of the earthquake, in part because we reoccupied nearly 200 GPS sites that had been observed following the Darfield earthquake, and in part because a number of space agencies collected synthetic aperture radar (SAR) data over the source area that we were able to use in differential interferometric SAR (DInSAR) processing. The geodetic data were collected one day to seven weeks following the February earthquake, so they include ground deformation due to aftershocks, in particular the M W 5.8 and M W 5.9 events that occurred within two hours of the mainshock.

To first order, the earthquake source can be modeled as a planar fault striking ~59° and dipping ~69° to the southeast.

The peak slip of 2.5–3 m is a mixture of reverse and right-lateral slip and is located ~7 km east-southeast of Christchurch city center at a depth of ~4 km. Slip of ~1 m reaches within ~1 km of the ground surface. The slip near the southwest end of the plane is approximately right-lateral with magnitude ~1 m. The geodetic data are significantly better fit by two fault planes, a compact region of oblique slip on the fault described above, plus right-lateral strike slip on a near-vertical fault to its southwest that coincides with the locations of the two major aftershocks and with a trend of smaller aftershocks. A lobe of ground uplift seen in some of the SAR data (e.g., Figure 4) just west of the main slip patch is not well modeled, and suggests some slip may also have occurred elsewhere, perhaps on a splay off the main fault plane.

GEODETIC DATA We use campaign GPS data collected between 28 February and 14 April from 57 sites (Figure 1) that were also occupied following the September 2010 Darfield earthquake (Beavan, Samsonov, Motagh, et al. 2010). We also use continuous GPS (cGPS) data from five regional sites operated by GeoNet (http://www.geonet.org.nz) for Land Information

Seismological Research Letters Volume 82, Number 6 November/December 2011 789

(A)

-42.5

KAIK

-43.0 Christchurch

-43.5

METH

MQZG

-44.0

m/yr

37 m -44.5

100 km

WAIM 171.0

-43.2

172.0

173.0

(B)

-43.4 METH

-43.6

MQZG -43.8

25 km

-44.0

171.5

172.0

172.5

173.0

(C) -43.45

-43.50

AHE

-43.55

C -43.60

5 km

172.50

172.60

172.70

172.80

▲▲ Figure 1. GPS sites used in analysis. A) Regional GPS sites (red triangles) and active faults (purple lines). Station CHAT is 800 km due east of Christchurch. Arrow shows Pacific plate motion relative to Australia. B) Red triangles show continuous sites (with LINZ sites labeled), blue triangles show 24-hour occupation sites, and green triangles show four-hour occupation sites (see text). Orange line shows surface rupture from 4 September 2010 Darfield earthquake. C) Crosses show short-occupation sites in Christchurch city and also indicate the approximate extent of the city. Displacements at sites with black crosses are large outliers to the dislocation model predictions and are downweighted in the inversion. AHE and C denote the Avon-Heathcote estuary and Cashmere.

New Zealand (LINZ) and eight sites in Christchurch operated by private companies. In addition, we use lower-accuracy campaign GPS data collected between 14 April and 27 April from 123 sites within Christchurch City and surrounding suburbs, eight of which were also observed in the higher-accuracy campaign dataset. The GPS displacement data are listed in Table S1 in the Supplementary Information. We use differential interferometric synthetic aperture radar (DInSAR; Table 1) from the Italian Cosmo-SkyMed (CSK) X-band (3.1 cm wavelength) radar satellite, both from an ascending track with images acquired on 19 February and 23 February, and a descending track with images acquired on 20 February and 16 March. We also have available two ascending interferograms from the Japanese ALOS/PALSAR L-band (23.6 cm wavelength) instrument, with time spans 10 January–25 February 2011 and 27 October 2010–14 March 2011; we use only the second of these in our modeling as it has better spatial coverage.

GPS Processing The “high accuracy” campaign GPS dataset is composed of a mix of 34 stations that have at least one 24-hour session both before and after the earthquake and 23 stations with at least one four-hour session both before and after (Figure 1). We processed these data together with the cGPS data by standard methods (e.g., Beavan, Samsonov, Denys, et al. 2010) using Bernese version 5.0 software (Dach et al. 2007) to give station coordinates and their estimated covariances for each day of observation. We placed the coordinates in the IGS05 reference frame by a translation of each daily solution to best fit the IGS05 coordinates of a set of regional Australian and Pacific sites at the epoch of survey. We combined the resulting pre-earthquake daily coordinate and covariance data using least-squares inversion software “adjcoord” (Bibby 1982; Crook 1992) to give minimally constrained pre-earthquake coordinates and covariances by holding station CHAT (Chatham Island, Figure 1) fixed. During this procedure, we multiplied the formal covariances from the Bernese software by 25 (i.e., uncertainties are multiplied by 5) to account for unmodeled temporal correlation between the 180-sec samples used in the final stages of the GPS processing (e.g., Darby and Beavan 2001). We used the same procedure on the post-earthquake data. We are able to make two pre-earthquake position estimates for a number of the cGPS stations in Christchurch. In one case we use pre-earthquake data at the same epoch as the pre-earthquake campaign data (September– October 2010); in the other we take pre-earthquake continuous data from shortly before the earthquake (January–February 2011). There are no significant systematic differences between the two estimates after taking interseismic motion into account (see Table S1). We also found that post-seismic deformation at GPS sites in the vicinity of the Darfield earthquake was small (<10 mm) in the one to eight weeks following that quake (Beavan, Samsonov, Motagh, et al. 2010). We take these two observations as evidence that we can treat campaign GPS observations from September to October 2010 as immediate pre-earthquake observations for the Christchurch earthquake, provided we take interseismic motion into account. Similarly,

790 Seismological Research Letters Volume 82, Number 6 November/December 2011

TABLE 1 InSAR Details

Data type

Track*

CSK asc

—

CSK desc

—

ALOS asc

336

ALOS asc (not used)

335

Dates, yyyymmdd

Multi-looks, pixels Baseline, m

Across-track

Along-track

Approx ground resolution, m

20

10

10

22

123

16

16

30

2100

3

6

40

359

3

6

40

20110219 20110222 20110220 20110316 20101027 20110314 20110110 20110225

* CSK track numbers are not available.

continuous GPS data (unpublished) from sites in Christchurch show that post-seismic deformation from a few days to three months after the February earthquake is small, so we are justified in combining all the post-earthquake geodetic data into a single post-earthquake solution. We combine the pre- and post-earthquake coordinates and covariances using in-house least-squares inversion software “disp” to generate coseismic displacements and their estimated uncertainties (Table S1).

Plate tectonic motion and reference frame rotation in the ~six months between the pre- and post-earthquake coordinate sets is accounted for by solving in the inversion for the rigid-body translation and rotation of three far-field sites nominally on the Pacific plate: CHAT (Chatham Island), KAIK (Kaikoura), and WAIM (Waimate) (Figure 1A).

The “low-accuracy” campaign GPS data consist of two 30-minute observations in October 2010 and similar observations in April 2011 (Figure 1C). The data were collected for LINZ by Opus International Consultants Limited and processed to baseline solutions using industry software. The baselines were provided to LINZ, and baseline errors were estimated from experience with similar surveys. The data were combined using LINZ’s geodetic adjustment software “SNAP” into a preearthquake and a post-earthquake solution. In each solution, three stations common to the high- and low-accuracy surveys (MQZG, 5508, B87X; see Table S1) were tightly constrained to their coordinates derived in the processing of the high-accuracy surveys. Based on adjustment results and experience, the resulting coordinates were assigned horizontal 1s uncertainties of 20 mm and a vertical 1s uncertainty of 30 mm.

SAR Processing The single-look complex CSK ascending (satellite moving north and looking eastward) data were processed using the SARscape software (http://www.sarmap.ch/), while the descending (satellite moving south and looking westward) CSK data were processed from raw data with the Jet Propulsion Laboratory (JPL)/Caltech ROI_pac software. The raw ALOS PALSAR data were processed using the GAMMA software (http:// www.gamma-rs.ch). The topographic contribution to the interferometric phase was removed using a 3 arc sec digital elevation

model (DEM) from the Shuttle Radar Topography Mission (SRTM). The DInSAR interferogram phases were then filtered using a weighted power spectrum technique (Goldstein and Werner 1998). The CSK data were unwrapped using a minimum cost flow algorithm (Chen and Zebker 2002), and the ALOS PALSAR data were unwrapped using a branch-cut region growing algorithm (Rosen et al. 1994). Finally, all interferograms were projected to a geographic grid using the SRTM DEM. Further details of the DInSAR processing are given in Table 1.

Before modeling, each of the interferograms is sampled with a quadtree algorithm (e.g., Jónsson et al. 2002). In this procedure, the scene is divided into four quadrants. If the root mean square (rms) scatter about the mean in any quadrant exceeds a given threshold, the quadrant is divided into four new quadrants, and the rms scatter about the mean is again tested. The process continues iteratively until convergence.

Data reduced in this manner represent the statistically significant portion of the signal with far fewer sampling points than the original, 300–400 points per interferogram in this case (see Figures S1–S3).

In the processed interferograms, the shorter wavelength CSK interference patterns (Figures 2, S1, S2) are incoherent throughout substantial parts of Christchurch city, presumably because the scattering properties of the ground changed due to ground failure and/or building damage. The longer-wavelength ALOS data are also incoherent over some parts of the city, but do show regions of coherence (Figure 3) where it is lacking in the CSK data (Figure 2). In particular, a coherent region of towardsatellite fringes is visible just west of the Avon-Heathcote estuary in both the track 335 and track 336 ALOS interferograms (e.g., Figure 3). However, we were not able to reliably phase connect this area with the rest of the interferogram, so it has been omitted from the unwrapped image we use in our modeling (Figure S3) by setting a high coherence threshold of 0.7.

We also performed pixel tracking, or sub-pixel correlation analysis, on the CSK ascending and descending SAR image pairs using the ROI_pac software (Pathier et al. 2006).

Cross-correlation of the SAR amplitude images was done with 64 × 64 pixel windows, giving measurements of line-of-sight

Seismological Research Letters Volume 82, Number 6 November/December 2011 791

“Product CSK © ASI, (Italian Space Agency), year of acquisition, 2011, distributed by e-GEOS (an ASI/Telespazio Company).”

10 km

Christchurch central business district

Ground displacement away from satellite

t Fligh direc tion tion direc 36° k o o rl le = Rada nce ang e Incid Ground displacement towards satellite ▲▲ Figure 2. The colored image shows an interferogram derived from CSK X-band radar images acquired on 19 and 23 February 2011, overlaid on shaded topography. Each color cycle (“fringe”) represents 1.55 cm of ground displacement in the line-of-sight (LOS) direction from the ground to the satellite. The total LOS displacement between the western edge of the image and central Christchurch is more than 20 cm. The image is incoherent over most of Christchurch city due to extensive ground and building damage. The order of the colors in the fringes indicates whether ground displacement is toward or away from the satellite, as shown by text in two regions.

See Figure 3 and Figures S1–S4 for other interferograms.

and along-track displacements with a spatial resolution of about 100 m. This technique measures the horizontal displacements in the along-track direction and the line-of-sight displacements (same component as InSAR) with a precision of about 5–20 cm (see Figure 4).

EARTHQUAKE SOURCE MODELING FROM DISPLACEMENT DATA GPS data provide a 3D coseismic displacement vector at a set of points, while DInSAR provides coseismic ground displacement in the line-of-sight from the ground to the satellite at points throughout the interference pattern, provided the pat-

tern remains coherent. For the modeling we define five different datasets: 1) 4-hr and 24-hr GPS; 2) 1-hr GPS; 3) CSK ascending image; 4) ALOS ascending image; and 5) CSK descending image. We apply an overall weighting to each dataset so that each one has approximately the same misfit ( χ 2 per degree of freedom) in the best-fitting model.

Single-fault Models The displacement data are modeled in two steps (Arnadottir and Segall 1994) to solve for the location and geometry of the fault plane and the amount and direction of slip. First, we used non-linear least-squares inversion software “disloc99” (Darby and Beavan 2001) to solve for the best-fitting uniform-slip,

792 Seismological Research Letters Volume 82, Number 6 November/December 2011

ALOS/PALSAR data from Japanese Space Agency

Region of fringes omitted from modeling (see text) AHE

5 km

▲▲ Figure 3. Original (wrapped) version of the ALOS track 335 ascending interferogram of 10 January–25 February 2011. Each fringe represents 11.8 cm of apparent ground motion in the line-of-sight direction to the satellite. A region of narrowly spaced fringes is visible due west of the Avon-Heathcote estuary (AHE). We have been unable to reliably phase connect these fringes with the remainder of the interferogram, so have left this region out of our current modeling. Compare with Figure 4, where a toward-satellite displacement is observed in this region using sub-pixel correlation techniques.

10 km

Region of towardssatellite displacement also seen in ALOS interferograms

meters

radar look

▲▲ Figure 4. Ground displacement observed along line of sight to CSK satellite on ascending track, using sub-pixel correlation. This technique provides measurements in regions where the DInSAR technique fails due to loss of coherence, though at lower resolution.

Compare with other ascending track images in Figures 2, 3, S1, and S3. In this image, red denotes displacement away from the satellite and blue denotes toward (opposite convention to other images in the paper).

Seismological Research Letters Volume 82, Number 6 November/December 2011 793

TABLE 2 Solutions for fault location, geometry and slip Uniform-slip, GPS only Parameter lat lon strike dip rake length top depth width slip MW

–43.541° 172.691° 56° 61° 147° 10.6 km 1.4 km 5.3 km 2.1 m 6.35

One fault

Two faults, GPS+SAR

1σ uncert.

GPS+SAR

Plane 1

Plane 2

0.1 km 0.2 km 1° 1° 2° 0.2 km 0.1 km fixed 0.1 m

–43.545° 172.690° 59° 66.5° see Fig. 5 16 km 1 km 7 km see Fig. 5 6.3

–43.535° 172.711° 58° 72° see Fig. 6 12 km 1 km 7 km see Fig. 6 6.25

–43.575° 176.666° 79.5° 87° see Fig. 6 8 km 2 km 6 km see Fig. 6 5.95

Uniform-slip GPS-only solution has bottom depth of fault fixed at 6 km in this model.

Lat and lon are the center of the fault trace if the fault plane were extended to the ground surface.

MW calculated assuming 3 × 1010 Nm –2 rigidity.

Formal 1s uncertainties for the uniform-slip solution were calculated using the method recommended by the authors of the non-linear least squares algorithm used by Darby and Beavan (2001).

rectangular fault plane in an elastic half-space, using the GPS data only (because disloc99 has not been modified to include DInSAR data). We found that it was necessary to constrain only the lower depth of faulting to obtain a solution for the other eight parameters describing the fault. Several sites, all in the eastern suburbs of Christchurch and other places where ground damage was prevalent, had very large residuals. We increased the estimated uncertainties on these data and repeated the inversion to give the solution in Table 2. If we fix the bottom depth to different values we find a strong trade-off between fault width and slip magnitude, but the centroid depth is well constrained by the modeling at about 3.5 km (i.e., if the bottom depth is fixed shallower, the solutions for top depth and slip magnitude are deeper and larger, respectively). The inferred M W is about 0.1 higher than seismological estimates (J. Ristau, personal communication 2011; see also Sibson et al. 2011, page 824 of this issue), but this is not surprising given that deformation due to aftershocks is included in the geodetic solution.

We extended the uniform-slip fault plane a few km in all directions and did a linear inversion for slip on this fault plane using both the GPS and DInSAR data, again assuming an elastic half-space model. We used inversion software based on Jónsson et al. (2002) as described by Beavan, Samsonov, Denys, et al. (2010), solving for a linear ramp on each of the DInSAR datasets. We then did a grid search in the vicinity of this solution, varying the location, strike, and dip of the fault and repeating the inversion at each step to find the lowest χ 2 solution. Some of the GPS data previously marked as outliers were now reasonably good fits to the resulting model, so we restored their original uncertainty values and repeated the inversion and grid search. The resulting fault location and slip distribution are shown in Figures 5 and S5, the fault param-

eters are given in Table 2, and the slip solution is tabulated in Table S2. The fault runs from near Cashmere (Figure 1C) at its southwest end toward the Avon-Heathcote estuary and a few kilometers offshore, with a strike of 59° and a dip of 66.5° to the southeast, a little steeper than in the uniform-slip, GPS-only solution. The main patch of slip, with maximum magnitude ~2.5 m, is centered at a depth of ~4 km beneath the estuary and is a mix of reverse faulting and right-lateral strike slip. The slip on the southwestern part of the fault, with a maximum magnitude of ~1 m, is predominantly right-lateral strike slip.

A preliminary, but very similar, version of this model using a subset of the data was published by Kaiser et al. (2011).

Two-fault Model Three observations suggest that a model consisting of more than one fault might provide a significantly improved fit to the observations: 1) the presence of a distinct lineation of small aftershocks (Bannister et al. 2011, page 839 of this issue) to the south of the inferred fault plane; 2) the location of two major aftershocks near the west end of this lineation; and 3) the pattern of misfits to some of the GPS data. We tried a two-fault model with one plane coincident with the one-fault model and another coincident with the aftershock lineation, then did a grid search varying the location and geometry of the faults and solving for the slip distribution at each step. The minimum misfit solution is shown in Figures 6 and S6, parameters for both planes are listed in Table 2, and the full solution is tabulated in Table S3. Again, some sites previously marked as misfits were now in good agreement with the model so we returned their uncertainties to their original values before the final inversion run; it is this final set of uncertainties that is given in Table S1. In this solution the southern fault plane is closely coincident with the aftershock lineation and with the

794 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A) -43.45

2.5 2.0 1.5 1.0 0.5 0.0

-43.55

-43.60

p up

Fault slip, m

degrees N

-43.50 er

ge

ed

200 mm observed 200 mm modelled

0

172.60

172.65 172.70 degrees E

(B)

172.75

2.5 m

2

2.5 2.0 1.5 1.0 0.5 0.0

4 6 8 0

SW

172.80

5

10

15

distance along strike, km

Slip, m

distance down dip, km

172.55

20

NE

▲▲ Figure 5. A) Locations of model fault and its slip magnitude (colored rectangles) assuming a single planar fault, GPS displacements observed (blue arrows) and modeled (red arrows), and aftershocks since September 2010 (crosses). Central Christchurch shown by solid black square. B) Slip distribution of hanging wall relative to footwall on model fault plane. Red-and-white four-pointed stars show locations of mainshock on 22 February and (in A) the two major aftershocks to its southwest a few hours later.

estimated hypocentral locations and focal planes of the two major aftershocks (S. Bannister, J. Ristau, personal communication 2011; see also Sibson et al. 2011, page 824 this issue).

The estimated M W is also close to that estimated seismologically for the aftershocks. The mainshock plane is somewhat steeper than in the single-fault solution, and the slip patch is more concentrated. By changing the degree of smoothing in the inversion the maximum slip can vary from less than 2.5 m to over 3 m, but the moment is stable.

We perform an F-test to determine whether the two-fault solution is significantly better than the one-fault solution. The weighted residual sum of squares, number of data, and num-

ber of parameters are 8,210, 1,657, 231 (one fault), and 5,400, 1,657, 278 (two faults). These values give a tiny probability that the two models fit the data equally well. However, the true number of parameters is overestimated due to smoothing of the solution so this result is not definitive.

Consistency of the Datasets We investigate the consistency of the datasets by re-running the solution with different weightings for the GPS and DInSAR data sets. If the DInSAR data are strongly downweighted (i.e., effectively a GPS-only solution), the location of maximum slip shallows by about 0.5 km but there is little other change. If

Seismological Research Letters Volume 82, Number 6 November/December 2011 795

-43.45

(A)

2.5 2.0 1.5 1.0 0.5 0.0

-43.55

-43.60

e

dg

re

e pp

Fault slip, m

degrees N

-43.50 u

200 mm observed 200 mm modelled

0

172.65 172.70 degrees E

(B) Main shock

2

172.75

172.80

2.5 m 2.5

2.0

4

1.5

1.0

6

0.5 0.0

8 SW 0

0

5 10 distance along strike, km

(C) Aftershocks

2

15

NE

2.5 m 2.5

2.0

4

1.5 1.0

6

Slip, m

distance down dip, km

172.60

Slip, m

distance down dip, km

172.55

0.5

0.0

8 W

0

5 10 distance along strike, km

15

E

▲▲ Figure 6. A) Locations of model faults and their slip magnitudes (colored rectangles), GPS displacements observed (blue arrows) and modeled (red arrows), and aftershocks since September 2010 (crosses). Slip distribution of hanging wall relative to footwall on model fault planes of B) 22 February mainshock and C) 22 February aftershocks. Red-and-white four-pointed stars show locations of mainshock and the two major aftershocks a few hours later.

796 Seismological Research Letters Volume 82, Number 6 November/December 2011

0

-43.45

km

0

10

20

-50

0

degrees N

-43.50

-43.55 300 200

0 -43.60

200 mm observed

100

200 mm modelled

172.55

172.60

172.65 172.70 degrees E

172.75

172.80

▲▲ Figure 7. Observed (blue arrows) and modeled (red arrows) vertical displacements for the model of Figure 6. Predicted model displacements are also shown as contours with 50 mm spacing. Central Christchurch shown by solid black square. An extensive region east of central Christchurch shows subsidence exceeding the model predictions, probably as a result of ground failure due to liquefaction, lateral spreading, and compaction

the GPS data are strongly downweighted, the maximum slip decreases by 10–15% and its depth increases by about 0.5 km.

In either case the goodness of fit of the non-downweighted data does not change significantly from the original solution. We take this as evidence that the solution is not strongly dependent on a particular dataset.

DISCUSSION The Christchurch earthquake occurred within the wider aftershock region of the September 2010 Darfield earthquake, and very close to a strongly felt M W 5.1 aftershock (http://www. geonet.org.nz/earthquake/quakes/3368445g.html) that occurred within a few days of the Darfield mainshock. This indicates that stress changes due to Darfield almost immediately caused significant earthquake activity in the vicinity of the future February earthquake. Calculations by ourselves and others (e.g., Zhan et al. 2011, page 800 of this issue) show positive but very small Coulomb stress changes from Darfield in the region of the February quake; these results do not highlight eastern Christchurch as a region of large Coulomb stress increase. The Christchurch event seems less complex than Darfield, with most of the surface deformation (away from the liquefaction regions) explicable by slip on two sub-parallel fault planes; the

Darfield event involved several reverse fault segments in addition to the main strike-slip fault.

An inversion for fault slip kinematics using strong-motion data is reported by Holden (2011, page 783 of this issue), using a fault geometry based on the geodetic solution. As well as revealing details of the rupture process, she finds a similar fault slip distribution, depth, and magnitude, though a slightly higher ratio of reverse faulting to strike-slip (rake 135° compared to 145°–150° on the main slip patch of the geodetic model) and a larger maximum slip (more than 4 m compared to 2.5–3 m for the geodetic model). This provides a degree of confidence in both the geodetic and strong-motion models, but indicates there are still differences to be resolved with future work.

Both the CSK ascending and descending datasets fit well with the majority of the GPS data, leading to generally low residuals between model and observations (Figures S1, S2). The ascending ALOS data have a slightly worse fit (Figure S3), but this mostly occurs in regions where the ALOS data are coherent and the CSK data are not. Some of the GPS stations in the low-lying areas between central Christchurch and the coast also have large residuals to the model (Figures 6–7). The GPS data also show a significant region of ground subsidence in central Christchurch (Figure 7) amounting to tens of centimeters in excess of what is modeled, even in regions where the model

Seismological Research Letters Volume 82, Number 6 November/December 2011 797

is still a good fit to the horizontal observations (Figure 6). We suspect this is due to liquefaction and compaction of underlying sediments even in regions where major observable ground surface damage did not occur.

The longer-wavelength ALOS data are coherent in parts of central and eastern Christchurch where CSK lost coherence because of severe ground damage, including major liquefaction. In particular, there is a clear region of ALOS fringes (Figure 3) west of the Avon-Heathcote estuary and just west of the main slip patch in the models of Figures 5–6. We obtained two ALOS pairs, one using a post-earthquake scene only two days after the earthquake and the other from nearly a month later. The two images are quite similar to each other, suggesting that ALOS is detecting real ground deformation (or at least real phase changes) in this area. In support of this, pixel-tracking (i.e., non-interferometric) analysis of the CSK ascending data (Figure 4) also shows a region of toward-satellite ground motion in this area. We have attempted to model these signals as a small shallow fault splaying off the main fault plane. Although this does improve the fit to the ALOS data without degrading fits to the other data, we have not found a solution that provides a clearly significant improvement. More complicated fault geometry may be necessary to fit all of the details of the surface deformation south and east of Christchurch. Also, the deformation detected by satellite radar and GPS in these regions would not have been purely fault related, but would include deformation due to ground damage and phase changes due to variations of water content in the near surface; the mixture of shallow ground damage and deeper fault slip may be difficult to unravel.

We have used a uniform elastic half-space for all our modeling. There is in fact significant topography in the region, and there are both depth and lateral variations in structure. North of the fault are flat lying gravels and muds over greywacke basement (Forsyth et al. 2008), beneath which is dehydrated oceanic plateau material (Reyners et al. 2011). To the south of the fault are hills that are the remnant of a late Miocene volcano that formed through the oceanic plateau crust. Future geodetic modeling should take into account this elastic structure and topography. However, because there are so many near-field data constraining the fault location, we doubt that our conclusions on the fault geometry and slip will be greatly changed by more sophisticated modeling.

The geodetic source model presented here is just one part of the still-unfolding story of the earthquake sequence that began in September 2010 and is continuing at the time of this writing with a damaging M W 6 aftershock on 13 June 2011.

Multiple different fault surfaces have been active so far, and each of the larger earthquakes has produced radiated energy well above the average expected for the size of the fault. How and why this large amount of energy has been released should become clearer with future research.

ACKNOWLEDGMENTS We thank GeoNet, Trimble Navigation NZ Ltd, Geosystems NZ Ltd, and Global Survey Ltd for providing continuous

GPS data, and Josh Thomas, Dave Collett, Paul Denys, Kirby MacLeod, and Linda Alblas for their assistance with the postearthquake GPS surveys. We thank Stephen Bannister and Caroline Holden for providing comments on the manuscript, and an anonymous reviewer for a number of suggestions that helped us improve the paper. CSK original data is copyright 2011 Italian Space Agency; part was provided by e-GEOS, an ASI/Telespazio company, and part was provided under CSK AO PI project 2271. ALOS original data is copyright 2010 and 2011 METI and JAXA, distributed by GeoGRID and PASCO. The inversions used Igor Pro (http://www.wavemetrics.com/); figures were prepared using Igor Pro and GMT (http://gmt.soest.hawaii.edu/). Much of this research was funded by the New Zealand government. Part of this research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

REFERENCES Arnadottir, T., and P. Segall (1994). The 1989 Loma Prieta earthquake imaged from inversion of geodetic data. Journal of Geophysical Research 99, 21,835–21,855.

Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).

Fine-scale relocation of aftershocks of the 22 February Mw 6.2 Christchurch earthquake using double-difference tomography.

Seismological Research Letters 82 (6), 839–845.

Beavan, J., S. Samsonov, P. Denys, R. Sutherland, N. Palmer, and M. Denham (2010). Oblique slip on the Puysegur subduction interface in the 2009 July M W 7.8 Dusky Sound earthquake from GPS and InSAR observations: Implications for the tectonics of southwestern New Zealand. Geophysical Journal International 183 (3), 1,265–1,286; doi: 10.1111/j.1365-246X.2010.04798.x.

Beavan, J., S. Samsonov, M. Motagh, L. Wallace, S. Ellis, and N. Palmer (2010). The Darfield (Canterbury) earthquake: Geodetic observations and preliminary source model. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 228–235.

Beavan, J., P. Tregoning, M. Bevis, T. Kato, and C. Meertens (2002).

Motion and rigidity of the Pacific plate and implications for plate boundary deformation. Journal of Geophysical Research 107 (B10); doi:10.1029/2001JB000282.

Bibby, H. M. (1982). Unbiased estimate of strain from triangulation data using the method of simultaneous reduction. Tectonophysics 82 (1–2), 161–174.

Chen, C. W., and H. A. Zebker (2002). Phase unwrapping for large SAR interferograms: Statistical segmentation and generalized network models. IEEE Transactions on Geoscience and Remote Sensing 40, 1,709–1,719.

Crook, C. N. (1992). ADJCOORD: A Fortran Program for Survey Adjustment and Deformation Modelling. New Zealand Geological Survey EDS Report 138, Department of Scientific and Industrial Research, Geology and Geophysics, Lower Hutt, New Zealand.

Dach, R., U. Hugentobler, P. Fridez, and M. Meindl (2007). Bernese GPS Software Version 5.0. Bern, Switzerland: Astron. Inst., University of Bern, 612 pp.

Darby, D. J., and R. J. Beavan (2001). Evidence from GPS measurements for contemporary interplate coupling on the southern Hikurangi subduction thrust and for partitioning of strain in the upper plate.

Journal of Geophysical Research 106 (B12), 30,881–30,891.

Forsyth, P. J., D. J. A. Barrell, and R. Jongens (2008). Geology of the Christchurch Area. Institute of Geological and Nuclear Sciences 1:250,000 Geological Map 16, 1 sheet + 67 pp. Lower Hutt, New Zealand: GNS Science.

798 Seismological Research Letters Volume 82, Number 6 November/December 2011

Fry, B., and M. Gerstenberger (2011). Large apparent stresses from the Canterbury earthquakes of 2010 and 2011. Seismological Research Letters 82, 833–838.

Fry, B., R. Benites, and A. Kaiser (2011). The character of accelerations in the Mw 6.2 Christchurch earthquake. Seismological Research Letters 82, 846–852.

Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011). The Darfield (Canterbury, New Zealand) M W 7.1 earthquake of September 2010: A preliminary seismological report. Seismological Research Letters 82 (3), 378–386; doi:10.1785/gssrl.82.6.378.

Goldstein, R. M., and C. L. Werner (1998). Radar interferogram filtering for geophysical applications. Geophysical Research Letters 25, 4,035–4,038.

Holden, C. (2011). Kinematic source model of the 22 February 2011 Mw 6.2 Christchurch earthquake using strong motion data.

Seismological Research Letters 82, 783–788.

Jónsson, S., H. Zebker, P. Segall, and F. Amelung (2002). Fault slip distribution of the Hector Mine earthquake estimated from satellite radar and GPS measurements. Bulletin of the Seismological Society of America 92, 1,377–1,389.

Kaiser, A., C. Holden, J. Beavan, D. Beetham, R. Benites, A. Celentano, D. Collett et al. (2011). The February 2011 Christchurch earthquake: A preliminary report. Submitted to New Zealand Journal of Geology and Geophysics.

Pathier, E., E. J. Fielding, T. J. Wright, R. Walker, B. E. Parsons, and S. Hensley (2006). Displacement field and slip distribution of the 2005 Kashmir earthquake from SAR imagery. Geophysical Research Letters 33, L20310; doi:20310.21029/22006GL027193.

Pettinga, J. R., M. D. Yetton, R. J. Van Dissen, and G. L. Downes (2001).

Earthquake source identification and characterisation for the Canterbury region, South Island, New Zealand. Bulletin of the New Zealand Society for Earthquake Engineering 34 (4), 282–317.

Quigley, M., R. Van Dissen, N. Litchfield, P. Villamor, B. Duffy, D. Barrell, K. Furlong, T. Stahl, E. Bilderback, and D. Noble (forthcoming). Surface rupture during the 2010 M W 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis. Geology 40 (1).

Quigley, M., P. Villamor, K. Furlong, J. Beavan, R. Van Dissen, N. Litchfield, T. Stahl, B. Duffy, E. Bilderback, D. Noble, D. Barrell, R. Jongens, and S. Cox (2010). Previously unknown fault shakes New Zealand’s South Island. Eos, Transactions, American Geophysical Union 91, 469–472.

Reyners, M., D. Eberhart-Phillips, and S. Bannister (2011). Tracking repeated subduction of the Hikurangi plateau beneath New Zealand. Earth and Planetary Science Letters; doi:10.1016/j. epsl.2011.09.011.

Rosen, P. A., C. W. Werner, and A. Hiramatsu (1994). Two-dimensional phase unwrapping of SAR interferograms by charge connection through neutral trees. Proceedings of the IGARSS’94, Pasadena, CA (8–12 August 1994).

Sibson, R., F. Ghisetti, and J. Ristau (2011). Stress control of an evolving strike-slip fault system during the 2010–2011 Canterbury, New Zealand, earthquake sequence. Seismological Research Letters 82 (6), 824–832.

Stirling, M. W., G. H. McVerry, and K. R. Berryman (2002). A new seismic hazard model for New Zealand. Bulletin of the Seismological Society of America 92, 1,878–1,903; doi:10.1785/0120010156.

Wallace, L., J. Beavan, R. McCaffrey, K. R. Berryman, and P. Denys (2007). Balancing the plate motion budget in the South Island, New Zealand, using GPS, geological and seismological data.

Geophysical Journal International 168 (1); doi:10.1111/j.1365246X.2006.03183.x.

Zhan, Z., B. Jin, S. Wei, and R. W. Graves (2011). Coulomb stress change sensitivity due to variability in mainshock source models and receiving fault parameters: A case study of the 2010–2011 Christchurch, New Zealand, earthquakes. Seismological Research Letters 82, 800–814.

GNS Science Lower Hutt, New Zealand P. O. Box 30368 Lower Hutt 5040 New Zealand j.beavan@gns.cri.nz

(J. B.)

Seismological Research Letters Volume 82, Number 6 November/December 2011 799

Coulomb Stress Change Sensitivity due to Variability in Mainshock Source Models and Receiving Fault Parameters: A Case Study of the 2010–2011 Christchurch, New Zealand, Earthquakes Zhongwen Zhan, Bikai Jin, Shengji Wei, and Robert W. Graves

Zhongwen Zhan,1,2 Bikai Jin, 3 Shengji Wei,1 and Robert W. Graves4

INTRODUCTION Strong aftershocks following major earthquakes present significant challenges for infrastructure recovery as well as for emergency rescue efforts. A tragic instance of this is the 22 February 2011 Mw 6.3 Christchurch aftershock in New Zealand, which caused more than 100 deaths while the 2010 Mw 7.1 Canterbury mainshock did not cause a single fatality (Figure 1). Therefore, substantial efforts have been directed toward understanding the generation mechanisms of aftershocks as well as mitigating hazards due to aftershocks. Among these efforts are the prediction of strong aftershocks, earthquake early warning, and aftershock probability assessment. Zhang et al. (1999) reported a successful case of strong aftershock prediction with precursory data such as changes in seismicity pattern, variation of b-value, and geomagnetic anomalies. However, official reports of such successful predictions in geophysical journals are extremely rare, implying that deterministic prediction of potentially damaging aftershocks is not necessarily more scientifically feasible than prediction of mainshocks.

A potentially more effective approach for aftershock hazard mitigation is described by Bakun et al. (1994) for the case of the Loma Prieta earthquake. This approach relies on the rapid detection of an aftershock using a dense observation network in the rupture area of the mainshock and subsequent broadcast of an alert to more distant sites. Recent progress in rapid determination of epicenter and magnitude involving a 1. Seismological Laboratory, California Institute of Technology, Pasadena, California 91125 U.S.A. 2. Mengcheng National Geophysical Observatory, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026 China 3. Key Laboratory of Dynamic Geodesy, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077 China 4. U.S. Geological Survey, 525 South Wilson Avenue, Pasadena, California, 91106 U.S.A.

small number of stations and short time window of P waveforms (Allen and Kanamori 2003; Wan et al. 2009; Wang et al. 2009) make the approach of earthquake early warning more effective for regions not very close to the rupture area of the mainshock. Such an approach might have been useful for aftershocks of the 2008 Wenchuan earthquake, where megacities such as Chengdu are about 90 km away and 20 seconds were available for rapid mitigation response. But in the case of the 2011 Christchurch earthquake, the populated region is only about 10 km from the epicenter, thus leaving little time for early warning.

For a situation such as Christchurch, aftershock probability assessment may provide a viable approach to address the hazard level. Several aftershock-triggering mechanisms, i.e., the static Coulomb stress theory (King et al. 1994; Stein 1999), the dynamic triggering theory (Felzer and Brodsky 2006), and viscoelastic relaxation theory (Freed et al. 2001), can be applied to assess aftershock probabilities. In this paper we will concentrate on how applicable the static Coulomb stress triggering mechanism is to the 2011 Christchurch aftershock and examine the sensitivity of the stress changes to mainshock slip distribution and aftershock fault orientation. The Coulomb stress theory has been broadly applied in aftershock studies (e.g., King et al. 1994; Parsons et al. 1999; Toda et al. 1998; Ma et al. 2005), earthquake sequencing (Stein et al. 1997; Xiong et al. 2010; Nalbant et al. 1998) and the triggering of large to moderate earthquakes (Parsons et al. 2000). Previous studies (e.g., Harris 1998, 2000; Freed 2004; King et al. 1994; Stein 1999) proposed a Coulomb stress change of 0.01 MPa to be the threshold for potential earthquake triggering.

The Coulomb stress triggering theory involves computing the change in normal traction and shear traction on a fault (receiving fault) caused by changes of the stress field due to the mainshock. Therefore, accurate information on the receiving fault geometry (strike, dip, rake, and focal depth) and source model of the mainshock are necessary for effective assessment

800 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.800

43.25˚S

0

5

10

15

20

depth(km)

0

5

10

15

M3

20

M5

M7

depth(km)

GCMT/Mw7.0 CAP/Mw6.3

43.5˚S

43.75˚S

km 0

10

171.75˚E

20

172˚E

172.25˚E

172.5˚E

172.75˚E

▲▲ Figure 1. Seismicity in the first three days after the 2010 Canterbury mainshock (color-scaled dots) and the 2011 Christchurch aftershock (dots in gray scale). The red lines are the free surface rupture trace from field observation. The big red star indicates the epicenter of the main event and the smaller red star is the location of the Mw 6.3 aftershock. The GCMT solution of mainshock and the cut-and-paste (CAP) mechanism of the aftershock are shown as beach balls. The black rectangles are the free surface projection of the two-segment model, which are used in the teleseismic finite fault inversion. The yellow rectangles are for the four-segment model used in the stochastic slip model.

of aftershock probability. Due to inadequate coverage of seismic and geodetic observation systems and inaccurate 3D Earth structure models, there are always errors in the source models of the mainshock. Moreover, fault geometries of future aftershocks are not precisely known, and aftershocks occurring on blind faults are particularly difficult to study due to lack of geological information about the faults. For example, the 1994 Northridge earthquake occurred on a blind (buried) fault; the study of its potential triggering by the 1971 San Fernando earthquake was only made possible after its rupture plane and hypocenter depth were resolved (Stein et al. 1994). The 2011 Christchurch earthquake was another case of such a blind earthquake, which has not yet been associated with any known geological faults. Thus, this event is a valuable case study of how effective the Coulomb stress mechanism is in triggering aftershocks, and its variability due to the uncertainties in receiving fault parameters and mainshock source models.

In this paper we examine the sensitivity of computed static Coulomb stress change levels to source parameterization by

considering various combinations of mainshock rupture models and aftershock fault orientations for the 2010 Canterbury and 2011 Christchurch earthquakes. General constraints on the mainshock source models and aftershock fault geometry are provided by teleseismic and geodetic data. We also investigate the sensitivity of the results to aftershock focal depth and apparent coefficient of friction. We conclude with a discussion of how these results can be used in combination with focal mechanism studies to help constrain aftershock rupture assessment using Coulomb stress change calculations.

GENERAL INFORMATION ON THE Mw 7.1 CANTERBURY EARTHQUAKE AND THE Mw 6.3 CHRISTCHURCH EARTHQUAKE The Australian and Pacific plates converge obliquely at about 40 mm yr−1 at New Zealand. Partly due to along-strike variations in the orientations of both the plate boundary and the direction of relative motion between the plates, the defor-

Seismological Research Letters Volume 82, Number 6 November/December 2011 801

mation takes on a larger strike-slip component southward (Wallace and Beavan 2006). Accordingly, the style of deformation changes southward, from subduction of the Pacific plate and back arc rifting in the North Island to nearly pure strike-slip in the Marlborough region to oblique convergence in the central South Island (causing formation of the central Southern Alps) and back to subduction of the Australian plate at the Fiordland subduction zone in the southwestern South Island (Wallace and Beavan 2006). The earthquake sequence we study in this paper occurred in the central South Island.

The 2010 Canterbury earthquake occurred at 4:35 a.m. local time on 4 September (16:35 UTC, 3 September), on a previously unrecognized fault system, the Greendale fault (Figure 1) (Quigley et al. 2010). This Mw 7.1 earthquake caused widespread damage throughout the area, but no deaths and only two injuries were reported despite the epicenter’s location about 40 km west of Christchurch (population ~386,000), New Zealand’s second-most populated city (Quigley et al.

2010). The 2011 Christchurch earthquake occurred at 12:51 p.m. on 22 February 2011 local time (21 February UTC), causing widespread damage and more than 100 fatalities. The earthquake was centered 2 km west of the town of Lyttelton and 10 km southeast of the center of Christchurch.

SOURCE MODELS OF THE 2010 CANTERBURY EARTHQUAKE The 2010 Mw 7.1 Canterbury earthquake ruptured the previously unrecognized Greendale fault in an east-west direction for ~30 km (Figure 1). The average displacement of this predominantly right-lateral strike-slip event is ~2.5 m, with maxima of ~5 m (Van Dissen et al. 2011). The first finite fault slip model of the main event was published by the U.S. Geological Survey (http://earthquake.usgs.gov/earthquakes/eqinthenews/2010/ us2010atbj/finite_fault.php), in which teleseismic body waves

were used for the inversion assuming a single fault plane. The GPS and InSAR data were collected later on and a static slip model was derived by Beavan et al. (2010). Compared with the single fault plane model, this static slip model is composed of six segments, consisting of the strike-slip Greendale fault and several thrust faults.

To address the variability of static triggering due to mainshock rupture models, we analyze: 1) a single fault plane model with uniform slip; 2) a two-segment slip model from teleseismic body wave inversion; and 3) two stochastic slip models. Despite its simplicity, a uniform slip model can provide a straightforward physical picture and can explain the main features of some earthquakes (e.g., Talebian et al. 2006). Also, it is a good reference for comparison with results generated from other slip models. First we use the Global Centroid Moment Tensor (GCMT) solution to define the fault geometry and the rake angle for the uniform slip model. We choose the fault plane with strike of 87° and dip of 85°, since the strike is consistent with the rupture trace on the free surface (Figure 1).

Slip with an amplitude of 3 m and rake of 172° is uniformly distributed on the rectangle fault plane, which is 42 km along

strike and 12 km along dip. However, because of the complexity of this earthquake, it is hard to fit the waveforms with this simple slip model (Figure 2). Poor waveform fits are also shown in the USGS results (http://earthquake.usgs.gov/earthquakes/ eqinthenews/2010/us2010atbj/finite_fault.php).

To investigate the potential for additional complexity in the rupture geometry, we derive a two-segment finite fault slip model by inverting teleseismic body waves. We collected 27 teleseismic P waves and 15 SH waves from the earthquake.

Stations are selected based on data quality and azimuthal coverage (Figure 3). To derive the finite fault model, we use the approach developed by Ji et al. (2002a, 2002b), which allows fitting of seismic waveforms in the wavelet domain. Nowadays, similar procedures are run routinely by several agencies, such as the USGS (http://earthquake.usgs.gov/earthquakes/eqinthenews/) and the Caltech Tectonics Observatory (http://www. tectonics.caltech.edu/slip_history/). By examining the seismicity in the first three days after the 2010 Canterbury earthquake, we can observe a linear distribution of aftershocks in the north-south direction crossing roughly perpendicular to the mapped surface rupture trace. The epicenter of the mainshock is also located within this linear band of seismicity (Figure 1). This suggests the possibility that more than one fault was involved in the rupture. Thus, we added one more fault plane with strike along this seismicity trend (strike of 345° and dip of 75°) into the finite fault inversion. The epicenter is specified to be on this fault plane with depth of ~7 km; thus we assume the earthquake initiated on the north-south trending fault and propagated to or triggered the rupture on the other fault later on. The waveform fitting of the two-fault plane model is much better than that of the single fault plane model, especially for the beginning portion of some P-wave records (Figure 3). For example, station PSI’s P waveform, which is not fitted in the single fault plane inversion, is now fitted well. The slip distribution on the first segment shows mainly thrust motion, which is required to fit positive first motions of some P waves. The largest slip patch is on the second fault plane and is dominated by strike-slip motion. Some thrust motion is also shown in the western part of the second fault plane. The rupture length and the location of thrust motion in our model are consistent with field observations and static inversion results (Beavan et al.

2010; Van Dissen et al. 2011).

The stochastic slip model is another approach for characterizing the slip distribution of an earthquake, and it has been widely applied in ground motion simulations (Mai and Beroza 2002; Liu et al. 2006; Graves and Pitarka 2010). Lavallée and Archuleta (2003) found that the slip distribution of the 1979 Imperial Valley earthquake could be well modeled with a stochastic model assuming power law of k-n, where k is the wavenumber. A stochastic approach has to be taken in the following two cases. The first case is when studying historical earthquakes, which lack seismic waveform or geodetic data for finite fault inversion, as with the 1811/1812 New Madrid earthquakes. The other case is when characterizing the rupture for scenario earthquakes. For the Canterbury earthquake, we follow the procedure by Graves and Pitarka (2010) to gener-

802 Seismological Research Letters Volume 82, Number 6 November/December 2011

P

BLDU P

CASY P

MAW P

SBA P

SNAA P

PMSA P

LCO P

PPTF P

RAR

P

1 44

TARA

3.0

−10 0

P

MAJO 13.8

P

GUMO 9.3

P

PMG 40.5

P

COEN 50.9

55 32

20 62

P

ERM

63 41

P

JOHN

13.6

128 87

29 70

P

HNR

156 63

P

KIP

2.6

181 64

30 32

P

WAKE

181 34

P

AFI

20.1

205 57

39 53

SH

XMAS

213 39

P

XMAS

9.4

267 45

P

CTAO 52.1

31.3

22.0

17.2

28.7

P

280 48

MBWA

10 20 30 40 50

11.7

−10 0

SH

309 31

SH

298 46

SH

291 70

SH

283 79

SH

283 39

SH

267 45

SH

213 39

SH

181 34

SH

181 64

SH

156 63

SH

128 87

SH

63 41

KSM 18.7

PSI 34.1

WRKA 29.0

BLDU 41.1

309 31

283 39

311 38

MTN

311 38

P

WRKA

9.9

320 40

283 79

SH

CTAO

330 61

P

PSI

35.0

333 85

291 70

354 62

COEN

339 89

P

KSM

22.1

339 35

298 46

SH

WAKE

354 62

P

MTN

100.3

39 53

CASY 27.3

SBA 11.3

SNAA 8.2

PMSA 18.5

LCO 13.4

PPTF

10 20 30 40 50

s

s

−10 0

144.3

40.2

64.8

81.8

42.2

57.2

113.7

132.8

42.3

184.6

54.6

95.0

57.6

106.8

10 20 30 40 50

s

▲▲ Figure 2. Waveform fits for the single fault plane inversion. Black lines are data in displacement and red are synthetic. Station names are displayed to the left of the traces along with the azimuths (above) and epicentral distances in degrees (below). Peak amplitude (in microns) for the data is indicated above the end of each trace.

ate stochastic rupture models. We generated two models, one model with only a single fault segment (Figure 4) and the other with four fault segments, which were derived by simplifying the model of Beavan et al. (2010) (Figure 5).

FAULT PARAMETERS OF THE 2011 CHRISTCHURCH EARTHQUAKE To study the effect of receiving fault geometry on Coulomb stress change, we need to determine the focal mechanism and focal depth of the 2011 Christchurch earthquake. There are many different approaches for studying earthquake source parameters using regional or teleseismic waveforms.

Two kinds of regional waveform data are generally used: surface waves and body waves. Since surface waves are generally much stronger than body waves, full waveform inversions are

mainly controlled by surface waves. Dreger and Helmberger (1993) used the long-period body waves recorded by a regional sparse network to invert for focal mechanism. Later, Zhao and Helmberger (1994) and Zhu and Helmberger (1996) developed the “cut and paste” (CAP) technique, which breaks broadband waveforms into Pnl and surface wave segments and inverts them independently, allowing for different bandpass filtering, time shifts, and weights. The CAP technique has been successfully applied to determine the depth and focal mechanism in many regions (e.g., Tan et al. 2006). However, regional data are not always accessible immediately after earthquakes, so inversion techniques using teleseismic waveforms become important and are routinely used to estimate source parameters for earthquakes of M 6 and above (e.g., the Global CMT solution, the USGS body wave moment tensor solution, and the USGS Wphase solution). Most of these automatic approaches involve

Seismological Research Letters Volume 82, Number 6 November/December 2011 803

267 45

P

213 39

P

205 57

P

181 34

P

181 64

P

156 63

P

128 87

P

63 41

P

55 32

P

39 53

P

30 32

P

29 70

P

20 62

P

1 44

CASY MAW SBA

SNAA

PMSA

LCO PPTF

RAR

XMAS AFI

KIP

JOHN TARA

−10 0

9.4

SH

39 53

P

354 62

P

339 35

P

339 89

P

333 85

P

330 61

P

320 40

XMAS

20.1

WAKE

2.6

HNR

13.6

ERM 3.0

MAJO 13.8

GUMO 9.3

PMG 40.5

P

COEN 50.9

P

CTAO 52.1

31.3

P

291 70

P

283 79

P

283 39

P

280 48

KSM 22.0

PSI 17.2

WRKA 28.7

MBWA

10 20 30 40 50

22.1

35.0

SH

311 38

SH

309 31

SH

298 46

SH

291 70

SH

283 79

SH

283 39

SH

267 45

SH

213 39

SH

181 34

SH

181 64

SH

156 63

SH

128 87

SH

63 41

CTAO 9.9

MTN 11.7

KSM 18.7

PSI 34.1

WRKA 29.0

BLDU 41.1

CASY 27.3

SBA 11.3

SNAA 8.2

PMSA 18.5

LCO 13.4

PPTF

−10 0

10 20 30 40 50

t(s)

−10 0

144.3

40.2

64.8

81.8

42.2

57.2

113.7

132.8

42.3

184.6

54.6

95.0

57.6

106.8

10 20 30 40 50

t(s)

F1: Strike= 345 deg

-10

0

t(s)

F2: Strike= 87 deg km

km -20

10

0

0

10 10 10

10

10 4

10

10

20

20 0

10

10

Depth km

354 62

COEN

309 31 298 46

SH

WAKE

311 38

P

MTN

100.3

10 20

Depth km

P

BLDU

cm 0

100

200

300

▲▲ Figure 3. Results of the two-segment finite fault inversion. The upper panel shows the displacement waveform fits in black for the data and red for the synthetic. Station names are displayed to the left of the traces along with the azimuths and epicentral distances in degrees. Peak amplitude (in microns) for data is indicated above the end of each trace. The lower panel shows the cumulative slip distribution (slip vectors with amplitude of slip also represented by color shading) and time contours of the rupture propagation as determined by the inversion. The rupture times are given relative to the origin time, and the red star indicates the epicenter. The strike of each segment is shown on the top.

804 Seismological Research Letters Volume 82, Number 6 November/December 2011

0

Slip (cm)

0 / 160 / 558 4

600 8

6

480

4

8

360

2

6

10

12

10

10

4

0

5

2

4

6

10

120

2

8

15

8

15

20

240

6

10

W (km)

4

2

5

25

30

35

40

45

50

0 55

60

85o Azi. (km) ▲▲ Figure 4. A stochastic single-segment slip model of the 2010 Canterbury earthquake. The background color shows the distribution of cumulative slip in centimeters and the green arrows show the slip vectors. Time contours of the rupture propagation are shown as black contours. The rupture times (numbers on the contour lines) are given relative to the origin time. The strike of the fault plane is shown at the bottom.

Slip (cm)

0 / 165 / 581 600

W (km)

6

8

8

360 240

2

W (km)

14

8

10

480

18

10

5

16

0

6

8

0

5

10

121o Azi. (km)

0

120

10

8

5

10

15

87o Azi. (km)

18

15

20 0

5

10

87o Azi. (km)

0

5

10

0

40o Azi. (km)

▲▲ Figure 5. Stochastic multiple-segment slip model of the 2010 Canterbury earthquake. The background color shows the distribution of cumulative slip in centimeters and the green arrows show the slip vectors. Time contours of the rupture propagation are shown as black contours. The rupture times (numbers on the contour lines) are given relative to the origin time. The strikes of the fault segments are shown at the bottom of each panel.

long-period waves so the solutions have poor resolution of earthquake depth. Also when the earthquake is shallow, strong trade-off among depth, focal mechanism, and magnitude can cause large uncertainties in source parameters (Dahlen and Tromp 1998). To overcome these problems, we extend the idea of the regional CAP technique to teleseismic cases (teleCAP).

In teleCAP, we cut 10–50 s period band P-wave segments in the vertical components and SH-wave segments in the transverse components and fit them independently, allowing different time shifts and weights. We choose the relative weights between P and SH waves so that they contribute almost equally to the final misfit function (e.g., Tan et al. 2006).

Synthetic seismograms are calculated with a 1D sourceside crustal model obtained from CRUST 2.0 (Bassin et al. 2000). Figure 6 shows the seismic stations used in the inversion. These stations are chosen based on their signal-to-

noise ratio (SNR) and azimuthal coverage. We find the best waveform-fitting source parameters by grid-searching earthquake magnitude, focal mechanism (strike, dip, and rake), depth, and source duration. Figure 7 shows the best waveform fitting, the corresponding focal mechanism (strike/dip/ rake = 174°/46°/42° or 52°/61°/128°) and magnitude (Mw 6.3). Compared with the Global CMT solution (strike/dip/ rake/magnitude = 167°/57°/32° or 59°/64°/143°, Mw 6.1), there is ~10 degree difference for strike/dip/rake and 0.2 difference in magnitude. These differences will be discussed later. Both P and SH wave amplitudes and waveforms at all azimuths are fit very well. Figure 8 shows the waveform misfit as functions of centroid depth and source duration; the best fitting depth is 5 km, which is the same as reported by New Zealand local seismologists using local stations. The best fitting source duration is 6 s, the same as in the Global CMT solu-

Seismological Research Letters Volume 82, Number 6 November/December 2011 805

ERM MAJO INU JHJ2

JNU

MIDW

CBIJ

WAKE

JOW

KIP

JOHN

YOJ TATO YHNB NACB SSLB YULB TPUB

GUMO PATS

HKPS

KWAJ

QIZ DAV LDM KKM

KUM IPMKOM BTDF PSI

SBM KSM

PMG

COCO

HNR AFI

COEN

KAPI

MTN KNRA

UGM XMI XMIS

XMAS

MANURABL

MBWA

WRAB

QIS

TAOE

CCTTAAO

RAR

PPTF

WRKA BBOO

GIRL KMBL MORW BLDU MUN

CASY

PAF

VNDA SBA

QSPA LCO

MAW CRZF

PLCA

PMSA EFI

TRQA

HOPE

▲▲ Figure 6. Seismic stations used in the inversion of fault parameters of the 2011 Christchurch earthquake. These stations are chosen based on their signal-to-noise ratio (SNR) and azimuthal coverage.

tion. For each candidate depth and source duration, we also plot its best fitting focal mechanisms and magnitudes in Figure 8, to show the trade-off between magnitude and other source parameters. Obviously, the earthquake magnitude decreases as depth increases, as expected from the free surface effects as discussed by Dahlen and Tromp (1998). If depth = 12 km (as determined in the Global CMT), the magnitude will be about Mw 6.15, which is close to the Mw 6.1 in the GCMT catalog.

Due to the trade-off between depth and focal mechanism for shallow events, the GCMT’s focal mechanism may also be somewhat biased. We conclude that teleCAP provides higherresolution source parameters for the 2011 Christchurch earthquake. However, it should be noted that teleCAP assumes a double-couple point source, as do most other approaches, so it cannot distinguish between the fault plane and auxiliary fault plane. The first three days’ aftershock distribution of the 2011 Christchurch earthquake shows a clear linear trend from EEN to WWS (Figure 1), which prefers the fault plane 52°/61°/128°.

In the following discussion we will use only this fault plane.

Epicenter location is another important source parameter that will greatly affect the computation of Coulomb stress change.

In this paper, we use the epicenter location from New Zealand GeoNet, which is based on data from a dense local seismic network and is presumably accurate.

COMPUTATION OF COULOMB STRESS FOR VARIOUS MAINSHOCK SOURCE MODELS AND RECEIVING FAULT GEOMETRIES Based on the Coulomb failure criterion (Jaeger et al. 2007, 475) and the theory of elastic dislocation (Okada 1992), we calculate the coseismic Coulomb failure stress change (Δσf ) caused by the mainshock for different mainshock slip models and for different receiving fault geometries. Following King et al. (1994), Δσf is given by Δσf = Δτs – μ′Δσn, where Δτs and Δσn are the changes in shear and normal stress, respectively, due to the mainshock, and μ′ is the apparent coefficient of friction. Here we use the rock mechanics sign convention in which compressive is positive.

In this study, we use the lithosphere model of dislocation sources embedded in an elastic multilayered half space (Wang et al. 2003, 2006) and adopt the program PSGRN/PSCMP (Wang et al. 2006) to compute the static Coulomb stress change produced by the mainshock. Since the influence from the curvature of Earth’s free surface is small for this local study (Xiong et al. 2010), the Earth surface is treated as flat in our model. The parameters of our multilayered model in Table 1 are based on Crust 2.0. A moderate value of apparent coefficient of friction μ′ = 0.4 is used in our calculation (King et al.

806 Seismological Research Letters Volume 82, Number 6 November/December 2011

PV

AFI 32.5/29.0

KIP 70.1/29.0

EFI 75.3/150.1

−5.00 83

PMSA 63.1/156.3

−3.00 80

HOPE 79.2/163.1

−5.00 89

−3.00 89

PPTF 41.0/62.8

450.40 73

−2.00 87

PV

−4.00 88

640.33 83

SH

555.40 63

407.95 89

TPUB 82.0/312.9

−4.00 85

DAV 66.0/307.2

HKPS 84.8/307.5

2.00 45

−1.00 78

−3.00 88

492.56 66

362.18 82

SH

309.63 71

−4.00 94

−5.00 95

611.19 79

400.43 96

409.05 96

−4.00 94

UGM 64.6/284.0

372.16 96 IPM 80.0/286.0

−5.00 93

−3.00 92

−5.00 96

−5.00 96

−5.00 96

361.52 87

−5.00 96

601.16 72

−5.00 93

516.71 85

−4.00 89

579.49 90

KOM 76.1/286.2

−4.00 93

601.04 79

442.52 94

298.02 71

526.55 81

526.51 80

KUM 80.8/286.3

−5.00 91

582.83 94

KSM 71.4/290.8

−5.00 94

603.31 75

−4.00 92

557.52 87

KNRA 46.5/292.8

402.61 87

PV

SH

SH

HNR 35.8/338.1 −4.00 87

−5.00 85 ERM 89.3/338.4

−4.00 94

−4.00 91

648.51 94

−5.00 88

438.91 72

−6.00 89

502.61 66

−6.00 85

437.99 81

PATS 51.9/341.7 −5.00 87

−5.00 94 GUMO 62.4/329.2

PSI 80.2/283.2

−5.00 92

407.62 64

625.06 76

WAKE 62.9/353.6

KWAJ 52.4/353.6

−6.00 90

RABL 43.3/329.3 609.67 81

−5.00 85

FM 174 46 42 Mw 6.30

INU 85.1/331.4 −5.00 95

612.09 90

−5.00 92

613.69 90

−5.00 91

625.35 67

CBIJ 75.8/332.1 −8.00 91 JHJ2 82.1/332.6

−5.00 93 YOJ 81.7/315.4 627.87 87

JOW 81.1/320.7

610.08 82

TATO 82.9/314.5

−4.00 90

399.08 95

WRKA 40.5/282.4

SH

BTDF 75.9/285.8

PV PMG 40.7/319.2

354.53 87

YHNB 82.7/314.3

−7.00 87

−4.00 92

524.35 85

JNU 85.4/325.8

−5.00 89

628.19 91

−5.00 91

MBWA 49.1/279.6

MANU 47.1/324.3

NACB 82.2/314.1

−3.00 92

XMIS 66.3/278.3

585.42 79

YULB 81.7/313.5

−3.00 77

BBOO 30.5/278.2

XMI 66.3/278.4

309.61 71

−5.00 98

554.84 74

−4.00 93

−4.00 91

332.78 90

SSLB 82.2/313.4

−4.00 96

QIZ 84.8/302.3

2.00 41

PV CTA 32.2/308.3

COEN 38.9/310.3

QIS 35.9/299.2

LDM 68.8/299.9

334.66 94

COCO 71.5/270.7

−1.00 79

PV

SH

GIRL 52.1/273.9

CTAO 32.2/308.3

−4.00 89

−5.00 90

CASY 40.0/213.8

595.84 82

−5.00 95

KKM 71.0/298.7

MAW 57.2/205.4

CRZF 76.1/217.7

WRAB 39.7/294.1

MTN 47.0/297.8

15.00 66

15.00 68

374.70 91

454.76 85

−2.00 92

KAPI 60.3/293.7

KMBL 41.5/269.6

VNDA 34.3/184.2 −1.00 87

BLDU 45.7/267.0

MORW 47.0/268.4

−2.00 59

15.00 43

−2.00 91

LCO 87.3/128.4

SBM 70.9/293.0

514.09 94

SBA 34.4/182.2

TAOE 53.6/64.2

PLCA 78.5/136.2

QSPA 46.5/180.0

MUN 45.3/265.1

−3.00 75

−1.00 65

PV PAF 65.3/224.6

−3.00 89

553.03 89

XMAS 52.8/38.7

RAR 32.0/54.5

SH

TRQA 84.7/139.7 −6.00 92

JOHN 62.2/19.3

PV

SH

MIDW 72.1/9.2

−6.00 93

MAJO 85.8/332.8 −5.00 88

608.63 87

−5.00 96

▲▲ Figure 7. Best teleseismic waveform fitting and the corresponding focal mechanism (strike/dip/rake = 174°/46°/42°) and magnitude (Mw 6.3). Black is the data and red is the synthetic. The red crosses on the focal mechanism beach ball show the locations of stations.

1994), and parameter sensitivity will be discussed. To show the influence of the uncertainty of focal depth, we calculate the Δσf caused by the mainshock on several horizontal planes at 2, 5, 10, and 15 km depths, respectively, each of which consists of 101 × 101 grid points. In the next several subsections, we will show the Δσf results for these different cases and discuss their variability. It should be noted that the effect of viscoelastic relaxation is not taken into account here. Because of the short time interval between the two events, its influence is believed to be relatively small, compared with the uncertainties of other parameters above. More accurate results can be achieved by taking this effect into account in the future.

Coulomb Stress Change Caused by Different Mainshock Slip Models The selection of an appropriate mainshock slip model is important for the Δσf distribution. Figure 9 displays the Δσf distribu-

tion for the four slip models discussed previously. In all cases, the slip models have a strong influence on the Δσf distribution along the Greendale fault in the near field. For example, the Greendale fault lies completely within a stress shadow in Figure 9A, but there are some parts significantly loaded (>1MPa) using the other three models in Figure 9B–D. However, these different slip models cause no significant difference in the far field. At the eastern and western ends of the Greendale fault, the stress changes more than 0.01 MPa caused by each model, and the Δσf distributions, are very similar. In summary, it can be inferred that a uniform slip model can explain the main features of the Δσf distribution in the far field; the more complicated slip models make the Δσf distribution heterogeneous in the near-field along the fault. At the hypocenter of the 2011 Christchurch earthquake, the Δσf are 0.013, 0.044, 0.033, and 0.053 MPa for the four different slip models, respectively—all above 0.01 MPa, the presumed threshold value for earthquake triggering.

Seismological Research Letters Volume 82, Number 6 November/December 2011 807

(A)

(B)

6.13

6.24

6.14

6.36

6.15 6.18 6.65

6.25

6.19

Misfit

Misfit

6.46 6.20 6.27

6.23

6.34

6.39 6.29

6.26

6.31

6.346.306.28

0

2

4

6

8

10

12

14

Depth (km)

1

2

3

4

5

6

7

8

9

Duration (s)

▲▲ Figure 8. Misfit of teleseismic waveforms as a function of earthquake focal depth and duration. The best fitting focal mechanisms and magnitudes are also plotted to show the trade-offs between them.

TABLE 1 Multilayered lithosphere model from CRUST 2.0.

Layer Upper crust Middle crust Lower crust Mantle

Thickness (km)

Vp (km·s –1)

Vs (km·s –1)

Density (kg·m –3)

14.3 9 11

6.0 6.6 7.2 8.0

3.5 3.7 4.0 4.6

2700 2900 3050 3300

Effect of Focal Depth on Coulomb Stress Change The sensitivity of our results to focal depth of the 2011 Christchurch earthquake is explored by comparing the Δσf distributions resolved at depths of 2, 5, 10, and 15 km. Here the focal depth is meant to be hypocentral depth, where rupture of the Christchurch earthquake initiated. The depth inferred from waveform inversion is essentially centroid depth, and the hypocentral depth is typically difficult to resolve unless with a dense local seismic network. For these calculations, we use the two-segment slip model of the mainshock along with the focal mechanism of the receiving fault (52°/61°/128°) derived from the teleCAP inversion. The results are shown in Figure 10. Some local changes of the Δσf distribution are observed in the far field, and complex changes happen in the near field.

For example, the area with positive Δσf at the east end of the Greendale fault is getting smaller when focal depth increases; and in the area of the mainshock thrust fault segment, Δσf changes polarity as the depth increases. The Δσf at the 2011 Christchurch earthquake epicenter are 0.035, 0.044, 0.065, and 0.094 MPa at 2, 5, 10, and 15 km depth, respectively, increas-

ing with the depth and all above 0.01 MPa. Similar results are obtained for the other slip models, as shown in Table 2.

Effect of Receiving Fault Geometry on Coulomb Stress Change To analyze the impact of receiving fault geometry on the Δσf, we make some significant changes in the strike and dip of the receiver fault (from focal mechanism) and compare their influence on the resulting Δσf distributions. The two-segment slip model of the mainshock and a focal depth of 5 km are adopted in this calculation, and the results are shown in Figures 11 (strike sensitivity) and 12 (dip sensitivity). In Figure 11, significant changes in the Δσf distribution can be observed as the strike varies. When the strike of the receiving fault is rotated counterclockwise by 30 degrees, the area with positive Δσf increases in the near- and far-fields, compared with Figure 10.

The opposite situation occurs when the strike of the receiving fault is rotated clockwise from the teleCAP solution. Similar changes are also obtained when the dip angle of the receiving fault is changed by ± 20 degrees, as shown in Figure 12.

The area with positive Δσf outside the northwest corner of the Greendale fault grows with increasing dip angle. In the near-field, the situation is a little more complex, but the whole region with positive Δσf gets larger. From these two figures, we conclude that the Δσf distribution can be quite sensitive to the assumed geometry of the receiving fault.

Sensitivity of Coulomb Stress Change to Coefficient of Friction The selection of an appropriate value for the apparent coefficient of friction μ′ is important because it controls the con-

808 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C)

(D)

▲▲ Figure 9. The coseismic Δσf caused by the Mw 7.0 earthquake with different slip models: A) a single fault plane model with uniform slip; B) a two-segment slip model; C) a stochastic model with single plane based on the National Earthquake Information Center (NEIC) solution; D) a stochastic model with four segments based on Beavan et al. (2010). The strike angle, dip angle, and rake angle of the receiver fault are 52°, 61°, and 128°, respectively. The calculated depth is 5 km, and the apparent coefficient of friction μ′ is 0.4. The two focal mechanisms show the location and mechanism of the mainshock and Mw 6.3 aftershock.

tribution of the normal stress change to the Δσf (Xiong et al.

2000; King et al. 1994). In general, μ′ is set to be different values for different types of faults. Xiong et al. (2010) set μ′ to a high value (0.8) for thrust faults, a moderate value (~0.6) for normal faults, and a lower value (0.2~0.4) for strike-slip faults. Since the 2010 Canterbury earthquake mainshock is primarily a strike-slip event (with some thrust component), we have set μ′ at 0.4 in the previous calculations. Here we consider values of μ′ of 0.0 and 0.8 to analyze their sensitivity on the computed Δσf distribution. In these calculations, we again use the two-segment slip model for the mainshock along with the teleCAP mechanism (52°/61°/128°) and focal depth (5 km) for the aftershock. The resulting Δσf at the epicenter of the 2011 Christchurch earthquake is 0.095, 0.044, and –0.008 MPa for μ′ = 0.0, 0.4, and 0.8, respectively, as shown in Figure 13. The decrease of Δσf with increasing μ′ indicates that the change of normal stress is positive (clamping the fault plane)

Christchurch fault plane. Other areas, such as northwest of the Greendale fault, exhibit an increase in Δσf with increasing μ′.

Obviously, the polarity change of Δσf can lead to significant uncertainty for evaluating seismic hazard, underscoring the need for accurate constraints on mainshock faulting mechanism and estimation of μ′.

DISCUSSION AND CONCLUSION Coulomb stress triggering is physically straightforward and has been widely applied in studying the distribution and probability of aftershocks. However, there can be substantial variability due to uncertainty in mainshock slip models and fault orientation of the subsequent aftershocks. In this paper, we calculate the coseismic static Coulomb stress change caused by the 2010 Canterbury earthquake for several different mainshock slip models, and various permutations of receiving fault geometry

Seismological Research Letters Volume 82, Number 6 November/December 2011 809

(A)

(B)

(C)

(D)

▲▲ Figure 10. The coseismic Δσf caused by the mainshock with the two-segment slip model at different depths, with μ′ = 0.4. A), B), C), and D) show results for depths of 2 km, 5 km, 10 km, and 15 km, respectively. The strike angle, dip angle, and rake angle of receiver fault are 52°, 61°, and 128°, respectively. The beach balls show the location and mechanism of the mainshock and Mw 6.3 aftershock.

TABLE 2 Δσf at the 2011 Christchurch earthquake epicenter, caused by different mainshock models with three µ ′ values at four focal depths. The focal mechanism of the receiving fault is 52°/61°/128°.

Depth Uniform fault plane model

Two-segment slip model

Stochastic model with single plane based on NEIC solution Stochastic model with four segments

µ′

2 km

5 km

10 km

15 km

0.0 0.4 0.8 0.0 0.4 0.8 0.0 0.4 0.8 0.0 0.4 0.8

0.062 0.010 –0.042 0.087 0.035 –0.018 0.110 0.004 –0.103 0.120 0.048 –0.023

0.062 0.013 –0.036 0.095 0.044 –0.008 0.143 0.033 –0.077 0.119 0.053 –0.013

0.066 0.021 –0.023 0.113 0.065 0.017 0.183 0.086 –0.010 0.114 0.058 0.002

0.081 0.036 –0.008 0.143 0.094 0.044 0.214 0.131 0.048 0.121 0.068 0.015

810 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C)

(D)

▲▲ Figure 11. The coseismic Δσf caused by the two-segment slip model at a focal depth of 5 km with μ′=0.4 for variations in receiver fault strike. A), B), C), and D) show results for strikes of 352°, 22°, 82°, and 112°, respectively. The dip and rake of the receiver fault are held constant at 61° and 128°, respectively, in these calculations. The beach balls show the location and mechanism of the mainshock and Mw 6.3 aftershock.

at different focal depths with three values of apparent coefficient of friction. We find that different slip models can result in significant differences in the amplitude and distribution of Δσf in the near field of the fault, but no substantial difference in the far field. On the other hand, focal depth and receiving fault geometry play a much stronger role on Δσf outside the immediate mainshock rupture zone. In our calculations for the 2011 Christchurch earthquake, Δσf can increase significantly (by a factor of 3) when the aftershock focal depth increases from 2 km to 15 km. Additionally, our results show a change of 30 degrees in receiving fault geometry can even cause polarity changes in Δσf . We also find the resulting Δσf at the epicenter of the 2011 Christchurch earthquake decreases significantly as the value of apparent coefficient of friction (μ′) increases. This emphasizes the need for careful consideration of the appropriate value of μ′ for different faulting environments. It should be noted that in this study we assume that the coseismic slip distribution of the Canterbury mainshock is responsible for the triggering of the Christchurch earthquake. Because the GPS measurements

after the Canterbury earthquake show very little postseismic motion (less than ~2% of coseismic) (Reyners 2011, this issue), postseismic deformation probably can be neglected. However we still cannot rule out the possibility that smaller aftershocks triggered the Christchurch earthquake as a secondary aftershock with larger magnitude (e.g., Felzer et al. 2002).

In general, we find the occurrence of the Canterbury earthquake with a reasonable set of parameter choices raises the Δσf on the Christchurch fault plane beyond the 0.01 MPa threshold, promoting the aftershock plane to break. To improve the accuracy of Δσf analysis, and hence the probability assessment of aftershocks, it is helpful to carefully study source parameters of historical earthquakes for each region to understand the potential receiving fault geometry. For M > 5.5 earthquakes, the teleCAP technique used in this paper shows promise for obtaining accurate focal mechanism and depth.

For M ~ 5 earthquakes not recorded with local broadband seismic stations, teleseismic P waves are typically above noise level in the short-period band (~1Hz), and teleCAP can be

Seismological Research Letters Volume 82, Number 6 November/December 2011 811

(A)

(B)

(C)

(D)

▲▲ Figure 12. The coseismic Δσf caused by the two-segment slip model at a focal depth of 5 km with μ′ = 0.4 for variations in receiver fault dip. A), B), C), and D) show results for dips of 21°, 41°, 61°, and 81°, respectively. The strike and rake of the receiver fault are held constant at 52° and 128°, respectively, in these calculations. The beach balls show the location and mechanism of the mainshock and Mw 6.3 aftershock.

also applied, although the variability of P-wave amplitude has to be taken into account (Ni et al. 2010; Chu et al. 2011).

For M ~ 5 earthquakes well recorded with local stations, the traditional CAP technique can be applied to estimate source parameters. For even smaller earthquakes (M 2–4), Tan and Helmberger (2007) have proposed a new amplitude correction technique to invert short-period (0.5–2 Hz) P waveforms for source parameters, and achieved success in the 2003 Big Bear sequence. Since focal mechanism itself cannot distinguish between the fault plane and auxiliary fault plane, additional information is needed, for example from aftershock distribution or earthquake rupture directivity (e.g., Luo et al. 2010).

Paleoseismology and geology can also provide important information on potential fault orientations, particularly for regions without active seismicity.

ACKNOWLEDGMENTS Constructive reviews provided by Jeanne Hardebeck, Morgan Page, and an anonymous reviewer were very helpful in revis-

ing the paper and making it acceptable for publication. This work is supported by China Earthquake Administration fund 200808078, and NSFC fund 40821160549, 41074032.

REFERENCES Allen, R. M., and H. Kanamori (2003). The potential of earthquake early warning in southern California. Science 300 (5,620), 786–789.

Bakun, W. H., F. G. Fischer, E.g. Jensen, and J. VanSchaack (1994). Early warning system for aftershocks. Bulletin of the Seismological Society of America 84 (2), 359–365.

Bassin, C., G. Laske, and G. Masters (2000). The current limits of resolution for surface wave tomography in North America. Eos, Transactions, American Geophysical Union 81, F897.

Beavan, J., S. Samsonov, M. Motagh, L. Wallace, S. Ellis, and N. Palmer (2010). The Darfield (Canterbury) earthquake: Geodetic observations and preliminary source model. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 228–235.

Chu, R., S. Ni, A. Pitarka, and D. V. Helmberger (2011). Inversion of source parameters for moderate earthquakes using short-period teleseismic P waves. Submitted to Geophysical Journal International.

812 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C)

▲▲ Figure 13. The coseismic Δσf caused by the two-segment slip model for different values of apparent coefficient of friction μ′ at a depth of 5 km with a receiver fault mechanism of 52°/61°/128°. A), B), and C) show results for μ′ of 0.0, 0.4, and 0.8, respectively.

Dahlen, F. A., and J. Tromp (1998). Theoretical Global Seismology.

Princeton, NJ: Princeton University Press.

Dreger, D. S., and D. V. Helmberger (1993), Determination of Source Parameters at Regional Distances with Single Station or Sparse Network Data, Journal of Geophysical Research 98, 8,107–8,125.

Felzer, K. R., T. W. Becker, R. E. Abercrombie, G. Ekström, and J. R. Rice (2002), Triggering of the 1999 MW 7.1 Hector Mine earthquake by aftershocks of the 1992 MW 7.3 Landers earthquake, Journal of Geophysical Research 107 B92190; doi:10.1029/2001JB000911.

Felzer, K. R., and E. E. Brodsky (2006). Decay of aftershock density with distance indicates triggering by dynamic stress. Nature 441, 735–738.

Freed, A. M. (2004). Earthquake triggering by static, dynamic, and postseismic stress transfer. Annual Review of Earth and Planetary Sciences 33, 335–367.

Freed, A. M., and J. Lin (2001). Delayed triggering of the 1999 Hector Mine earthquake by viscoelastic stress transfer. Nature 411, 180– 183.

Graves, R., and A. Pitarka (2010). Broadband ground motion simulation using a hybrid approach. Bulletin of the Seismological Society of America 100, 2,095–2,123; doi: 10.1785/0120100057.

Harris, R. A. (1998). Introduction to special section: Stress triggers, stress shadows, and implications for seismic hazard. Journal of Geophysical Research 103, 24,347–24,358.

Harris, R. A. (2000). Earthquake stress triggers, stress shadows, and seismic hazard. Current Science 79 (9), 10.

Jaeger, J. C., N. G. W. Cook, and R. W. Zimmerman (2007).

Fundamentals of Rock Mechanics, fourth edition. Wiley-Blackwell.

Ji, C., D. J. Wald, and D. V. Helmberger (2002a). Source description of the 1999 Hector Mine, California, earthquake, part I: Wavelet domain inversion theory and resolution analysis. Bulletin of the Seismological Society of America 92 (4), 1,192–1,207.

Ji, C., D. J. Wald, and D. V. Helmberger (2002b). Source description of the 1999 Hector Mine, California, earthquake, part II: Complexity of slip history. Bulletin of the Seismological Society of America 92 (4), 1,208–1,226.

King, G. C. P., R. S. Stein, and J. Lin (1994). Static stress changes and the triggering of earthquakes. Bulletin of the Seismological Society of America 84, 935–953.

Lavallée, D., and R. J. Archuleta (2003). Stochastic modeling of slip spatial complexities for the 1979 Imperial Valley, California, earthquake. Geophysical Research Letters 30 (5), 1,245; doi:10.1029/2002GL015839.

Liu, P., R. J. Archuleta, and S. H. Hartzell (2006). Prediction of broadband ground-motion time histories: Hybrid low/high frequency method with correlated random source parameters. Bulletin of the Seismological Society of America 96, 2,118–2,130.

Seismological Research Letters Volume 82, Number 6 November/December 2011 813

Luo, Y., Y. Tan, S. Wei, D. Helmberger, Z. Zhan, S. Ni, E. Hauksson, and Y. Chen (2010). Source mechanisms and rupture directivity of the May 18, 2009 Mw 4.6 Inglewood, California earthquake. Bulletin of the Seismological Society of America 100 (6), 3,269–3,277.

Ma, K.-F., C.-H. Chan, and R. S. Stein (2005). Response of seismicity to Coulomb stress triggers and shadows of the 1999 Mw = 7.6 Chi-Chi, Taiwan, earthquake. Journal of Geophysical Research 110, B05S19.

Mai, P. M., and G. C. Beroza (2002). A spatial random field model to characterize complexity in earthquake slip. Journal of Geophysical Research 107 (B11); doi:10.1029/2001JB000588.

Nalbant, S. S., A. Hubert, and G. C. P. King (1998). Stress coupling between earthquakes in northwest Turkey and the north Aegean Sea. Journal of Geophysical Research 103, 24,469–24,486.

Ni, S., A. Pitarka, and D. V. Helmberger (2010). Rapid source estimation from global calibrated paths. Seismological Research Letters 81 (3), 498–504; doi: 10.1785/gssrl.82.6.498.

Okada, Y. (1992). Internal deformation due to shear and tensile faults in a half-space. Bulletin of the Seismological Society of America 82, 1,018–1,040.

Parsons, T., R. S. Stein, R. W. Simpson, and P. A. Reasenberg (1999).

Stress sensitivity of fault seismicity: A comparison between limitedoffset oblique and major strike slip faults. Journal of Geophysical Research 104, 20,183–20,202.

Parsons, T., S. Toda, R. S. Stein, A. Barka, and J. H. Dieterich (2000).

Heightened odds of large earthquakes near Istanbul: An interaction-based probability calculation. Science 288, 661.

Quigley, M., P. Villamor, K. Furlong, J. Beavan, R. Van Dissen, N. Litchfield, T. Stahl, B. Duffy, E. Bilderback, D. Noble, D. Barrell, R. Jongens, and S. Cox (2010). Previously unknown fault shakes New Zealand’s South Island. Eos, Transactions, American Geophysical Union 91, 469–472; doi:10.1029/2010EO490001.

Reilinger, R. E., S. Ergintav, R. Bürgmann, S. McClusky, O. Lenk, A. Barka, O. Gurkan et al. (2000). Coseismic and postseismic fault slip for the 17 August 1999, M = 7.5, Izmit, Turkey earthquake.

Science 289, 1,519–1,524.

Reyners, M. (2011). Lessons from the destructive Mw 6.3 Christchurch, New Zealand, earthquake. Seismological Research Letters 82(3), 371–372.

Scholz, C. H. (1990). The Mechanics of Earthquakes and Faulting. New York: Cambridge University Press, 439 pp.

Shan, B., X. Xiong, Y. Zheng, and F. Diao (2009). Stress changes on major faults caused by Mw7.9 Wenchuan earthquake, May 12, 2008. Science in China, Series D—Earth Sciences 52 (5), 593–601; doi:10.1007/s11430-009-0060-9.

Stein, R. S. (1999). The role of stress transfer in earthquake occurrence.

Nature 402 (6,762), 605–609.

Stein, R. S., A. A. Barka, and J. H. Dieterich (1997). Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering. Geophysical Journal International 128, 594–604.

Stein, R. S., G. C. P. King, and J. Lin (1994). Stress triggering of the 1994 M = 6.7 Northridge, California, earthquake by its predecessors, Science 265, 1,432–1,435.

Talebian, M., J. Biggs, M. Bolourchi, A. Copley, A. Ghassemi, M. Ghorashi, J. Hollingsworth, J. Jackson, E. Nissen, B. Oveisi, B. Parsons, K. Priestley, and A. Saiidi (2006). The Dahuiyeh (Zarand) earthquake of 2005 February 22 in central Iran: Reactivation of an intramountain reverse fault. Geophysical Journal International 164 (1), 137–148.

Tan, Y., L. Zhu, D. V. Helmberger, and C. K. Saikia (2006). Locating and modeling regional earthquakes with two stations. Journal of Geophysical Research 111, B01306; doi:10.1029/2005JB003775.

Tan, Ying, and Don Helmberger (2007). A New Method for Determining Small Earthquake Source Parameters Using Short-Period P Waves.

Bulletin of the Seismological Society of America 97 (4): 1,176–1,195; doi:10.1785/0120060251.

Toda, S., J. Lin, M. Meghraoui, and R. S. Stein (2008). 12 May 2008 M = 7.9 Wenchuan, China, earthquake calculated to increase failure stress and seismicity rate on three major fault systems.

Geophysical Research Letters 35, L17305.

Toda, S., R. Stein, P. Reasenberg, J. Dieterich, and A. Yoshida (1998).

Stress transferred by the Mw = 6.5 Kobe, Japan, shock: Effect on aftershocks and future earthquake probabilities. Journal of Geophysical Research 103, 24,543–24,565.

Van Dissen, R., D. Barrell, N. Litchfield, P. Villamor, M. Quigley, A. King, K. Furlong, et al. (2011). Surface rupture displacement on the Greendale fault during the Mw 7.1 Darfield (Canterbury) earthquake, New Zealand, and its impact on man-made structures. Proceedings of the Ninth Pacific Conference on Earthquake Engineering, 14–16 April 2011, 186–193.

Wallace, L. M., and J. Beavan (2006). A large slow slip event on the central Hikurangi subduction interface beneath the Manawatu region, North Island, New Zealand. Geophysical Research Letters 33, L11301; doi:10.1029/2006GL026009.

Wan, K., S. Ni, X. Zeng, and S. Paul (2009). Real-time seismology for the 05/12/2008 Wenchuan earthquake of China: A retrospective view.

Science in China, Series D—Earth Sciences 52 (2), 155–165.

Wang, R., F. Lorenzo Martín, and F. Roth (2003). Computation of deformation induced by earthquakes in a multi-layered elastic crust—FORTRAN programs EDGRN/EDCMP. Computers & Geoscience 29 (2), 195–207.

Wang, R., F. Lorenzo-Martín, and F. Roth (2006). PSGRN/PSCMP—a new code for calculating co- and post-seismic deformation, geoid and gravity changes based on the viscoelastic-gravitational dislocation theory. Computers & Geoscience 32, 527–541.

Wang, W., S. Ni, Y. Chen, and H. Kanamori (2009). Magnitude estimation for early warning applications using the initial part of P waves: A case study on the 2008 Wenchuan sequence. Geophysical Research Letters 36, L16305; doi:10.1029/ 2009GL038678.

Xiong, X, B. Shan, Y. Zheng, and R. Wang (2010). Stress transfer and its implication for earthquake hazard on the Kunlun fault, Tibet.

Tectonophysics 482, 216–225.

Zhao, L. S., and D. V. Helmberger (1994). Source estimation from broadband regional seismograms. Bulletin of the Seismological Society of America 84, 91–104.

Zhang, G., L. Zhu, X. Song, Z. Li, M. Yang, N. Su, and X. Chen (1999).

Predictions of the 1997 strong earthquakes in Jiashi, Xinjiang, China. Bulletin of the Seismological Society of America 89 (5), 1,171–1,183.

Zhu, L., and D. V. Helmberger (1996). Advancements in source estimation techniques using broadband regional seismograms. Bulletin of the Seismological Society of America 86, 1,634–1,641.

814 Seismological Research Letters Volume 82, Number 6 November/December 2011

Seismological Laboratory California Institute of Technology Pasadena, California 91125, U.S.A. zwzhan@gmail.com

(Z. Z.)

E

InSAR and Optical Constraints on Fault Slip during the 2010–2011 New Zealand Earthquake Sequence William D. Barnhart, Michael J. Willis, Rowena B. Lohman, and Andrew K. Melkonian

William D. Barnhart, Michael J. Willis, Rowena B. Lohman, and Andrew K. Melkonian Cornell University

Online material: Images of InSAR data, model residuals; tables including offsets and resampled InSAR data

INTRODUCTION Our study used space-based interferometric synthetic aperture radar (InSAR) and feature tracking on sub-meter-resolution optical imagery pairs to characterize surface deformation resulting from the 4 September 2010 Mw 7.1 Darfield, 22 February 2011 Mw 6.3 Christchurch, and 13 June 2011 Christchurch earthquakes (dates in local time), each of which occurred in the Canterbury region of the South Island of New Zealand. A rapid, coordinated international emergency response is often required when strong-motion earthquakes hit urban areas. Unfortunately in these cases relief workers often have little information about the location or the extent of damage. Remote sensing can rapidly provide maps of certain key variables (i.e., building damage, potential loading of nearby faults, etc.) to relief workers on the ground. These maps can cover broad areas on time scales that are only limited by the revisit time of the satellite or aircraft. Critically, imagery types such as satellite-based synthetic aperture radar (SAR) have long repeat times of up to 46 days at present, although the existence of overlapping tracks and multiple satellite platforms effectively reduces the repeat time somewhat. Here we demonstrate the impact of commercial optical imagery that can be acquired within hours to days after an earthquake, with the goal of supporting relief efforts in future earthquakes on a more rapid timescale than can be achieved with SAR imagery alone. We demonstrate that these sub-meter-resolution scenes are feasible tools for deriving near-fault surface displacements for use in fault slip inversions, even in areas of heavy agricultural activity.

The Darfield and Christchurch earthquakes present an opportunity to observe postseismic deformation related to multiple moderate (<Mw 7.5) earthquakes occurring in close spatial and temporal proximity with an unprecedented set of seismic and geodetic constraints spanning the two events. While there are many examples of earthquakes of this size occurring in close proximity, including the 1992 Landers and 1999 Hector Mine doi: 10.1785/gssrl.82.6.815

earthquakes, the shorter time interval between the Darfield and Christchurch events means that many instruments that were deployed after the first earthquake were still in place to observe the second and third events. We perform inversions of these data for the spatial distribution of fault slip that occurred during each of these earthquakes and assess the potential contribution of the static Coulomb stress change that occurred during the Darfield event to the eventual rupture of the Christchurch earthquake.

TECTONIC SETTING The 2010–2011 Canterbury earthquake sequence occurred east of the dominantly strike-slip Pacific-Australian plate boundary, on previously unrecognized faults within the topographically smooth Canterbury Plains (Figure 1). The Darfield earthquake (Mw 7.0–7.1) ruptured nearly 40 km of the northern Canterbury Plains, partially on the now recognized Greendale fault, leaving extensive surface ruptures and ground warping (e.g., Quigley et al. 2010 and this study). Though surface ruptures suggest dominantly right-lateral strike-slip motion, aftershock locations and focal mechanisms, firstmotion focal mechanisms, and subsequent geodetic modeling show that the event consisted of a complex rupture sequence involving NE-SW striking reverse faults in addition to E-W striking right-lateral strike-slip faults (e.g., Beavan et al. 2010; Gledhill et al. 2011). The 22 February 2011 (hereafter, 22-Feb) Christchurch earthquake (Mw 6.3, Figure 1) that followed five months later occurred as part of a sequence of aftershocks to the east that illuminated numerous zones characterized by both E-W striking strike slip and NE-SW striking reverse slip (Figure 1). The 22-Feb Christchurch event was dominantly reverse slip and occurred near the contact between the volcanic Banks Peninsula and poorly consolidated sediments underlying Christchurch. Unlike the Darfield earthquake, the 22-Feb Christchurch earthquake led to significant urban damage and casualties due both to its shallow source, its exceptionally strong ground motion (e.g., Bradley and Cubrinovski 2011; Fry et al.

2011; Iizuka et al. 2011, page 875 of this issue), and proximity to the cities of Christchurch and Lyttelton (Figures 1 and

Seismological Research Letters Volume 82, Number 6 November/December 2011 815

−43˚00'

335 Path 36

3 Path

13-June-2011 22-Feb-2011 Mw 6.0 Mw 6.3

k Trac

4-Sep-2010 Mw 7.1

195

−44˚00'

N 23 51 Track

/yr

3 Track

m 6m

3

40 km 171˚00'

172˚00'

173˚00'

▲▲ Figure 1. Study region and spatial coverage of data. Focal mechanisms are Global Centroid Moment Tensor (GCMT; Dziewonski et al. 1981) solutions for the 4 September 2010, 22 February 2011, and 13 June 2011 Canterbury earthquakes. Faults (thin lines) and seismicity (black dots) are from GNS Geonet (http://geonet.org. nz) with earthquake spanning the period 3 September 2010 to 1 August 2011. Boxes indicate extent of InSAR data (black) and optical imagery (white). Image overlaid on SRTM digital elevation model (Farr and Kobrick 2000). Inset shows map location with major tectonic features and Nuvel-1A plate motion of Australia relative to fixed Pacific (DeMets et al. 1994).

2). Another significant event (Mw 6.0) occurred 13 June 2011 (hereafter 13-June) near the Christchurch earthquake epicenter, causing further damage in the city of Christchurch. The Darfield earthquake exhibited a large stress drop of ~160 bars while the Christchurch events exhibited more moderate stress drops of 50–60 bars (Fry et al. 2011, page 846 of this issue).

The city of Christchurch (Figure 1) is located on the eastern Canterbury Plains, an alluvial plain of Cretaceous through present sediments overlying the Late Paleozoic to Mid Cretaceous Torlesse terrane (e.g., Mackinnon 1983). The Banks Peninsula, an extinct Miocene volcanic structure, punctuates the eastern edge of the Canterbury Plains near Christchurch (Timm et al. 2009). Several other Cenozoic volcanic structures exist throughout the South Island, including near the city of Dunedin. Paleoseismic and GPS studies suggest that up to 80% of the 38 mm/yr relative Australian-Pacific plate motion occurring within the central South Island of New Zealand (DeMets et al. 1994; Beavan et al. 1999; Wallace et al. 2007) is accommodated by the Alpine fault (Berryman et al. 1992; Norris and Cooper 2001) while the Porter’s Pass/Amberly fault system, north of our study area, accommodates ~10–15% (3–8 mm/ yr, Beavan et al. 1999; Wallace et al. 2007). The rates and rate uncertainties in the central South Island allow for up to 10 mm/ yr of unaccounted strike-slip motion, which has been attributed

to model errors or uplift in the foothills of the Southern Alps and strike-slip motion in the Canterbury Plains (e.g., Beavan et al. 1999; Sutherland et al. 2006; Wallace et al. 2007). Several large (>M 7.1) earthquakes are associated with the Porter’s Pass fault zone in the Southern Alps foothills (Howard et al.

2005) while other large Quaternary events are documented to the north in the Marlborough fault system (Cowan 1991).

Documentation of active faults in the Marlborough fault system and Canterbury Plains reveals dominantly right-lateral and reverse-slip motion on shallow to steeply (>50 degrees) dipping planes (see Pettinga et al. 2001 and associated references).

Prior to the 2010–2011 earthquake sequence, the strongest historical ground motion in Christchurch was attributed to an M 7–8 event (Stirling et al. 1999), and the Canterbury Plains in this focus area were characterized by low to moderate rates of seismicity (e.g., Pettinga et al. 2001). Reflection-seismic surveys in the vicinity of the Darfield event revealed offsets and folding of Quaternary sediments older than 24 ka by thrust faults (Dorn et al. 2010), leading those authors to suggest that infrequent events >M 7 with long recurrence intervals could be possible.

DATA: AVAILABILITY AND PROCESSING RESULTS Characteristics of the radar and optical data that we used in this work are summarized in Tables 1 and 2, respectively.

Multiple pairs of SAR imagery with at least two different look angles span each earthquake (spatial coverage shown in Figure 1), as well as the period in between them, allowing some redundancy in the data and the assessment of whether individual features in the data are associated with the earthquake or with noise. Because of the limited number of acquisitions, we only use ascending tracks, which restricts our ability to constrain the three-dimensional deformation field for each event. For the Darfield earthquake, we successfully obtained SAR pixel offsets, which constrain displacements in the horizontal, alongtrack direction and provide an additional component of the three-dimensional deformation field (e.g., Fialko et al. 2001).

We processed interferograms using the Caltech/JPL (Jet Propulsion Laboratory) InSAR processing package ROI_PAC (Rosen et al. 2004), using a digital elevation model from the Satellite Radar Topography Mission (SRTM, Farr and Kobrick 2000). PALSAR imagery from the ALOS satellite was provided

by Japanese Aerospace Exploration Agency (JAXA) through an agreement with NASA and the Alaska Satellite Facility (ASF). ENVISAT imagery was acquired through a Category-1 agreement with the European Space Agency (ESA).

All interferograms of the Darfield coseismic and postseismic period we analyze are found in Figures S1 and S3 of this manuscript. The strong shaking, liquefaction, and high strain gradient resulted in interferograms that require some manual phase unwrapping to connect coherent zones separated by regions of decorrelation. In these cases, we ensured that the phase unwrapping was consistent across spatially overlapping interferograms by inspection and comparison to the predicted displacement field resulting from our inversion.

816 Seismological Research Letters Volume 82, Number 6 November/December 2011

N

−43˚30'

5 km

Christchurch

Figure 3 −43˚36'

Greendale

c

b

100 m

A

172˚06'

B

N

m

N

91

D

5.8m 200 m C

20 m

D

▲▲ Figure 2. Examples of surface ruptures from the Darfield earthquake visible in postseismic WorldView 1 optical imagery. A) Overview map with surface rupture (thick black line, Quigley et al. 2010) and Global CMT solution for the Darfield event (Dziewonski et al. 1981).

Roads (gray lines) and railroads (dashed lines) from http://www.diva-gis.org /. B) Example of surface rupture (arrows added by authors) and interpretive field text courtesy of local farmer (exists in field, not added by authors). C) En-echelon rupture jump of ~90 m (arrow), hedgerow offset by rupture (circle). D) Zoom view of hedge and canal offsets of ~5.8 m. Optical imagery copyright 2011 Digital Globe, provided through the NGA Commercial Imagery Program.

The large number of pixels (several million) in the final InSAR data products would be prohibitively computationally expensive to ingest into any inversion scheme. Therefore, we subsample the data using the procedure outlined in Lohman and Simons (2005) so that we retain a set of spatial averages with 138 to 376 points for each interferogram (Figures S1 and S2).

Because we were not able to unambiguously unwrap coherent phases across the Darfield rupture for Envisat Track 51, we treat the regions north and south of the Darfield rupture as two separate data sets. Peak line of sight (LOS) offsets during the Darfield earthquake were around two meters, with horizontal pixel tracking results of up to five meters. There were at least three distinct strike-slip fault planes and two zones of thrust faulting activated during the Darfield earthquake (e.g., Quigley et al. 2010; Beavan et al. 2010; Gledhill et al. 2011). This rupture complexity is apparent in the complicated, multi-lobed deformation field imaged with InSAR and aftershock locations (Figure 1).

The 22-Feb and 13-June Christchurch earthquakes exhibit a much simpler appearance in the InSAR observations (Figure S2), although there is a large region of decorrelation within the city of Christchurch, and some of the deformation

occurred offshore where it cannot be imaged with InSAR. The steep gradients of deformation suggest a shallow, near-surface slip source, as supported by our inversion described below.

Peak observed LOS deformation associated with the 22-Feb Christchurch earthquake is 0.52 meters. We were unable to obtain subpixel, horizontal offsets from SAR imagery for the 22-Feb event, which suggests either there was no surface rupture (as confirmed by field observations) or any surface offsets were below the noise level of subpixel offset tracking (typically on the order of a meter).

High-resolution (~0.5 m resolution) optical imagery (Figure 2) from commercial satellites was made available to scientists via the U.S. National Geospatial Agency and the National Science Foundation (Table 2). We also explored the use of data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), which has lower spatial resolution but is available on a more consistent, global basis, particularly for preseismic imagery that may not be acquired as part of the background mission for commercial satellites. Previous work using cross-correlation of optical imagery has been primarily limited to ASTER and SPOT imagery (e.g., Avouac et al. 2006; Crippen

Seismological Research Letters Volume 82, Number 6 November/December 2011 817

TABLE 1 Pairs of SAR imagery used in this study for both traditional InSAR and horizontal offsets obtained through pixel tracking (asterisks). B⊥ is the perpendicular baseline for each pair, in meters. Dates are in GMT. Satellite

Date1

Date2

Track/Path

Frame/Scene

B⊥ (m)

Christchurch EQ: 22 February 2011 ALOS ALOS

2011Jan10 2010Oct27

2011Feb25 2011Mar14

335 336

6300 6290/6300

421 1178

2010Mar11 2010Jan24 2010Sep01 2010Jul09

2010Sep11 2010Oct27 2010Oct06 2010Sep17

336 336 323 51

6300 6300 6309 6309

1215* 1893 236 532*

2010Sep11 2010Sep11 2010Oct27

2010Oct27 2011Mar14 2011Mar14

336 336 336

6300 6300 6300

231 1407 1173

2011July08

195

6291

14

Darfield EQ ALOS ALOS ENVI ENVI Post Darfield EQ ALOS ALOS ALOS

Christchurch EQ: 13 June 2011 ENVI

2011June08

* indicates date pairs where both traditional interferometry and pixel tracking offsets were generated All scenes are ascending tracks TABLE 2 Optical data used in this study spanning each event. Dates are in GMT. Optical imagery copyright 2010 Digital Globe, provided by the NGA Commercial Imagery Program.

Satellite

Date

Spatial Resolution (m)

Band

Pre/Post-seismic

Christchurch EQ WorldView1 WorldView1

2010Sep21 2011Feb26

0.5 0.5

Panchromatic Panchromatic

Pre Post

2009Oct23 2010Sep21 2006Feb11 2010Sep18

0.5 0.5 15 15

Panchromatic Panchromatic

Pre Post Pre Post

Darfield EQ GeoEye WorldView2 ASTER ASTER

1992; Michel and Avouac 2002; Leprince et al. 2007; Kääb and Debella-Gilo 2010), with spatial resolutions of 15 and 2.5–10 m, respectively. For comparison, the GeoEye imagery used here has a pixel size of 0.5 m. We performed normalized cross-correlation of imagery (e.g., Melkonian 2011) processed using the ampcor program contained within the ROI_PAC software package (Rosen et al. 2004). Results for the higher resolution commercial data are described below, but we were unable to clearly resolve subpixel offsets for either of the earthquakes based on ASTER imagery due to striping within the data.

The GeoEye-1 satellite acquired pre-event high-resolution imagery on 23 October 2009. The panchromatic 15 km × 15 km scene is down-sampled from 41-cm resolution to 50-cm resolution for civilian use. The satellite, launched in

September 2008, has precise pointing capabilities providing scenes that are geolocated with a circular error of probability (CEP) of about six meters without the use of ground control points. We extract the radiometrically corrected JPEG2000 imagery from its National Imagery Transmission Format (NTF) wrapper using the Geographic Data Abstraction Library (version 1.8, http://www.gdal.org/). The resulting 8.5 Gb 16-bit unsigned integer geotiff is geocoded, reprojected to Universal Transverse Mercator (UTM) coordinates and registered and orthorectified to a 90-m SRTM digital elevation model (Farr and Kobrick 2000). The post-event imagery comes from the Worldview-1 satellite. This satellite, launched in September 2007, has a revisit time of 1.9 days and began imaging the Canterbury region almost immediately after the

818 Seismological Research Letters Volume 82, Number 6 November/December 2011

earthquake. Unfortunately clouds hampered acquisition until 21 September 2010, 17 days after the earthquake. We extracted the 17.9-km-swath-wide, half-meter panchromatic imagery using identical procedures as with the GeoEye-1 imagery.

Difficulties arise using this high-resolution imagery due to agricultural changes in the intervening year and different sun elevations and azimuths that result in a variable degree of shadowing from houses and hedgerows. Much of the imagery decorrelates over this time interval, in part because there have been dramatic changes in land use that are visible in the form of radical differences in relative brightness between fields and different plowing patterns between the two images. However, the hedgerows themselves, which are visually distorted across the fault in the postseismic images (Figures 2B–D), act as coherent features that provide very strong offsets from image to image.

Since the hedgerows are effectively linear and have a similar brightness along their length, the offsets are better-resolved in a direction perpendicular to each hedgerow than they are along their length. Therefore, we obtain good characterization of the E-W deflection of N-S-trending hedgerows across the fault, but poor results for E-W motion of hedgerows and roads that trend in a near E-W direction. Since the horizontal displacements in the E-W direction are much larger than those in the N-S direction for this earthquake, the most useful features in the imagery pixel tracking have been the N-S-trending roads and hedgerows.

Figure 3 summarizes the results of optical imagery pixel tracking for the Darfield earthquake. Colored dots (Figure 3A) indicate the magnitude of displacement in an E-W direction of a 10 × 10 pixel box that was allowed to move for 32 pixels in any direction, posted at 5-pixel spacing. Peak displacements across the fault (Figure 3B) agree with what one would pick from the trend of the hedgerow using the postseismic imagery alone (Figure 2D). Although offsets in this example are only recoverable from anthropogenic features, processing images with shorter temporal baselines (days to months) produces coherent offsets in vegetated regions, validating that this technique can be used in remote regions if appropriate acquisitions are available. Unfortunately, the only imagery available with these short temporal baselines is located away from the Darfield fault trace.

MODELING RESULTS Source Modeling Darfield Earthquake For the Darfield and Christchurch earthquakes, we invert the geodetic observations for spatially distributed fault slip using planar fault geometries that we infer using a combination of nonlinear inversion and independent data such as surface ruptures, aftershocks, etc. For the Darfield earthquake, we use four steeply south-dipping planes (Table S1) to model the primarily right-lateral strike-slip motion (Figures 4A and 4C) using a linear inversion for spatially distributed fault slip on a set of 328 triangular dislocations (Meade 2007) with minimum moment regularization constraints. Beavan et al. (2010) demonstrated that shallow (~4 km) thrust slip in addition to right-lateral slip is

required to fully account for all features in the deformation field; however, our primary goal in interpreting the Darfield earthquake deformation field is to drive modeling of Coulomb stress change at the location of the Christchurch earthquake. At these distances, the effects of the shallow thrust faults are not likely to have a strong effect on Coulomb stress change (e.g., King 2009).

Our Darfield fault model location is based on mapped surface ruptures (Quigley et al. 2010) while dips are constrained by focal mechanisms of right-lateral aftershocks. We extend our faults to the east and west to account for significant deformation apparent in the interferograms beyond mapped surface ruptures. Our best-fit slip distribution and model residual is shown in Figure 4A, with a moment magnitude of Mw 7.0. Slip magnitudes and depth ranges agree well with previous inversions by Beavan et al.

(2010) using InSAR and GPS observations. We are unable to fit some features in the data near the center and easternmost end of the rupture (Figure S1). The misfit is influenced by a combination of errors in model geometry, exclusion of NE-SW-dipping reverse faults, spatially correlated atmospheric noise, ionospheric perturbations, and contributions from significant postseismic deformation evident in postseismic interferograms (Figure S3) and, therefore, likely present in varying degrees in the coseismic interferograms used in our inversions.

Figure 3B illustrates the predicted E-W horizontal offsets from our best-fit model at the location of the optical image pixel-tracking results. The predicted displacements across the fault are significantly smaller (~2.5 m compared with 5 m), which is not surprising given that there was a data gap in the InSAR imagery approaching the fault and that the regularization placed on our inversion tends to reduce slip in regions that have less coverage by the data. The discrepancy may also be due, in part, to variable amounts of postseismic slip between the interferograms and the optical imagery. Overall, the difference between the observed displacements and those predicted using inversions based on InSAR data and an elastic half-space model highlights both the importance of using near-field data when it exists as well as the potential for issues in using elastic models in regions where the deformation is clearly anelastic. However, these issues are likely to primarily affect the inversion for slip in the shallow subsurface and will not contribute much to the predicted Coulomb stress study discussed below.

Christchurch Earthquakes To obtain a fault model for the Christchurch earthquakes, we use the Neighborhood Algorithm (Sambridge 1999) to invert ALOS-PALSAR and Envisat interferograms (Table 1) for single fault dislocations (Table S1). We then fix this best-fit geometry and extend the fault along-strike and down-dip to avoid spurious edge effects before performing a linear inversion for distributed slip. Model trace locations are shown in Figure 4C. We use an automated, resolution-based fault parameterization (Barnhart and Lohman 2010) that generates smaller fault patches near the surface, where there is more constraint from data, than at depth and offshore, which allows us to more efficiently explore a range of potential fault geometries and constraints on slip than if we used a uniform fault patch size distribution. For the 22 February

Seismological Research Letters Volume 82, Number 6 November/December 2011 819

N

A E-W Horiz. Displ. (m)

6

X’ B

X’ B

3 0

-3

1 km

X

X

-10 0 10 20 East Displ. (m)

-6 0

100

200

300

400

Horizontal distance (m)

▲▲ Figure 3. Pixel tracking results from Darfield coseismic optical imagery pairs. A) Calculated east-west pixel offsets overlaid on WorldView 1 postseismic scene (location in Figure 2A, Table 2). Positive = east motion, negative = west motion. Pixels with a signal-tonoise ration less than 3.5 and standard deviation in the E-W component greater than 0.025 m have been masked. Black lines are mapped fault trace based on expression of surface rupture in the postseismic image. B) Profile X–X′ across the fault showing values of pixel offsets (black dots) and displacement predicted by the slip distribution shown in Figure 4A. All pixel locations, E-W, and N-S offsets are available in Table S3. Optical imagery copyright 2011 Digital Globe, provided through the NGA Commercial Imagery Program.

7

7

14

Slip (m)

Depth (km)

A 0

0 I: S121E 75S

0

B 0

0

0

0

7

7

7

14 II: N89E 85S 14 III: N82E 85S 14 IV: N84E 85S 13 0 10 0 7.3 0 Along-Strike Length (km) East West

0

V: N57E 70S

18 0

West

East

30

−43˚30'

9

−43˚45'

2 Slip (m)

Depth (km)

C

V

IV

I II

VI

III

172˚00'

11

30km 172˚30'

173˚00'

▲▲ Figure 4. InSAR-based coseismic slip distributions. A) Darfield earthquake slip distribution. Arrows indicate motion of northern block relative to the southern block (right = right-lateral, up = reverse). Text describes strike and dip of each plane. Roman numerals correspond to fault model location in (C). B) 22-Feb Christchurch earthquake slip distribution. Text describes strike and dip of the plane.

C) Model surface trace locations (I–IV: Darfield earthquake, V: 22-Feb Christchurch earthquake, VI: 13 June Christchurch earthquake).

Dots are aftershocks from Geonet catalog for the period 3 Sept 2010 to 1 August 2011. Image overlaid on shaded SRTM DEM (Farr and Kobrick 2000).

820 Seismological Research Letters Volume 82, Number 6 November/December 2011

Coulomb Stress Change In order to model the potential effects of static Coulomb stress change of the Darfield earthquake on the 22-Feb Christchurch earthquake, we use the Darfield earthquake slip distribution described above (Figure 4A), which predicts a static Coulomb stress change on a fault with the orientation and rake inferred for the Christchurch earthquake as shown in Figure 5A. In our calculation, all slip inverted for the Christchurch earthquake occurs within the region of positive Coulomb stress change (Figure 5A, black curve). This suggests that static Coulomb stress change from the Darfield earthquake indeed encouraged the Christchurch earthquake. Peak calculated static Coulomb stress change is 3.1 bars while the minimum is –4.5 bars.

To obtain statistics describing the significance of these inferred static Coulomb stress changes, we apply a Monte Carlo error propagation technique similar to that described in Lohman and Barnhart (2010). We begin by simulating 500 noisy data sets by adding spatially correlated noise with a spatial scale of 100 km to the predicted LOS surface displacements from our best-fit slip distribution, using the same covariance as we infer from the original Darfield data. We then invert for slip on the same four-fault geometry used above for each synthetic data set. Lastly, we calculate the static Coulomb stress change on the fault geometry and slip orientation inferred for the Christchurch earthquake for each realization of the synthetic data. This method allows us to quantify errors in predicted Coulomb stress change (Figure 5B) induced by data noise, such as correlated atmospheric water vapor. As can be seen in Figure 5B, the expected variation due to these sources is far less than the inferred increase in stress resolved on the target fault plane that ruptured during the Christchurch earthquake. Other

Along-Strike East Length (km)

30

3

a

0

9

6

18 0

9

18

b

Coulomb Stress Change (bars)

West

-4

0.4

StDev (bars)

Depth (km)

0 0

Depth (km)

event, we obtain a distributed slip model with 182 triangular dislocations (Figure 4B), with Laplacian smoothing constraints to regularize the inversion. We fix the slip rake direction to 64 degrees, as reported by the Global Centroid Moment Tensor (GCMT) solution (http://www.globalcmt.org; Dziewonski et al. 1981). Inversions in which we allow rake to vary reveal similar solutions. Our best-fit model strikes N57E and dips 70S beneath the Banks Peninsula. This fault geometry agrees well with the GCMT south-dipping focal solution (N59E, 64S) and distributions of aftershocks analyzed through the double-difference method (Bannister et al. 2011, page 839 of this issue). The slip model suggests peak slip of 2.1 m with the main rupture area occurring between 2 and 11 km and has a moment magnitude Mw 6.4 (Figure 4B). Some very shallow slip is observed in the model, although this region corresponds to areas offshore where no geodetic data is available and is probably an artifact of the inversion. Our slip model supports the ground and pixel-offset observations of no surface rupture during the Christchurch earthquake; however, data gaps in the InSAR observations within the city of Christchurch may inhibit our inversions from inferring any slip at the surface. Because only one pair of images is available to constrain slip during the 13-June event (Table 1, Figure S2), we do not present a distributed slip model. We show the location of our best-fit single patch model in Figure 4C.

0.05

▲▲ Figure 5. A) Static Coulomb stress change on the Christchurch earthquake fault plane predicted by the slip distribution inferred for the Darfield earthquake (Figure 4A). Positive Coulomb stress change encourages rupture, negative discourages rupture.

Black outline shows extent of Christchurch earthquake slip with magnitude > 0.7 m. B) 1σ standard deviation of static Coulomb stress change, calculated using 500 realizations of the Darfield earthquake slip distribution.

errors due to variations in fault plane geometry, crustal elastic structure, or to the contribution from the rest of the aftershock sequence likely also contribute.

DISCUSSION Certain attributes of this earthquake sequence suggest reactivation of poorly developed faults. A particularly interesting attribute of seismicity during the 2010–2011 Canterbury earthquake sequence is the activity of steeply dipping (>50 degrees) reverse faults. First motion focal solutions for the Darfield earthquake reveal reverse-motion rupture on a steep, eastdipping plane (Gledhill et al. 2011) before slip propagated to E-W-striking strike-slip faults. In addition, aftershock locations and focal mechanisms located in NE-SW-trending zones at the ends and center of the Darfield rupture reveal steeply dipping reverse-motion planes, and steep reverse faults are necessary to model geodetic observations of both the Darfield (Beavan et al.

2010) and Christchurch earthquakes. Traditional Andersonianstyle faulting predicts that faults should form at angles of ~30 degrees to the principal shortening direction (Anderson 1951), which results in reverse faults dipping 30 degrees with a horizontal shortening direction and normal faults dipping 60 degrees with a vertical shortening direction. While Anderson’s theory predicts the angles at which faults form relative to the local stress field, preexisting faults can reactivate and new faults

Seismological Research Letters Volume 82, Number 6 November/December 2011 821

will not be formed if it is energetically more favorable to slip on non-optimally oriented planes (Anderson 1951). Reactivation of non-Andersonian faults is observed in numerous tectonic environments including Iran (e.g., Byerlee 1978) and the Aegean (e.g., Berberian 1995). The steep dip of reverse faults observed in aftershock and mainshock focal mechanisms along with geodetically derived fault geometries for the Darfield and Christchurch earthquakes strongly suggest Cretaceous-Oligocene faults, formed during formation of the Torlesse terrain and later breakup of the Rangitata Orogen (Jackson 1994), were seismically reactivated during the 2010–2011 Canterbury earthquake sequence. Likewise, the high stress drops, particularly for the Darfield event, calculated for each event (Fry et al. 2011, page 846 of this issue) suggest reactivation of high-friction faults under low strain rates compared to faults in the Marlborough fault zone or Puysegur and Hikurangi subduction zones.

The lack of many aftershocks west of the Darfield earthquake in the Southern Alps (Figure 1), where thrust faults are oriented more N-S and dip at lower angles (<40 degrees) (Mackinnon 1983; e.g., Dorn et al. 2010) compared to the east implies that despite non-Andersonian dips, faults active during the 2010–2011 earthquake sequence are favorably oriented for rupture within the current stress field in the Canterbury Plains. Our calculation of positive Coulomb stress change in the location of Christchurch earthquake (Figure 5A) implies the Darfield earthquake likely expedited the timing of the Christchurch earthquake.

CONCLUSIONS This earthquake sequence demonstrates the need for reassessment of seismic hazards in the eastern South Island, New Zealand, through continued GPS and seismic reflection studies (such as that by Dorn et al. 2010) to identify faults active in the Quaternary beneath the smooth Canterbury Plains. As noted before, the Banks Peninsula stands out conspicuously on the eastern edge of the Canterbury Plains. The location of this earthquake sequence relative to the Miocene volcanic structure suggests the structure’s location may have a strong influence on stress release in this region, as also suggested by Sibson et al.

(2011, page 824 of this issue). Unmapped, potentially seismogenic faults may exist in association with other volcanic structures throughout the eastern South Island such as near Dunedin.

The steadily increasing availability and shortening latency time associated with optical imagery is opening up a wide range of opportunities for its use in earthquake analysis and response.

Comparing sequential optical images may allow rapid mapping of landslide locations, which will be useful for future studies of strong-motion shaking and will allow hazards assessment teams to move directly to affected regions and assess local hillslope stability. Subpixel offsets from optical imagery have the potential of allowing identification of destabilized slumps that did not fully fail but may pose a significant risk of motion during subsequent aftershocks or rainfalls. In extreme cases, these may induce further damage or local tsunamis.

The fine resolution of the optical imagery will also enable the mapping of liquefaction within the region that experienced strong shaking. Traditional mapping of the regions of liquefaction can be based on their appearance in the imagery and on regions of decorrelation in pixel offset tracking.

ACKNOWLEDGMENTS We would like to thank the National Science Foundation for funding that supported access to the optical imagery used during this project. The Polar Geospatial Center and the National Geospatial-Intelligence Agency provided information about and access to the optical imagery. We thank an anonymous reviewer for helpful remarks on the manuscript. W. Barnhart is supported on a NASA graduate student fellowship. Portions of Figures 1, 2, 3, and 4 were made using GMT software (Wessel and Smith 1998). Envisat SAR images were acquired through the research user category from Eurimage, Italy. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. government and National Science Foundation.

REFERENCES Anderson, E. M. (1951). The Dynamics of Faulting and Dyke Formation with Applications to Britain. Edinburgh: Oliver and Boyd.

Avouac, J.-P., F. Ayoub, S. Leprince, O. Konca, and D. V. Helmberger (2006). The 2005, Mw 7.6 Kashmir earthquake: Sub-pixel correlation of ASTER images and seismic waveforms analysis. Earth and Planetary Science Letters 249 (3–4), 514–528; doi:16/j. epsl .2006.06.025.

Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).

Fine-scale relocation of aftershocks of the 22 February Mw 6.2 Christchurch earthquake using double-difference tomography.

Seismological Research Letters 82, 839–845.

Barnhart, W. D., and R. B. Lohman (2010). Automated fault model discretization for inversions for co-seismic slip distributions. Journal of Geophysical Research 115; doi:201010.1029/2010JB007545.

Beavan, J., M. Moore, C Pearson, M. Henderson, B. Parsons, S. Bourne, P. England et al. (1999). Crustal deformation during 1994–1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault. Journal of Geophysical Research 104 (B11), 25,233–25,255; doi:199910.1029/1999JB900198.

Beavan, J., S. Samsonov, L. Wallace, S. Ellis, and N. Palmer (2010). The Darfield (Canterbury) earthquake: Geodetic observations and preliminary source model. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 228–235.

Berberian, M. (1995). Master “blind” thrust faults hidden under the Zagros folds: Active basement tectonics and surface morphotectonics.

Tectonophysics 241 (3–4), 193–195; doi:16/0040-1951(94)00185-C. Berryman, K. R., S. Beanland, A. F. Cooper, H. F. Cutten, R. J. Norris, and P. R. Wood (1992). The Alpine fault, New Zealand: Variation in Quaternary structural style and geomorphic expressions. Annales Tectonicae 6, 126–163.

Bradley, A. B., and M. Cubrinovski (2011). Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake. Seismological Research Letters 82, 853–865.

Byerlee, J. D. (1978). Friction of rocks. Pure and Applied Geophysics 116, 615–626.

Cowan, H. A. (1991). The North Canterbury earthquake of September 1, 1888. Journal of the Royal Society of New Zealand 21 (1), 13–24.

822 Seismological Research Letters Volume 82, Number 6 November/December 2011

Crippen, R. E. (1992). Measurement of subresolution terrain displacements using SPOT panchromatic imagery. Episodes 15, 56–61.

DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein (1994). Effect of recent revisions to the geomagnetic reversal time-scale on estimates of current plate motions. Geophysical Research Letters 21 (20), 2,191–2,194.

Dorn, C., A. G. Green, R. Jongens, S. Carpentier, A. E. Kaiser, F. Campbell, H. Horstmeyer, J. Campbell, M. Finnemore, and J. Pettinga (2010). High-resolution seismic images of potentially seismogenic structures beneath the northwest Canterbury Plains, New Zealand. Journal of Geophysical Research 115; doi:201010.1029/2010JB007459.

Dziewonski, A., T. A. Chou, and J. H. Woodhouse (1981). Determination of earthquake source parameters from waveform data for studies of global and regional seismology. Journal of Geophysical Research 286, 2,825–2,852.

Farr, T., and M. Kobrick (2000). Shuttle Radar Topographic Mission produces a wealth of data. Eos, Transactions, American Geophysical Union 81, 583–585.

Fialko, Y., M. Simons, and D. Agnew (2001). The complete (3D) surface displacement field in the epicentral area of the 1999 Mw 7.1 Hector Mine Earthquake, California, from space geodetic observations. Geophysical Research Letters 28 (16), 3,063–3,066; doi:200110.1029/2001GL013174.

Fry, B., R. Benites, and A. Kaiser (2011). The character of accelerations in the Mw 6.2 Christchurch earthquake. Seismological Research Letters 82, 846–852.

Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011). The Darfield (Canterbury, New Zealand) Mw 7.1 Earthquake of September 2010: A Preliminary seismological report. Seismological Research Letters 82 (3), 378–386; doi:10.1785/gssrl.82.6.378.

Howard, M., A. Nicol, J. Campbell, and J. R. Pettinga (2005). Holocene paleoearthquakes on the strike-slip Porters Pass Fault, Canterbury, New Zealand. New Zealand Journal of Geology and Geophysics 48 (1), 59–74; doi:10.1080/00288306.2005.9515098.

Iizuka, H., Y. Sakai, and K. Koketsu (2011). Strong ground motions and damage conditions associated with seismic stations in the February 2011 Christchurch, New Zealand, earthquake. Seismological Research Letters 82, 875–822.

Jackson, J. (1994). Active tectonics of the Aegean region. Annual Review of Earth and Planetary Sciences 22 (1), 239–271; doi:10.1146/ annurev.ea.22.050194.001323.

Kääb, A., and M. Debella-Gilo (2010). Sub-pixel precision image matching for measuring surface displacements on mass movements using normalized cross-correlation. Remote Sensing of Environment 115 (1), 130–142; doi:10.1016/j.rse.2010.08.012.

King, G. (2009). Fault interaction, earthquake stress changes, and the evolution of seismicity. In Earthquake Seismology, 225–255, H. Kanamori, ed. Elsevier.

Leprince, S., S. Barbot, F. Ayoub, and J.-P. Avouac (2007). Automatic and precise orthorectification, coregistration, and subpixel correlation of satellite images, application to ground deformation measurements. IEEE Transactions on Geoscience and Remote Sensing 45 (6), 1,529–1,558; doi:10.1109/TGRS.2006.888937.

Lohman, R. B., and W. D. Barnhart (2010). Evaluation of earthquake triggering during the 2005–2008 earthquake sequence on Qeshm Island, Iran. Journal of Geophysical Research 115; doi:201010.1029/2010JB007710.

Lohman, R. B., and M. Simons (2005). Some thoughts on the use of InSAR data to constrain models of surface deformation: Noise structure and data downsampling. Geochemistry Geophysics Geosystems 6, doi:10.1029/2004GC000841.

Mackinnon, T. C. (1983). Origin of the Torlesse terrane and coeval rocks, South Island, New Zealand. Geological Society of America Bulletin 94 (8), 967–985; doi:10.1130/0016-7606(1983)94<967:OOTTT A>2.0.CO;2.

Meade, B. J. (2007). Algorithms for the calculation of exact displacements, strains, and stresses for triangular dislocation elements in a uniform elastic half space. Computers & Geosciences 33 (8), 1,064– 1,075; doi:10.1016/j.cageo.2006.12.003.

Melkonian, A. (2011). Measuring glacier velocities and elevation change rates from ASTER data for Juneau icefield, Alaska. Master’s thesis, Cornell University, Ithaca, NY. Michel, R., and J.-P. Avouac (2002). Deformation due to the 17 August 1999 Izmit, Turkey, earthquake measured from SPOT images. Journal of Geophysical Research 107 (B4); doi:10.1029/2000JB000102; http:// europa.agu.org/?view=article&uri=/journals/jb/jb0204/2000JB000 102/2000JB000102.xml&t=2002. Last accessed June 24, 2011.

Norris, R. J., and A. F. Cooper (2001). Late Quaternary slip rates and slip partitioning on the Alpine fault, New Zealand. Journal of Structural Geology 23 (2–3), 507–520; doi:16/S0191-8141(00)00122-X. Pettinga, J., M. Yetton, R. Van Dissen, and G. Downes (2001).

Earthquake source identification and characterization for the Canterbury region, South Island, New Zealand. Bulletin of the New Zealand Society for Earthquake Engineering 34, 282–317.

Quigley, M., R. Van Dissen, P. Villamor, N. Litchfield, D. Barrell, K. Furlong et al. (2010). Surface rupture of the Greendale fault during the Darfield (Canterbury) earthquake, New Zealand: Initial findings. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 236–242.

Rosen, P. A., S. Hensley, G. Peltzer, and M. Simons (2004). Updated Repeat Orbit Interferometry package released. Eos, Transactions, American Geophysical Union 85 (5), 47.

Sambridge, M. (1999). Geophysical inversion with a neighbourhood algorithm: Searching a parameter space. Geophysical Journal International 138 (2), 479–494.

Sibson, R., F. Ghisetti, and J. Ristau (2011). Stress control of an evolving strike-slip fault system during the 2010–2011 Canterbury, New Zealand, earthquake sequence. Seismological Research Letters 82, 824–832.

Stirling, M., M. Yetton, J. Pettinga, K. R. Berryman, and G. Downes (1999). Probabilistic Hazard Assessment and Earthquake Scenarios for the Canterbury Region, and Historic Earthquakes in Christchurch: Stage I (Part B) of Canterbury Regional Council’s Earthquake Hazard and Risk Assessment Study. Canterbury Regional Council Report No. U99/18.

Sutherland, R., K. Berryman, and R. Norris (2006). Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand. Geological Society of America Bulletin 118 (3–4), 464–474; doi:10.1130/B25627.1.

Timm, C., K. Hoernle, P. Van Den Bogaard, I. Bindeman, and S. Weaver (2009). Geochemical evolution of intraplate volcanism at Banks Peninsula, New Zealand: Interaction between asthenospheric and lithospheric melts. Journal of Petrology 50 (6), 989–1,023; doi:10.1093/petrology/egp029.

Wallace, L. M., J. Beavan, R. McCaffrey, K. Berryman, and P. Denys (2007). Balancing the plate motion budget in the South Island, New Zealand, using GPS, geological and seismological data.

Geophysical Journal International 168 (1), 332–352; doi:10.1111/ j.1365-246X.2006.03183.x.

Wessel, P., and W. H. F. Smith (1998). New, improved version of Generic Mapping Tools released. Eos, Transactions, American Geophysical Union 79, 579.

Department of Earth and Atmospheric Sciences Cornell University 2120 Snee Hall Ithaca, New York 14850 U.S.A. wdb47@cornell.edu

(W. D. B.)

Seismological Research Letters Volume 82, Number 6 November/December 2011 823

Stress Control of an Evolving Strike-Slip Fault System during the 2010–2011 Canterbury, New Zealand, Earthquake Sequence Richard Sibson, Francesca Ghisetti, and John Ristau

Richard Sibson,1 Francesca Ghisetti, 2 and John Ristau3

INTRODUCTION

TECTONIC/GEOLOGIC SETTING

Large earthquakes within seismogenic crust are generally thought to require the pre-existence of large fault structures. Such fault structures appear to evolve by the progressive growth and amalgamation of smaller faults and fractures (Cowie and Scholz 1992). In the course of their evolution some components of an evolving fault system may be inherited from previous tectonic episodes while others may be newly formed in the prevailing tectonic stress field. With increasing displacement and amalgamation of sub-structures, fault structures tend to become “smoother,” less complex, and perhaps weaker (Wesnousky 1988).

The 2010–2011 Canterbury earthquake sequence occurred within the upper crust of the South Island of New Zealand around 100 km southeast from the fast-moving (20– 30 mm/yr) Alpine and Hope fault strike-slip components of the Pacific-Australia transform fault system linking into the southern Hikurangi Margin subduction zone (Figure 1). As of 15 July 2011, the sequence has included three major shocks: the Mw 7.1 Darfield earthquake (3 September 2010 UTC) followed by an Mw 6.2 event on 21 February 2011 UTC and an Mw 6.0 event on 13 June 2011 UTC, along with a rich aftershock sequence that includes 27 shocks with Mw > 5.0. Rupturing occurred on previously unrecognized faults that appear to be components of a highly segmented E-W structure concealed beneath alluvial cover and/or Neogene volcanics. Some subsurface information is, however, available from seismic reflection lines and gravity surveys (e.g., Field et al. 1989).

Here we seek to demonstrate how this complex sequence has likely arisen through reactivation under the contemporary tectonic stress field of a mixture of comparatively newly formed and older inherited fault structures.

The 2010–2011 Canterbury earthquakes occurred within 30 ± 5 km thick continental crust belonging to the buoyant Chatham Rise plateau contained within the Pacific plate (Eberhart-Phillips and Bannister 2002). Local geology (Figure 2) comprises a basement of highly deformed Mesozoic Torlesse metagraywackes and their metamorphosed equivalents at greater depth, unconformably overlain by a Late Cretaceous– Neogene cover sequence up to 2.5 km thick (Forsyth et al.

2008). Polyphase deformation within this basement assemblage includes accretion, folding and thrusting along the Gondwana margin, extensional fault structures from Late Cretaceous rifting of the Zealandia microcontinent, and Neogene transpression across the Alpine fault system.

The cover sequence consists of Late Cretaceous–Paleogene terrestrial-marine sedimentary units (including varying thicknesses of Late Cretaceous Mt. Somers calc-alkaline volcanics and Eocene basalts) overlain by a regressive MiocenePliocene clastic sequence that contains the predominantly basaltic Late Miocene (11–6 Ma) Banks Peninsula volcanics.

Thickness variations are partly attributable to deposition as a Late Cretaceous–Paleocene syn-rift sequence accompanying extensional rifting along the Gondwana margin, which imposed an extensive fault fabric within the basement (Laird and Bradshaw 2004). Neogene shortening has led to varying reactivation of these inherited fault systems. Over the area of the Canterbury Plains the older units are largely obscured by Pliocene and Quaternary alluvial gravels up to a few hundred meters thick (Forsyth et al. 2008).

1. Department of Geology, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand 2. Terrageologica, 129 Takamatua Bay Rd., RD1, Akaroa 7581, New Zealand 3. GNS Science, Te Pu Ao, P.O. Box 30-368, Lower Hutt, New Zealand

CONTEMPORARY STRESS FIELD Available evidence on the contemporary regional stress field in the central South island (Sibson et al., forthcoming) comes from two principal sources summarized in Table 1: 1) stress inversions from earthquake focal mechanisms together with one breakout determination from the Galleon-1 borehole; and 2) axes of maximum contractional strain-rate derived

824 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.824

ult Fault Fa ope H ne pi Al /yr

m 38 m

0

.5 -1

+ + A

+

P

G

-1

H

.5 -1

+

05 - 0.

-1.5

-2

5 - 0.

-1 -1.5

5 -2.

0

44 S -2

-2

-0.5

-1

BANKS PENINSULA

+ -2

0

-1

5

?

Pegasus Bay -2

-1.5

R

0

Ra ng - 2 io r a .

CHRISTCHURCH

-0.5

+

-2

-1.5

-1

0

-1.5

.5 -1

.5 -1

0

0

43 S

5 -0. -1

+

-2.5

0

0

172 E

38 mm/yr Pacific-Australia Plate slip vector

173 E Structural contours of top to basement (depth in km relative to s.l.)

-2

1 2 0 to >2 km

3

-2 to < -2.5 km

Seismic sequence from Sept. 4, 2010 to June 15, 2011

4

(location and magnitude from GEONET, http://www.geonet.org.nz)

G

5 6

0 to - 2 km

M6-7.1

+

Exploration well

M5-5.9

Surface rupture of Sept. 4, 2010 earthquake (Greendale Fault) M4-4.9

▲▲ Figure 1. Tectonic setting of the 2010–2011 Canterbury earthquake sequence. Mapped faults are from 1:250,000 geological maps (Forsyth 2001; Rattenbury et al. 2006; Cox and Barrell 2007; Forsyth et al. 2008). Offshore faults are from Barnes (1994), Mogg et al.

(2008) and from preliminary data released from the 2011 offshore survey in the Pegasus Bay (NIWA and Otago University, http://www. gns.cri.nz/Home/News-and-Events/Media-Releases/Fault-structures-revealed). Subsurface geometry beneath the alluvial cover sequence of the Canterbury Plains and offshore is illustrated by the structural contours of depth to basement. Top of basement is modified from Hicks (1989) and Mogg et al. (2008) and incorporates basement depths in exploration wells, depth-converted seismic reflection lines (http://www.nzpam.govt.nz), and outcrop data. A, R, and H are respectively the Ashley, Rakaia, and Hinds fault systems, interpreted as inherited normal faults. P is the Porters Pass fault system and G is the right-lateral surface break of the 4 September 2010, M w 7.1 earthquake on the Greendale fault (Quigley et al. 2010). 1) right-lateral faults; 2) W-dipping reverse faults (triangles in hanging wall); 3) E-dipping reverse faults (triangles in hanging wall); 4) normal faults (ticks in hanging wall); 5) inferred blind faults; 6) structurally controlled escarpment inferred from Bouguer gravity gradients from Bennett et al. (2000).

Seismological Research Letters Volume 82, Number 6 November/December 2011 825

Q-G BPV PG

M-P

CRET

PG

MSV

T

CRET

T

T

T

T T

T

▲▲ Figure 2. Synoptic tectonostratigraphic column for the Canterbury Plains (not to scale). T = Torlesse basement assemblage; CRET = Late Cretaceous terrestrial sequence; MSV = Mt. = Paleogene marine sequence; Somers volcanics; PG M-P = Miocene-Pliocene marine-terrestrial sequence; BPV = Miocene Banks Peninsula volcanics; Q-G = Quaternary gravels.

from geodetic retriangulation and more modern GPS studies. Congruence between the two sources suggests a remarkably uniform regional stress field throughout the Canterbury region with maximum compressive stress σ1 horizontal and oriented WNW-ESE (115° ± 5°). The apparent parallelism between σ1 stress trajectories and maximum contractional strain-rate is explicable if the latter can be treated as a mea-

sure of maximum incremental shortening subparallel to σ1 (cf.

Keiding et al. 2009). This σ1 orientation is also consistent with predominantly reverse-slip reactivation of structures trending NNE-NE along the Southern Alps range front. However, the dominance of strike-slip faulting in the Canterbury earthquake sequence suggests that the regional stress field is that of an “Andersonian” wrench (strike-slip) regime (Anderson 1905, 1951) with principal compressive stresses oriented: σ1: 0°/115° ± 5°; σ2: vertical; σ3: 0°/025° ± 5°. Some local stress heterogeneity is, of course, to be expected, perhaps through buttressing by the Banks Peninsula volcanic edifice.

From the combination of strike-slip and reverse faulting throughout the region the likelihood is that the stress field is of the form σ1 > σv = σ2 ~ σ3, with some local variance between σv = σ2 and σv = σ3. Brittle structures developing within the current “wrench” regime would be expected to have the orientations shown in the Figure 3 inset. Newly forming strike-slip faults should be vertical, lying at 25°–35° to σ1 in accordance with the Coulomb criterion (Anderson 1905). Note that this also reflects the optimal attitude for reactivating existing faults over a broad range of sliding friction (0.8 > μ s > 0.4) (Sibson 1985). Grain-scale microcracks and macroscopic extension fractures would be statistically aligned perpendicular to σ3 with an orientation 115° ± 5°/V. Planes of maximum shear stress controlling the orientation of subvertical ductile shear zones at depth below the seismogenic zone should lie at ±45° to σ1 along trends of 070° (dextral) and 160° (sinistral).

PRINCIPAL COMPONENTS OF THE SEQUENCE The principal components of the 2010–2011 Canterbury earthquake sequence are summarized in a seismotectonic cartoon illustrating the relationship of the different structures to the inferred σ1 stress trajectories (Figure 3). Regional CMT mechanisms for Mw ≥ 4.0 shocks within the sequence are illustrated in Figure 4. The sequential development and structural characteristics of the larger ruptures are discussed below.

TABLE 1 Stress Indicators in Canterbury, Central South Island (see also Sibson et al., forthcoming).

Region

Method

σ 1 azimuth Reference

Stress Determinations North Canterbury-Marlborough Inversion of focal mechanisms Epicentral area 1994 Arthur’s Pass Mw 6.7 Inversion of aftershock focal mechanisms Central S. Alps Offshore S. Canterbury

Inversion of focal mechanisms Borehole breakouts

115 ± 16° 118 ± 4°

Balfour et al. 2005 Robinson and McGinty 2000 110 – 120° Leitner et al. 2001 114 ± 9° Sibson et al. 2012

Maximum Contractional Strain Rates Okarito west of Alpine F. Godley Valley east of Alpine F. Canterbury Plains NW of ChCh Central Southern Alps Canterbury-Otago block

Determination from repeated triangulation 117 ± 6° Pearson 1994 Determination from repeated triangulation 116 ± 14° Pearson 1994 Determination from GPS campaigns 116 ± 9° Pearson et al. 1995 Determination from GPS campaigns 105 – 115° Beavan and Haines 2001 Rotating elastic block model 110 ± 8° Wallace et al. 2007

826 Seismological Research Letters Volume 82, Number 6 November/December 2011

σ3

σ1

N

C

Mw 7.1

d

d

Mw 6.2

u

u

70°

55°

Mw 6.0

10 km

BPV

σ3

σ1

σ1

115 ± 5°

σ3

145 ± 5°

▲▲ Figure 3. Seismotectonic cartoon of the 2010–2011 Canterbury earthquake sequence in relation to the surface outcrop of Banks Peninsula volcanics (BPV), central Christchurch city (C), and the inferred regional stress field. Expected orientations of newly formed structures (ellipse = extension fracture; solid lines = Coulomb shears; dashed lines = ductile shears) shown in inset at lower left. Epicenters of major shocks denoted by stars; thick bold line = Greendale fault surface rupture; thinner dash-dot lines with filled teeth indicating dip direction = subsurface fault models for the Mw 6.2 and Mw 6.0 aftershocks (Beavan et al. 2011, page 789 of this issue); dotted line = microearthquake lineament; dark shading = area of intense aftershock activity in dilational stepover; gray-shaded bands = aftershock lineaments; hollow-toothed lines = belt of reverse-slip aftershocks (dip unconstrained).

▲▲ Figure 4. Regional CMT focal mechanisms for Mw > 4.0 shocks within the 2010–2011 Canterbury earthquake sequence.

Seismological Research Letters Volume 82, Number 6 November/December 2011 827

The 2010 Mw7.1 Darfield mainshock (16:35:46 on 3 September 2010 UTC) appears to be a composite rupture that initiated at a depth of 11 km below the Canterbury Plains, ~ 6 km north of a segmented surface rupture with dextral strike-slip < 5 m that developed during the earthquake and was mapped for nearly 30 km west-east across the plains (Quigley et al. 2010). This rupture, on what is now termed the Greendale fault (Figures 1 and 3), occurred in an extremely low-relief area of the plains without any prior topographic expression of the structure. Measured dextral displacements averaging 2.5 m but ranging up to ~ 5 m were consistent with pure strike-slip on a subvertical fault with an enveloping surface for the left-stepping rupture segments striking ~ 085°. While near-field focal mechanism and geodetic analyses suggest that initial rupturing involved reverse-slip on NE-SW striking planes (Gledhill et al. 2011; GeoNet regional moment tensor solution catalogue http://www.geonet.org.nz/resources/earthquake/), teleseismic analyses from long-period waves (USGS, http://earthquake. usgs.gov; Global CMT Project, http://www.globalcmt.org/ CMTsearch.html) yield moment tensor mechanisms consistent with near-vertical dextral strike-slip on a rupture paralleling the mapped surface trace of the Greendale fault (Figure 4).

Aftershocks, largely restricted to the upper crust at depths less than 12 km, were initially concentrated in an E-W swath around the surface rupture trace of the Greendale fault, though a number of subsidiary clusters and lineaments are also evident (Bannister et al. 2011, page 839 of this issue; Gledhill et al.

2011).

An aftershock cluster dominated by reverse-slip events abuts the Southern Alps foothills west and south of the western curving termination of the Greendale fault surface rupture (Figures 3 and 4). A strong subsidiary belt of activity with a mixture of strike-slip and reverse-slip mechanisms trends NNW from the mainshock epicenter toward the foothills and the Porters Pass system of strike-slip faults. Of particular structural interest is a diffuse aftershock lineament trending 145°–155° that developed south of the surface rupture in the first two weeks of the sequence and extended out to the coast and beyond. A strong concentration of activity including five of the Mw > 5.0 events is associated with the eastern end of the Greendale fault rupture and the area just south of it. From the mixture of strike-slip and normal fault mechanisms localized within this rhomboidal area of distributed activity (Gledhill et al. 2011), it appears to represent a dilational stepover to an en échelon ENE-trending aftershock lineament that extended eastward along the Banks Peninsula volcanic rangefront.

Dilational stepovers (jogs) of this kind are well known for focusing long-continued aftershock activity with delayed slip transfer to the en échelon fault segment (Sibson 1986).

On 22 February (23:51:42 on 21 February 2011 UTC) central and eastern Christchurch were devastated by an Mw 6.2 aftershock located along the dominant ENE lineament following the northern outcrop margin of the Banks Peninsula volcanics. No surface fault break was observed, but Beavan et al. (2011, page 789 of this issue) modeled GPS and SAR data on surface deformation, in terms of dextral-reverse slip of up to

3 m on a buried rupture with a length of 12 km and a width of 7 km oriented ~060°/70° SE and extending to within 1 km of the surface beneath the Heathcote-Avon estuary (cf. Barnhart et al. 2011, page 815 of this issue). Beavan et al. (2011, page 789 of this issue) also suggest subsidiary strike-slip rupturing on a plane oriented ~ 080°/87° S (Figure 3). Aftershock activity was concentrated in the hanging wall of the main rupture, extending a little out to sea north and east of Banks Peninsula (Bannister et al. 2011, page 839 of this issue).

A further Mw 6.0 aftershock occurred on 13 June (02:20:50 on 13 June 2011 UTC) with an epicenter some 5 km further to the ENE but again without surface rupture. The regional CMT solution yielded nodal planes suggesting either dextral-reverse slip on a fault oriented 068°/84° SSE, or sinistral-reverse slip on 161°/67° WSW with the slip vector raking 6° SSE (GeoNet catalog; http://www.geonet.org.nz). Support for this latter rupture orientation comes from an aftershock lineament that extended progressively along a trend of 140°–150°, subparallel to the SE-SSE trending lineament from the Greendale fault rupture (Figures 3 and 4). A “single-fault” interpretation of surface deformation recorded by GPS and DinSAR (J. Beavan, personal communication 2011) suggests left-reverse oblique slip of up to 1.5 m on a rupture plane oriented 155°/55° SW that extends from 1 km to 9 km in depth and along strike for ~ 14 km, cutting across the eastern end of the 22 February rupture.

BASEMENT FAULT FABRIC Known fault traces extending into the basement (including Late Quaternary–Holocene active segments) identified by surface mapping in the Canterbury region and from seismic profiling, both onshore and offshore (Forsyth et al. 2008; Barnes 1994) (Figure 1) are clustered in two dominant groups based on their displacement characteristics (Figure 5).

1. Faults possessing dominant right-lateral components in the rangefront and foothills of the Southern Alps are subvertical, and oriented across the azimuthal range 050°– 100° (Figure 5). ENE faults cluster along the active Porters Pass system, and E-W faults are mainly represented by the new rupture trace along the Greendale fault (Figure 1). Dominant orientations at 070°–100° are also shown by faults with components of normal separation, together with normal fault escarpments inferred from gradients in Bouguer gravity with surface traces indicating high-angle dips (70°–80°) to the north and south.

2. Faults dominated by reverse-slip include two groups of opposite-facing structures: 000°–050° trending faults dipping west, and 035°–090° trending faults dipping southeast (Figures 1 and 5). North-trending reverse faults dipping at moderate angles (40°–60°) to the west are well developed along the S. Canterbury rangefront (Figure 1).

ENE-trending reverse faults dipping south at high angles (60°–70°) occur only in the north of the Canterbury region (Figure 1). Both groups include Quaternary-active segments.

828 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A) Range Front and Foothills

5%

10 %

% 14

Reverse Faults

N

E

Right-lateral Faults

W 5%

23%

10% % 15 % 20

Bouguer gravity anomalies (Hicks 1989; Bennett et al. 2000) together with information from exploration wells and seismic lines. These data have been used to contour the top of basement below the cover sequence (Figure 1). Two major depocenters (Pegasus-Rangiora basin to the north and Rakaia-Hinds basin to the south) are identified elongated in an easterly orientation and separated by an intervening structural high coincident with Banks Peninsula, where uplifted basement graywackes are exposed beneath the Miocene volcanics. Exploration wells have penetrated late syn-rift sequences infilling these basins.

Discontinuous E-W fault traces mapped in the Quaternary gravels of the Canterbury Plains along the Ashley, Rakaia, and Hinds fault systems (Figure 1) are thus interpreted as surface traces of buried basement faults belonging to the structural domain of the Chatham Rise.

STRUCTURAL ANALYSIS

Right-lateral and Normal Faults

24%

10 %

20 %

40 %

30 %

42%

Reverse Faults

Observed slip senses on the three major ruptures within the earthquake sequence are consistent with the inferred pattern of σ1 stress trajectories (Figure 3). However, some stress heteroge(B) Canterbury Plains, Banks Peninsula and neity is evident, especially near rupture terminations and fault Adjacent Offshore intersections (Figure 4). In Anderson’s (1905, 1951) application of the Coulomb criterion for brittle shear failure to the iniN tiation of faults within intact isotropic crust, strike-slip faults forming in a wrench stress regime (σv = σ2) should be vertical and lie at ± ~30° to the σ1 orientation. In contrast, large-displacement strike-slip faults commonly lie at far higher angles (often >45°) to regional σ1 trajectories and are distinctly “nonAndersonian” (Mount and Suppe 1987; Balfour et al. 2005).

It follows that vertical, low-displacement strike-slip faults at Andersonian orientations are possibly newly formed structures W E in the contemporary stress field, but they could also be inher5% ited faults that happen to be optimally oriented for frictional % reactivation. Following the same argument, oblique-slip rup0 1 tures most likely result from the reactivation of inherited faults 15% in the prevailing stress field. % 0 The principal ruptures of the Canterbury earthquake 2 sequence can be viewed with these considerations in mind (Figure 3). First, the subvertical Greendale fault rupture lying at 25°–35° to regional σ1 is at optimal Andersonian orientation, S implying that it is either a comparatively newly formed strike▲▲ Figure 5. Rose diagrams of fault strike azimuths within the slip fault or an inherited structure that is optimally oriented Canterbury region covered by Figure 1, weighted for mapped for reactivation in the present stress field. It should be borne in length: A) Area of the Southern Alps rangefront and foothills mind that most of the inherited dip-slip faults within the basewhere Torlesse basement is exposed; B) Area of the Canterbury ment are likely to have dips that are substantially less than verPlains, Banks Peninsula, and offshore where basement is largely tical, though the Greendale rupture could occupy an amalgam concealed. Top half of each plot is for faults where reverse-slip of opposite-dipping structures. Note further, however, that at is dominant; bottom half is for faults with predominantly right-Figure 5 termination, the Greendale rupture trace curves its western lateral and/or normal slip. into parallelism with the σ1 stress trajectories (Figure 3), a propagation characteristic of low-displacement shear fractures and Offshore, the northwestern edge of the Chatham Rise preconsistent with the local existence of CMT mechanisms with serves a strong extensional fabric defined by closely spaced components of normal slip. Moreover, total strike-slip displaceE-W striking, S-dipping normal faults that bound half-grabens ment on the Greendale fault appears not to be large because infilled with up to 2 km of inferred Late Cretaceous syn-rift it has not been recognized to continue along strike into the sediments (Barnes 1994). Projection of these structures westbedrock geology of the Southern Alps foothills. In fact, at its ward below the Canterbury Plains is conjectural but is based on

S

Seismological Research Letters Volume 82, Number 6 November/December 2011 829

16

disallowable from frictional lock-up

14 12 10 8 6

dip > 75° n = 61

SINISTRAL

allowable dextral GREENDALE FAULT

18

DEXTRAL

allowable sinistral

45°

favorably oriented

45°

disallowable from frictional lock-up

σ1 favorably oriented

4 2 0

030° 040° 050° 060° 070° 080° 090° 100° 110° 120° 130° 140° 150° 160° 170° 180° 190° 200°

STRIKE AZIMUTH

▲▲ Figure 6. Azimuthal distribution of nodal plane strikes for close-to-pure strike-slip CMT focal mechanisms (both planes dipping >75°) from the Canterbury earthquake sequence (GeoNet catalog http://www.geonet.org.nz), shown in relation to the inferred σ1 direction.

western termination the fault appears to transform into local areas of normal faulting to the north and reverse faulting to the south (Figure 4).

While the dominant rupture in the 22 February aftershocks clearly involves dextral-reverse oblique slip, the subordinate subvertical plane (080°/87° S) lying subparallel to the Greendale fault (Beavan et al. 2011, page 789 of this issue) is at close to the ideal Andersonian orientation for strike-slip.

This part of the sequence may therefore represent competition between inherited and newly formed fault segments. The two diffuse aftershock lineaments trending 140°–155° (Figure 3) are appropriately oriented for left-lateral strike-slip on vertical faults conjugate to the right-lateral Greendale fault with which they form a dihedral angle of ~ 50°–70°. Combining the CMT focal mechanism (161°/67° WSW) with the fault model for the 13 June Mw 6.0 aftershock (153°/55° SW) suggests predominantly left-lateral strike-slip on a moderately-to-steeply dipping plane with the slip vector raking only 6°, not too dissimilar to the ideal Andersonian relationship. However, the suggestion of a nonvertical rupture with a degree of oblique slip makes it likely that rupturing involved the reactivation of an inherited basement structure. These arguments are explored further by examining the distribution of strike azimuths, with respect to the inferred σ1 direction, of aftershock nodal planes for closeto-pure strike-slip CMT focal mechanisms (GeoNet catalog, http://www.geonet.org.nz) where both nodal planes dip >75°

(Figure 6). Because of the ambiguity as to which nodal plane represents the rupture plane, the distribution repeats at 90° intervals, separating potential dextral from potential sinistral strike-slip faults. Theoretical and field studies suggest that faults containing the σ2 direction undergo frictional lock-up at 55°–60° to σ1 (Collettini and Sibson 2001), reducing the allowable range of strike-slip fault orientations. Potential strike-slip orientations are thus reduced to three categories: dark-shaded columns are inadmissible because of frictional lock-up; lightshaded columns are positively discriminated as either dextral or sinistral strike-slip ruptures; and moderate-shaded columns could represent either dextral or sinistral strike-slip. Several features of the distribution are notable. First, despite its length and continuity, the Greendale fault trend is not dominant in strike-slip aftershock orientations. Moreover, a significant proportion of the positively discriminated mechanisms involve sinistral strike-slip on faults that commonly strike 135°–145°, conjugate to the dextral Greendale fault. However, by far the dominant azimuthal trend is 070° and/or 160°. Note first that these trends lie at ±45° to inferred σ1 defining the orientations of vertical planes with maximum shear stress, the expected orientation for ductile shear zones developing in the basement below the brittle seismogenic crust (Figure 3). However, the 070° trend also lies subparallel to the Hope fault and the present interplate slip vector, suggesting the possibility of some kinematic control.

830 Seismological Research Letters Volume 82, Number 6 November/December 2011

DISCUSSION The 2010–2011 Canterbury earthquake sequence developed within a segmented fault system under an Andersonian wrench stress regime (σ1: 0°/115° ± 5°; σ2: vertical; σ3: 0°/025° ± 5°) (Figure 3). Rupturing predominantly involved dextral strikeslip on subvertical E-W faults with varying degrees of reverseslip on differently oriented (mostly ENE-WSW) fault segments. Local normal and reverse slip faulting also occurred at stress heterogeneities at strike-slip rupture tips. Some ruptures clearly involve reactivation of inherited basement faults but other comparatively low-displacement structures may be newly formed within the contemporary stress field.

Subordinate SE-SSE trending aftershock lineaments appear to represent a set of predominantly left-lateral strike-slip faults conjugate to the main dextral structures (Figure 3). The intersection angle of 50°–70° between the conjugate fault sets (±25°–35° to inferred σ1) is consistent with Andersonian frictional fault mechanics. Note that this Andersonian conjugate relationship differs from the orthogonal relationship recognized for conjugate strike-slip faults in central Honshu, Japan, and southern California, which possibly reflects control of the active brittle structures by orthogonal ductile shear zones in the basement (Thatcher and Hill 1991).

Analysis of the strike-slip focal mechanisms from the Canterbury sequence (Figure 6) suggests that the subordinate set of steep sinistral strike-slip faults may be quite widespread.

This has significance for rupture segmentation because sinistral displacements along the conjugate faults will create contractional jogs that impede slip along the main E-W dextral faults.

In this regard, the 2010–2011 Canterbury sequence has similarities to the 2000 Western Tottori earthquake sequence in southwestern Honshu. The Western Tottori Mw 6.7 mainshock involved sinistral rupturing along a previously unrecognized NNW-SSE strike-slip fault, but high-resolution aftershock mapping showed the mainshock lineament to be offset in a series of contractional jogs by Andersonian conjugate dextral faults (Fukuyama et al. 2003). As in the Canterbury sequence, such contractional jogs may act as high-strength asperities because ruptures bypassing them likely have to break through comparatively intact rock. It is notable that particularly high apparent stresses and ground accelerations are associated with the intersection zone at the eastern end of the main E-W aftershock distribution in the Canterbury sequence where the 22 February Mw 6.2 rupture is apparently cross-cut by the 13 June Mw 6.0 rupture (Fry and Gerstenberger 2011, page 833 of this issue).

Overall, the fault system responsible for the Canterbury earthquake sequence appears to be controlled by the orientation of the tectonic stress field in the upper crust rather than conforming with local plate boundary kinematics. On this basis the earthquakes can be regarded as intraplate events remote from the main Alpine-Marlborough fault system defining the onshore plate boundary. Continuance of conjugate faulting has the important implication that displacement weakening leading to preferential failure has not yet reached

the stage where one of the fault sets has become totally dominant and superseded the other. Amalgamation of inherited and newly formed fault components of low total displacement, together with segmentation from the cross-cutting of the major E-W dextral structures by conjugate left-lateral faults, has led to a rough, immature fault system capable of generating high stress-drop ruptures.

ACKNOWLEDGMENTS The writers extend their thanks and appreciation to John Beavan, Stephen Bannister, and Martin Reyners for much helpful discussion and advice.

REFERENCES Anderson, E. M. (1905). The dynamics of faulting. Transactions of the Edinburgh Geological Society 8, 387–402.

Anderson, E. M. (1951). The Dynamics of Faulting and Dyke Formation with Application to Britain. 2nd ed. Edinburgh: Oliver & Boyd, 206 pp.

Balfour, N. J., M. K. Savage, and J. Townend (2005). Stress and crustal anisotropy in Marlborough, New Zealand: Evidence for low fault strength and structure-controlled anisotropy. Geophysical Journal International 163, 1,073–1,086.

Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).

Fine-scale relocation of aftershocks of the 22 February Mw 6.2 Christchurch earthquake using double-difference tomography.

Seismological Research Letters 82, 839–845.

Barnes, P. M. (1994). Continental extension of the Pacific plate at the southern termination of the Hikurangi subduction zone: The North Mernoo fault zone, offshore New Zealand. Tectonics 13, 753–754.

Barnhart, W., M. J. Willis, R. B. Lohman, and A. Melkonian (2011).

InSAR and optical constraints on fault slip during the 2010–2011 New Zealand earthquake sequence. Seismological Research Letters 82, 815–823.

Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters 82, 789–799.

Beavan, J., and J. Haines (2001). Contemporary horizontal velocity and strain-rate fields of the Pacific-Australia plate boundary zone through New Zealand. Journal of Geophysical Research 106, 741– 770.

Bennett, D., R. Brand, D. Francis, S. Langdale, C. Mills, B. Morris, and X. Tian (2000). Preliminary results of exploration in the onshore Canterbury Basin, New Zealand. New Zealand Petroleum Conference Proceedings, 19–22 March 2000. Wellington, NZ: Crown Minerals, 12 pp. http://www.nzpam.govt.nz/cms/petroleum/conferences/conference-proceedings-2000.

Collettini, C., and R. H. Sibson (2001). Normal faults, normal friction?

Geology 29, 927–930.

Cowie, P., and C. H. Scholz (1992). Growth of faults by accumulation of seismic slip. Journal of Geophysical Research 97, 11,085–11,095.

Cox, S. C., and D. J. A. Barrell (2007). Geology of the Aoraki Area.

Institute of Geological and Nuclear Sciences 1:250,000 geological map 15, 1 sheet and 71 pp. Lower Hutt, New Zealand: GNS Science.

Eberhart-Phillips, D., and S. Bannister (2002). Three-dimensional crustal structure in the Southern Alps region of New Zealand from inversion of local earthquake and active source data. Journal of Geophysical Research 107; doi:10.1029/2001JB000567.

Seismological Research Letters Volume 82, Number 6 November/December 2011 831

Field, B. D., G. H. Browne, B. Davy, R. H. Herzer, R. H. Hoskins, J. L. Raine, G. J. Wilson, R. J. Sewell, D. Smale, and W. A. Watters (1989). Cretaceous and Cenozoic sedimentary basins and geological evolution of the Canterbury region, South Island, New Zealand.

New Zealand Geological Survey Basin Studies 2. Wellington, New Zealand: Department of Scientific and Industrial Research, 94 pp. + enclosures.

Forsyth, P. J. (2001). Geology of the Waitaki Area. Institute of Geological and Nuclear Sciences 1:250,000 geological map 19, 1 sheet and 64 pp. Lower Hutt, New Zealand: GNS Science.

Forsyth, P. J., D. J. A. Barrell, and R. Jongens (2008). Geology of the Christchurch Area. Institute of Geological and Nuclear Sciences 1:250,000 geological map 16, 1 sheet and 67 pp. Lower Hutt, New Zealand: GNS Science.

Fry, B., and M. C. Gerstenberger (2011). Large apparent stresses from the Canterbury earthquakes of 2010 and 2011. Seismological Research Letters 82, 833–838.

Fukuyama, E., W. L. Ellsworth, F. Waldhauser, and A. Kubo (2003).

Detailed fault structure of the 2000 Western Tottori, Japan, earthquake sequence. Bulletin of the Seismological Society of America 93, 1,468–1,478.

Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011). The Darfield (Canterbury, New Zealand) Mw 7.1 earthquake of 4 September 2010: A preliminary seismological report. Seismological Research Letters 82, 378–386.

Hicks, S. R. (1989). Structure of the Canterbury Plains, New Zealand, from gravity modelling. Research Report 222, Geophysics Division, Department of Scientific and Industrial Research, Wellington, New Zealand.

Keiding, M., B. Lund, and T. Árnadóttir (2009). Earthquakes, stress, and strain along an obliquely divergent plate boundary: Reykjanes Peninsula, southwest Iceland. Journal of Geophysical Research 114, B09306; doi:10.1029/2008JB006253.

Laird, M. G., and J. D. Bradshaw (2004). The break-up of a long-term relationship: The Cretaceous separation of New Zealand from Gondwana. Gondwana Research 7, 273–286.

Leitner, B., D. Eberhart-Phillips, H. Anderson, and J. Nabelek (2001).

A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms and stress observations. Journal of Geophysical Research 106, 2,193–2,220.

Mogg, W. G., K. Aurisch, R. O’Leary, and G. P. Pass (2008). The Carrack-Caravel prospect complex: A possible sleeping giant in the deep Canterbury Basin, New Zealand. Proceedings of the Petroleum Exploration Society of Australia Eastern Australasian Basins Symposium III, Sydney, Australia, 14–17 September 2008, 369–378.

Mount, V. S., and J. Suppe (1987). State of stress near the San Andreas fault. Geology 15, 1,143–1,146.

Pearson, C. (1994). Geodetic strain determinations from the Okarito and Godley-Tekapo regions, central South Island, New Zealand.

New Zealand Journal of Geology and Geophysics 37, 309–318.

Pearson, C., J. Beavan, D. Darby, G. H. Blick, and R. I. Walcott (1995).

Strain distribution across the Australian-Pacific plate boundary in the central South Island, New Zealand, from 1992 GPS and earlier terrestrial observations. Journal of Geophysical Research 100, 22,071–22,081.

Quigley, M., P. Villamor, K. Furlong, J. Beavan, R. Van Dissen, N. Litchfield, T. Stahl, B. Duffy, E. Bilderback, D. Noble, D. Barrell, R. Jongens, and S. Cox (2010). Previously unknown fault shakes New Zealand’s South Island. Eos, Transactions, American Geophysical Union 91, 469–472.

Rattenbury, M. S., D. B. Townsend, and M. R. Johnston (2006). Geology of the Kaikoura Area. Institute of Geological and Nuclear Sciences 1:250,000 geological map 13, 1 sheet and 70 pp. Lower Hutt, New Zealand: GNS Science.

Robinson, R., and P. J. McGinty (2000). The enigma of the Arthur’s Pass, New Zealand, earthquake. 2. The aftershock distribution and its relation to regional and induced stress fields. Journal of Geophysical Research 105, 16,139–16,150.

Sibson, R. H. (1985). A note on fault reactivation. Journal of Structural Geology 7, 751–754.

Sibson, R. H. (1986). Rupture interaction with fault jogs. In Earthquake Source Mechanics, ed. S. Das, J. Boatwright, and C. H. Scholz, 157– 167. American Geophysical Union Monograph 37 (Maurice Ewing Series 6). Washington, DC: American Geophysical Union.

Sibson, R. H., F. C. Ghisetti, and R. A. Crookbain (forthcoming).

“Andersonian” wrench faulting in a regional stress field during the 2010–2011 Canterbury, New Zealand, earthquake sequence.

In Stress Controls on Faulting, Fracturing and Igneous Intrusion in the Earth’s Crust—Commemorating the Work of Ernest Masson Anderson, ed. D. Healy et al. Geological Society of London special publication.

Thatcher, W., and D. P. Hill (1991). Fault orientations in extensional and conjugate strike-slip environments and their implications. Geology 19, 1,116–1,120.

Wallace, L. M., J. Beavan, R. McCaffrey, K. Berryman, and P. Denys (2007). Balancing the plate motion budget in the South Island, New Zealand, using GPS, geological and seismological data.

Geophysical Journal International 168, 332–352.

Wesnousky, S. G. (1988). Seismological and structural evolution of strike-slip faults. Nature 335, 340–343.

832 Seismological Research Letters Volume 82, Number 6 November/December 2011

Department of Geology University of Otago P.O. Box 56 Dunedin 9054, New Zealand rick.sibson@otago.ac.nz

(R. S.)

Large Apparent Stresses from the Canterbury Earthquakes of 2010 and 2011 B. Fry and M. C. Gerstenberger

B. Fry and M. C. Gerstenberger GNS Science

INTRODUCTION An earthquake of Mw 6.1–6.31 (Beavan et al. 2011, page 789 of this issue) that struck Christchurch, New Zealand, on 22 February (21 February, UTC) produced recorded ground motion acceleration over 2 g. The event caused widespread damage with dense recordings of non-linear site behavior. Globally, dense near-field recordings of shallow intraplate earthquakes are rare. It is possible that extreme ground motions are common with this type of earthquake and that their rarity is merely a function of inadequate seismic sampling in the near field of such low-probability, high-potency events. To better define the nature of these events, we calculate apparent stress (τa) of the three largest earthquakes in the Canterbury sequence and compare them to global and regional data. We then place recorded PGA and spectral accelerations into the context of regional and global ground motion prediction equations and discuss the implications of high-stress events for future seismic hazard estimates for the region. For the February event, we also briefly explore the implications of directivity on measured ground motions in central Christchurch.

The earthquakes that occurred in the Canterbury region of the South Island, New Zealand, from September 2010 to the present have disproportionately large energy magnitudes (Me) to their moment magnitudes (Mw). They have produced the largest ground motions ever measured in New Zealand.

The sequence began with the Mw 7.1 earthquake that occurred about 40 km west of the city of Christchurch on 4 September 2010. The maximum recorded ground acceleration recorded during the event was over 1.25 g, which was experienced near the intersection of the triggering thrust on which the rupture began and the strike-slip Greendale fault that carried most of the moment in the earthquake (Gledhill et al. 2010). Peak ground accelerations (PGA) in the central business district of Christchurch averaged between about 0.2 and 0.3 g. These motions were sufficient to generate liquefaction in areas of the city. The highest recorded acceleration in the greater metropolitan area was 0.61 g in a suburb on the southern edge of the city that has since proved to be prone to strong site amplification.

On 22 February 2011, an Mw 6.3 thrust earthquake occurred 1. Mw estimates for this earthquake have ranged from 6.1 (USGS) to 6.2 (Beavan et al. 2011, page 789 of this issue). To be conservative in our comparison to observed ground motions, we have used Mw 6.3 in all calculations. doi: 10.1785/gssrl.82.6.833

on a structure below the southern suburbs of the city at about 7 km epicentral distance from the center of Christchurch. This earthquake produced extreme motions in Christchurch (Fry et al. 2011, page 846 of this issue). Maximum PGA, considering both horizontal and vertical components, was over 2.2 g with two other recordings in the city greater than 1 g and average PGA in the central business district between about 0.6 and 0.8 g. This intense shaking damaged many buildings in the central business district of the city (~5–8 km epicentral distance) and triggered widespread liquefaction (Kaiser et al.

2011). On 13 June 2011, the city was again subject to intense shaking from a nearby, shallow Mw 6.0 earthquake (Beavan et al. 2011, page 789 of this issue). Measured accelerations from that event were also extreme, with measured PGA over 2 g in a southeastern suburb of the city. Taken together, this sequence has produced widespread destruction and more than 180 fatalities in Christchurch.

HIGH APPARENT STRESS (τa) The faults that failed in the September 2010 Mw 7.1, the February 2011 Mw 6.3, and the June 2011 Mw 6.0 earthquakes were likely very strong, with high amounts of friction.

Typically, faults in slowly deforming areas with long earthquake recurrence intervals exhibit this attribute, as increasing deformation typically decreases fault strength by reducing heterogeneities on the fault surface (e.g., Ben-Zion and Sammis 2003). Subsequently, the radiated energies (Es) for the three events were high for their given moments. Radiated energy can be determined from high-frequency velocity records (Boatwright and Choy 1986) and can be used to directly calculate Me (Choy and Boatwright 1995). Compared to the seismic moment, which is derived from displacement records, energy magnitudes are more indicative of the shaking potential of an earthquake. Es estimates from analysis of broadband P waves provide Me of Me = 7.99, Me = 6.75, and Me = 6.7 for the three events (George Choy, personal communication).

Apparent stress is defined as the product of rigidity (μ) and Es per unit moment (τa = (μ × Es)/Mo) (Wyss and Brune 1968), or the amount of stress per unit moment. There is considerable debate regarding the scaling of τa with earthquake moment.

Aki (1957) asserts that earthquakes are self-similar, implying that τa is not dependent on seismic moment. This assertion is supported by numerous other studies (e.g., Boatwright and

Seismological Research Letters Volume 82, Number 6 November/December 2011 833

TABLE 1 Magnitudes and Stress Calculations for the Three Largest Canterbury Events Date 9/03/2010 2/22/2011 6/13/2011

Mw Mw (regional) (teleseismic) 7.10 6.30 6.00

6.97 6.12 6.00

Me

τa (Mpa)

7.30 7.99 6.30 6.75 6.00 6.70

15.85 4.10 6.26

Ms

Choy 1992; Choy and Boatwright 1995; Singh and Ordaz 1994; Baltay et al. 2011). Many others have proposed a moment dependence on τa (e.g., Abercrombie 1995; Kanamori et al.

1993). τa has important implications for earthquake hazard studies, as increasing τa leads to stronger ground motions. Using teleseismically determined Es and Mo (George Choy, personal communication), we solve for τa of the three largest Canterbury events (Table 1). We assume constant bulk regional properties for density and shear-wave velocity to determine rigidity. These quantities are informed by the region’s 3D shear-wave velocity model (Eberhart-Phillips et al. 2010). Values of rigidity might be underestimated for the February and June events, as it is possible that these events occurred on surfaces that are cut by intrusions from the 6 Ma volcanic activity that resulted in the volcanic edifice in the vicinity of the earthquakes. In this scenario, the faults would be locally strengthened by the intrusions and the energy release would be dominated by a subregion of the fault, greatly increasing the localized τa .

Due to the frequency dependent nature of both scattering and attenuation, it is difficult to measure Es over a broad frequency range (Ide et al. 2003). In this study, we utilize esti-

mates of Mo that are derived from teleseismic data to maintain consistency with the teleseismically determined Es. Greater attenuation of high-frequency energy also dictates that teleseismic estimates of Es are minimum values of actual radiated energy in highly attenuating regions. Ideally, energy and moment estimates from regional data would be used to estimate τa . However, such techniques require refined knowledge of local attenuation structure and site responses that were previously only coarsely resolved. Ongoing studies are refining these properties (Kaiser et al. 2011) and should allow for regional estimates in the near future.

We estimate the τa of the September event to be the highest of the three earthquakes, at ~16 MPa. τa of the February and June events are ~5 and ~6 MPa respectively. Intraplate earthquakes typically have larger τa than interplate events. This is also true of the South Island, New Zealand, where recent large events along the Puysegur subduction zone (Fry et al. 2010) have τa ~ 0.2MPa. We also compare the three Canterbury events to data from four sequences of Hokkaido, Japan, events compiled by Baltay et al. 2011 (Figure 1) and the 2007 M 6.8 event that occurred on the Hikurangi subduction zone, North Island, New Zealand. The Iwate Miyagi event is a particularly appropriate analogue as the character of the recorded near-field waveforms is similar to that of the 22 February Canterbury event (Fry et al. 2011, page 846 of this issue).

The stresses calculated from the Canterbury events are remarkably high compared to global averages. The Canterbury earthquakes are also on average higher than τa values obtained from reported stress drops (most between 10 and 15 MPa) of intraplate events in eastern North America (Atkinson and Boore 2006). For shallow subduction events, Choy et al. (2001)

▲▲ Figure 1. Apparent stress plotted as a function of Mw for data from Japan (Baltay et al. 2011) and New Zealand. Ch 04 is the 2004 Chuetsu earthquake and aftershock sequence; CO 07 is the 2007 Chuetsu-Oki earthquake and aftershock sequence; Kam is the repeating earthquake sequence off-shore Kamaishi, Iwate; IM 08 is the 2008 Iwate-Miyagi earthquake and aftershock sequence; GB 07 is the 2007 M 6.8 Gisborne, New Zealand, earthquake; Puy are recent earthquakes on the Puysegur subduction zone, South Island, New Zealand; Can are the largest events of the 2010–2011 Canterbury sequence.

834 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 2. Observed vs. predicted ground motions from McVerry at al. (2006) for the 22 February 2011, Mw 6.3 earthquake. In all panels, the prediction is shown with stress drop scaling (green solid line) and without (gray solid line). For both, the 16th and 84th percentile motions are also shown in the same color. (A) PGA—both raw (black circles) and processed (open triangles) PGA are shown; (B) 0.5-s spectral accelerations; (C) 1.0-s spectral accelerations; and (D) 2.0-s spectral accelerations.

find average apparent stresses to be around 0.2–0.3 MPa. Shallow events worldwide have a spread from about 6 MPa to 0.04 MPa, with an average of about 0.5 MPa. While the data for shallow, strike-slip events is limited, strike-slip mechanisms near subduction zones show much higher τa; these events have an average of around 3 MPa (Choy et al. 2001). Typically, high τa is indicative of spatially small yet strong asperities yielding short rise times and strong, high-frequency waves in the near field. Pervasive Cretaceous-age high-angle EW-trending fault zones that are hundreds of kilometers long are present offshore of Christchurch (Wood and Herzer 1993). It is likely that these faults continue in the crust beneath the Canterbury Plains and are capable of being reactivated as strike-slip faults.

In this case, the fault strength would likely come from healing of the fault over time. Another possible fault origin for the February event is reactivation of faults generated during the 12 Ma–6 Ma emplacement of the Banks Peninsula volcanic rocks to the south of the city (see Figure 1 of Bannister et al.

2011, page 839 of this issue). The relative immaturity of faults in this hypothesis would contribute to the observed high τa . It is also possible that the faults that were active in the February and June events were cross-cut by volcanic intrusions. Even if the faults were originally weak, cross-cutting dikes would yield

high stresses when broken. Precise locations of aftershocks fall along the northern and western flanks of the extinct volcano (Bannister et al. 2011, page 839 of this issue) and subsequently do not discriminate these competing hypotheses for the origin of fault strength.

OBSERVED GROUND MOTIONS If the τa of the Canterbury earthquakes is larger than the average τa of earthquakes used in generating the New Zealand ground motion prediction equation (GMPE) (McVerry et al.

2006), measured ground motions should be larger than predicted ones.

Figure 2 shows plots of observed, maximum vector ground motions from the 22 February 2011 earthquake compared to predictions for Class D, deep or soft soil (Standards New Zealand, 2004) and for an oblique mechanism; this is the standard GMPE used in the New Zealand National Seismic Hazard Model (Stirling et al. 2002). Distances are calculated using the closest distance to the current estimated fault plane (Beavan et al. 2011, page 789 of this issue). For comparison, we have applied a stress drop scaling term as proposed by Atkinson and Boore (2006). Stress drop is typically estimated

Seismological Research Letters Volume 82, Number 6 November/December 2011 835

September 4, 2010, Mw=7.1

SA(g)

10

10

A)

1

1

0.1

0.1

SA(1s)

PGA

0.01

B)

0.01 1

10

100

10

100

June 22, 2011, Mw=6.0 10

10

D)

C)

1

0.1

0.1

0.01

0.01

SA( g)

1

SA(1s)

PGA 0.001

0.001 1

10

100

Closest Distance From Fault (km)

1

Closest Distance From Fault (km)

▲▲ Figure 3. Observed vs. predicted ground motions from McVerry at al. (2006) for the September 2010 Mw 7.1 earthquake (Panels A and B) and the June 2011 Mw 6.0 earthquake (Panels C and D). In all panels, the prediction is shown with stress drop scaling (green solid line) and without (gray solid line). For both, the 16th and 84th percentile motions are also shown in the same color. (A) September PGA—both raw (black circles) and processed (open triangles) PGA are shown; (B) September 1.0-s spectral acceleration; (C) June PGA; (D) June 1-s spectral acceleration.

as ~4 × τa based on empirical calculations. The stress drop scaling is both frequency and magnitude dependent and relies on an estimate of the ratio of the reference stress drop implicit in the GMPE to the stress drop of the target earthquake. For the McVerry et al (2006) relationship, which is primarily based on New Zealand earthquakes, and the Canterbury events, we have calculated a scaler using a ratio of 1.5 (e.g., 15 MPa/10 MPa).

Additionally, in Figure 2A, for PGA, we have shown both the raw and the processed PGAs using the standard processing as done by GeoNet. The two most notable features in the plots are: (1) the sharp shift in the decrease in ground motions after about 10 km resulting in an overprediction of the ground motions, and (2) the clustered increase in ground motions for the near-field data. This effect is likely driven by a biased sampling of data in the near field toward locations subject to strong directivity, with that bias diminishing when including more

distant stations and sampling over a much wider area outside the directivity region.

Figure 3 shows the PGA and 1-s spectral acceleration plots for the September and June earthquakes. Raw PGA values are plotted only for the September event. This event consisted of strike-slip displacement on the Greendale fault with smaller moments carried by surrounding thrust faults; for the distance calculations, only the dominant Greendale fault was used (Beavan et al. 2010). The error introduced by this should be insignificant based on the spatial distribution of stations available. Fault rupture models of the June rupture are still preliminary and will likely introduce large (a few kilometers) errors into the distance calculations; we have therefore used the epicenter of the relocated mainshock (Bannister et al. 2011, page 839 of this issue) projected to 1 km depth, the estimated top of the rupture. The distances are likely to be overestimated. For

836 Seismological Research Letters Volume 82, Number 6 November/December 2011

10

Obs SA(1s) AS AS+SD

1

AB 2006

SA(g)

AB+SD

0.1

0.01

0.001

1

10

Closest Distance To Fault (km)

100

▲▲ Figure 4. Observed vs. predicted ground motions from Abrahamson and Silva (2008) and Atkinson and Boore (2006) for the 22 February Mw 6.3 earthquake. Both GMPEs are shown with and without stress drop scaling.

both earthquakes, similar trends are seen as to the February event with a drop off and overprediction of accelerations at distances greater than 10 km.

For comparison, using OpenSHA (Field et al. 2003), we have plotted the median 1-s spectral acceleration curves for the February event using Atkinson and Boore (2006) and Abrahamson and Silva (2008) GMPEs in Figure 4. Both curves are plotted using a reverse mechanism and Vs 30 = 230 m/s. For Abrahamson and Silva, we have used parameters based on data from two average sites in Christchurch. Both have been plotted with and without the same stress drop scaling that was applied to McVerry et al. (2006). The behavior of the models is similar to the McVerry et al. (2006) model with underpredictions in the near field and overpredictions beyond 10 km.

DISCUSSION The implications for probabilistic seismic hazard assessment (PSHA) for the region are not yet clear, and for any interpretation of the data it must be acknowledged that this remains only a sample of three earthquakes and one must be wary of over-interpretation; this is of particular concern when quick scientific response is necessary for advising the emergency management process. Due to the nature of the ongoing aftershock activity in the Canterbury region, the hazard is likely to be

dominated by earthquakes less than M 6.5 with a preliminary PSHA (Gerstenberger et al. 2011) indicating that the 10% in 50-year ground motions are dominated by smaller earthquakes at distances of 10 km or less. This indicates that more work is necessary to understand relative contributions of both stress drop and directivity to the near-field motions. Figures 2 and 3 indicate that if directivity effects are specifically added to the current stress drop scaling, possibly through increased variability in ground motion, we may overestimate the hazard for the region.

For future improvements to New Zealand hazard estimates it is likely that having a catalog of Me and therefore τa will help us to make regional refinements in ground motion estimates. This can be done by incorporating Es estimates into routine processing and developing a refined 3D attenuation model and database of static station-dependent site effects (e.g., Boatwright et al. 2002). By calculating both regional moment tensors and radiated energy from regional data, better estimates of strong high-frequency ground motions can be made.

ACKNOWLEDGMENTS This manuscript greatly benefited from the review of Gail Atkinson. We would like to thank George Choy for providing the estimates of radiated energy.

Seismological Research Letters Volume 82, Number 6 November/December 2011 837

REFERENCES Abercrombie, R. E. (1995). Earthquake source scaling relationships from ~1 to 5 M L using seismograms recorded at 2.5 km depth. Journal of Geophysical Research 100, 24,015–24,036.

Abrahamson, N. A., and W. J. Silva (2008). Summary of the Abrahamson & Silva NGA ground-motion relations. Earthquake Spectra 24 (1), 67–97.

Aki, K. (1967). Scaling law of seismic spectrum. Journal of Geophysical Research 72, 1,217–1,231.

Atkinson, G. M., and D. M. Boore (2006). Earthquake ground-motion prediction equations for eastern North America. Bulletin of the Seismological Society of America 96, 2,181–2,205.

Baltay, A., S. Ide, G. Prieto, and G. Beroza (2011). Variability in earthquake stress drop and apparent stress. Geophysical Research Letters 38, L06303; doi:10.1029/2011GL046698.

Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).

Fine-scale relocation of aftershocks of the 22 February Mw 6.2 Christchurch earthquake using double-difference tomography.

Seismological Research Letters 82, 839–845.

Beavan, R. J., S. Samsonov, M. Motagh, L. M. Wallace, S. M. Ellis, and N. Palmer (2010). The Darfield (Canterbury) earthquake: Geodetic observations and preliminary source model. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 228–235.

Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters 82, 789–799.

Ben-Zion, Y., and C. G. Sammis (2003). Characterization of fault zones.

Pure and Applied Geophysics 160, 677–715.

Boatwright, J., and G. Choy (1986). Teleseismic estimates of the energy radiated by shallow earthquakes. Journal of Geophysical Research 91, 2,095–2,112.

Boatwright, J., and G. Choy (1992). Acceleration source spectra anticipated for large earthquakes in northeastern North America.

Bulletin of the Seismological Society of America 82 (2), 660–682.

Boatwright, J., G. Choy, and L. Seekins (2002). Regional estimates of radiated seismic energy. Bulletin of the Seismological Society of America 92, 1,241–1,255.

Choy, G. L., and J. L. Boatwright (1995). Global patterns of radiated seismic energy and apparent stress. Journal of Geophysical Research 100, 18,205–18,228.

Choy, G. L., J. L. Boatwright, and S. Kirby (2001). The Radiated Seismic Energy and Apparent Stress of Interplate and Intraplate Earthquakes at Subduction Zone Environments: Implications for Seismic Hazard Estimation. USGS Open-File Report 01-005, 10 pp.

Eberhart-Phillips, D., M. E. Reyners, S. C. Bannister, M. P. Chadwick, and S. M. Ellis (2010). Establishing a versatile 3-D seismic velocity model for New Zealand. Seismological Research Letters 81 (6), 992–1,000; doi:10.1785/gssrl.82.6.992.

Field, E. H., T. H. Jordan, and C. A. Cornell (2003). OpenSHA: A developing community-modeling environment for seismic hazard analysis. Seismological Research Letters 74 (4), 406–419.

Fry, B., S. Bannister, J. Beavan, L. Bland, B. Bradley, S. Cox, J. Cousins, N. Gale, G. Hancox, C. Holden, R. Jongens, W. Power, G. Prasetya,

M. Reyners, J. Ristau, R. Robinson, S. Samsonov, K. Wilson, and the GeoNet team (2010). The Mw 7.6 Dusky Sound earthquake of 2009: Preliminary report. Bulletin of the New Zealand Society for Earthquake Engineering 43 (1), 24–40.

Fry, B., R. Benites, and A. Kaiser (2011). The character of accelerations in the Mw 6.2 Christchurch earthquake. Seismological Research Letters 82, 846–852.

Gerstenberger, M., M. Cubrinovski, G. McVerry, M. Stirling, D. Rhoades, B. Bradley, R. Langridge, T. Webb, B. Peng, J. Pettinga, K. Berryman, and H. Brackley (2011). Probabilistic Assessment of Liquefaction Potential for Christchurch in the Next 50 Years. GNS Science report 2011/15, 30 pp. Lower Hutt, New Zealand.

Gledhill, K., J. Ristau, M. E. Reyners, B. Fry, and C. Holden (2010).

The Darfield (Canterbury, New Zealand) Mw 7.1 earthquake of September 2010: A preliminary seismological report. Seismological Research Letters 82 (3), 378–386; doi:10.1785/gssrl.82.6.378.

Ide, S., G. C. Beroza, S. G. Prejean, and W. L. Ellsworth (2003).

Apparent break in earthquake scaling due to path and site effects on deep borehole recordings. Journal of Geophysical Research 108 (B5), 2,271; doi:10.1029/2001JB001617.

Kaiser, A., C. Holden, J. Beavan, D. Beetham, R. Benites, A. Celentano, D. Collett et al. (2011). The February 2011 Christchurch earthquake: A preliminary report. Submitted to New Zealand Journal of Geology and Geophysics.

Kanamori, H., J. Mori, E. Hauksson, T. H. Heaton, L. K. Hutton, and L. M. Jones (1993). Determination of earthquake energy release and M L using TERRAscope. Bulletin of the Seismological Society of America 83, 330–346.

McVerry, G. H., J. X. Zhao, N. A. Abrahamson, and P. G. Somerville (2006). Response spectral attenuation relations for crustal and subduction zone earthquakes. Bulletin for the New Zealand Society of Earthquake Engineering 39, 1–58.

Singh, S. K., and M. Ordaz (1994). Seismic energy release in Mexican subduction zone earthquakes. Bulletin of the Seismological Society of America 72, 2,003–2,016.

Standards New Zealand (2004). Structural Design Actions, Part 5: Earthquake Actions—New Zealand. New Zealand Standard NZS 1170.5:2004. Wellington, New Zealand.

Stirling, M. W., G. H. McVerry, and K. R. Berryman (2002). A new seismic hazard model for New Zealand. Bulletin of the Seismological Society of America 92 (5), 1,878–1,903.

Wood, R.A., and R. H. Herzer (1993). The Chatham Rise, New Zealand.

In South Pacific Sedimentary Basins, ed. P. F. Ballance, 329–349.

Vol. 2 of Sedimentary Basins of the World. Amsterdam: Elsevier Science Publishers.

Wyss, M., and J. N. Brune (1968). Seismic moment, stress, and source dimensions for earthquakes in the California-Nevada region.

Journal of Geophysical Research 73 (14), 4,681–4,694.

838 Seismological Research Letters Volume 82, Number 6 November/December 2011

GNS Science 1 Fairway Drive Avalon, Lower Hutt, New Zealand b.fry@gns.cri.nz

(B. F.)

E

Fine-scale Relocation of Aftershocks of the 22 February Mw 6.2 Christchurch Earthquake Using Double-difference Tomography Stephen Bannister, Bill Fry, Martin Reyners, John Ristau, and Haijiang Zhang

Stephen Bannister,1 Bill Fry,1 Martin Reyners,1 John Ristau,1 and Haijiang Zhang2

Online material: Hypocenters of 2,177 earthquakes recorded during 21 February 21–31 March 2011

INTRODUCTION On 22 February 2011 New Zealand time (21 February UTC), the M W 6.2 Christchurch earthquake occurred just 7 km southeast of the center of Christchurch city, New Zealand (Fry et al. 2011, Holden 2011, page 783 of this issue). There were 181 confirmed fatalities, and the damage to Christchurch city is estimated to be NZ$15 billion–$NZ20 billion (US$12 billion–US$16 billion). The event was well-recorded by the broadband and strong-motion national-scale GeoNet network (Petersen et al. 2011) as well as by the Canterbury regional strong-motion network (Avery et al. 2004). Since the 22 February earthquake, more than 2,700 further aftershocks have been recorded up to 1 May 2011, including 21 events with local magnitude (ML) greater than 5. Here we describe the initial relocation analysis for these aftershocks.

The Mw 6.2 Christchurch earthquake is part of the larger aftershock sequence of the Mw 7.1 Darfield earthquake, which occurred at 16:35 3 September UTC, 2010. Seismological, GPS, and InSAR data all suggest that the earthquake rupture process for the Mw 7.1 Darfield earthquake involved failure of multiple fault segments (Beavan et al. 2010; Gledhill et al.

2011). A surface rupture for that earthquake, now termed the Greendale fault, extending ~29.5 km and located ~4 km south of the epicenter, is consistent with strike-slip faulting with an average horizontal surface displacement of ~2.5 m (Quigley et al. 2010). The vast majority of the 7,400+ aftershocks following the Darfield earthquake are shallow, at less than 15 km depth. Figure 1 shows that, although many of the aftershocks occurred near the surface trace of the Greendale fault, intense clusters of aftershock activity have also occurred 1. GNS Science, Lower Hutt, New Zealand 2. Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, U.S.A. doi: 10.1785/gssrl.82.6.839

at the western and eastern ends of the Darfield fault trace, as well as north-northwest from the Darfield epicenter.

The distribution of the aftershocks (Figure 1A) indicates a complex fault system, most of which was previously undetected; prior to the 2010–2011 activity there was negligible recorded seismicity in the region, as illustrated in Figure 1B for the time period 2000–2010. However, some faults have been inferred from geological mapping studies (Howard et al.

2005; Pettinga et al. 2001) as well as from onshore and offshore seismic reflection work (Dorn et al. 2010; Barnes 1995, 1996; Barnes et al. 2011; Wood et al. 1989).

Following the destructive 22 February earthquake new studies have begun to characterize the location and geometry of possible hidden faults beneath the region. This new work includes a combination of offshore marine seismic surveys (Barnes et al. 2011), additional gravity acquisition and interpretation, new aeromagnetic surveys across sections of the Canterbury Plains (Figure 1B), some active-seismic reflection surveys, and relocation analysis of the existing earthquake aftershock data (this study). Here we provide relocation analysis for more than 2,100 aftershocks that have occurred since the Mw 6.2 February earthquake. Separate analysis and processing of the aeromagnetic and reflection seismic survey data is underway.

SEISMIC DATA Seismic waveform data for the 22 February earthquake, and aftershocks, recorded by the New Zealand national seismograph network (Petersen et al. 2011) and the CanNet (Canterbury) strong-motion network (Avery et al. 2004) are publicly available through GeoNet (http://www.geonet.org.nz).

The strong-motion data coverage is excellent, with 15 strongground-motion recorders (Figure 2) within 8 km of the top of the fault plane of the February Mw 6.2 earthquake, 13 of these recording vertical ground accelerations greater than 0.2 g.

Many of the strong-motion recorders usually triggered for aftershocks of M L > 4 (90 events since 21 February to 1 May),

Seismological Research Letters Volume 82, Number 6 November/December 2011 839

(A) 0

10

−43.25°

20 km

−43.5°

M7.1

−43.75°

Mag 2 Mag 3 Mag 4 Mag 5 172°

173°

172.5°

(B)

Christchurch city

−43.25°

−43.5°

Canterbury Plains −43.75°

Banks peninsula 172°

172.5°

173°

▲▲ Figure 1. A) Seismicity in the time period 3 September 2010 through April 30, 2011. The Mw 7.1 Darfield and Mw 6.2 February Christchurch earthquakes are marked as stars. Earthquakes occurring after February 21 are filled in black, earlier events are gray. B) Seismicity in the preceding 10 years, from 1 January 2000 to 1 September 2010, as cataloged by GeoNet, illustrating the low level of seismicity before the 2010 Darfield earthquake.

840 Seismological Research Letters Volume 82, Number 6 November/December 2011

−43.25° OXZ

0

10

20 km

−43.5°

−43.75°

172°

172.5°

173°

▲▲ Figure 2. Seismometer stations on the Canterbury Plains. Open triangles show the sites of temporary stations operating during September–October 2010; filled triangles show short-period and broadband seismometer sites; filled circles show strong-motion seismometer sites.

but also triggered for many of the 100+ M L > 3 aftershocks near the city.

Additional waveform data were also collected by a temporary seismic array that was deployed immediately after the 22 February event. This array (Figure 2), consisting of six shortperiod and three strong-motion seismometer sites, was placed to the south and north of Christchurch to provide greater azimuthal coverage of the aftershock region and to assist with location and seismic tomography analysis.

RELOCATION ANALYSIS We relocate aftershocks of the Christchurch Mw 6.2 earthquake using the waveform and travel-time data currently available. We anticipate that additional waveform data currently being collected (Yoshihisa Iio, personal communication 2011) will assist in further improving the regional velocity model for the Canterbury Plains (Figure 1B), with subsequent iterative improvement also expected for the aftershock locations.

Additional travel-time picking by GeoNet analysts over the next few months will also assist in iterative improvement of the aftershock locations.

Initial event locations and phase picks were obtained from GeoNet, with some additional travel time picking carried out for the temporary stations deployed in late February. Most of

the initial GeoNet event locations, before the relocation analysis described here, have event depths at standard “fixed” depths of 2, 5, or 12 km. Most of the larger-magnitude events that triggered the strong-motion recorders had 20 or more phase picks. Travel-time picking was carried out by GeoNet analysts.

During phase picking it was noted that waveform data for some earthquake-station paths indicate multiple phase arrivals, in particular to GeoNet station OXZ located approximately 40 km to the west of Christchurch. This suggests strong multipath effects from high-velocity basement or sub-basement structure.

In our analysis we specifically downweight S-phase travel-time picks from station OXZ to allow for possible ambiguities of the S-phase arrivals.

We invert data from aftershocks recorded following the Christchurch Mw 6.2 earthquake, using the double-difference tomography approach of Zhang et al. (2009), which builds on earlier work by Zhang and Thurber (2003). The technique minimizes the residuals between observed and calculated arrival-time differences for pairs of closely located earthquakes, while also minimizing the residuals of absolute arrival times.

The algorithm solves for the hypocentral parameters of the earthquakes, also allowing some modification of the P-wave and S-wave velocity structure used for travel-time calculation.

Catalog-based differential times CBDT were calculated between events initially separated by less than 10 km, for all

Seismological Research Letters Volume 82, Number 6 November/December 2011 841

stations less than 150 km from Christchurch, using the manually picked P- and S-arrival times. Cross-correlation and bispectrum (BS) (Du et al. 2004) methods were then used to calculate the waveform-based differential times (WBDT) for all event-station pairs, after pre-filtering, following the technique of Du et al. (2004). These derived differential times were weighted based on the quality of the arrival time measurements. Absolute travel times and the two types of differential times (CBDT and WBDT) were then combined and simultaneously inverted using the approach of Zhang et al. (2009) in an iterative least-squares procedure that utilizes the LSQR minimization method (Paige and Saunders 1982).

In the first stage of the relocation analysis, the best 1,008 events were used primarily to refine the velocity model in the vicinity of Christchurch. These events were selected from those earthquakes recorded in the six weeks following the Darfield Mw 7.1 earthquake, as well as from the three weeks immediately following the February Mw 6.2 earthquake, when new temporary seismometer stations were deployed south and north of Christchurch. Initial event selection was based on the number of phases and initial standard errors. The dataset formed for this first stage of inversion contained 22,062 absolute phase times and 271,910 catalog-based differential times.

The initial 3D velocity model was based on the most recent version of the 3D New Zealand velocity model (Eberhart-Phillips et al. 2010). We interpolated the New Zealand–wide velocity model to a denser rectilinear grid centered on Christchurch city using Delaunay triangulation and then carried out a series of inversion runs using the tomoDDPS approach of Zhang et al. (2009), slowly decreasing the inversion node spacing as the event and station density allowed, with appropriate smoothing and weighting constraints (Zhang et al. 2009). The dense strong-motion array across Christchurch city, and the high number of events, led to high ray path coverage immediately south of the city, and enabled quite a fine node spacing to be used close to the aftershocks. The velocity model after this first stage had a minimum grid spacing of 1.5 km along the xand y-axes in the section of the volume nearest Christchurch city, and vertical nodes at 1, 2, 3, 4, 5.5, 8, 12, 24, and 30 km depth, fully encompassing the aftershock volume for the Christchurch Mw 6.2 earthquake. We expect further modification and improvement of this velocity model in the next few months as further waveform data is collected and analyzed (Yoshihisa Iio, personal communication 2011). Detailed resolution analysis, for example using checkerboard approaches, has yet to be carried out. However, we expect that the resolution will be quite spatially variable, reflecting the station and event distribution, with the best resolution just south and west of Christchurch city, where the ray path coverage is highest.

In a subsequent second stage we located 4,660 events, all with more than eight phase arrival observations, using the tomoDDPS software of Zhang et al (2009). This dataset consisted of 43,980 absolute phase times and 534,281 differential times. Relocation was carried out using the velocity model developed in the first phase described above. Final epicenters are shown in Figure 3, and details of the relocated hypocenters

for February and March 2011 are provided in supplementary table S1.

DISCUSSION Figure 3 shows the epicenters of the relocated events, while Figures 4 and 5 show projections of the events onto vertical planes AA′ (Figure 4) and BB′ (Figure 5).

A feature of the relocated aftershocks is that they do not clearly define the fault plane of the 22 February earthquake as determined from the centroid moment tensor (Sibson et al. 2011, page 824 of this issue) and geodetically (Beavan et al. 2011, page 789 of this issue); the projection of the geodetically preferred fault plane is overlain on cross-section AA′ (Figure 4). Some vertical alignments of the aftershocks are visible, although they are not directly on the inferred Mw 6.2 fault plane. One of these alignments, for example at X = 6–7 km on Figure 4, may be associated with a secondary strike-slip fault several kms south of the Mw 6.2 earthquake, which has also been inferred geodetically (Beavan et al. 2011, page 789 of this issue).

The refined velocity model highlights P-wave velocities (Vp) greater than 6 km/s beneath part of the Banks Peninsula, as shallow as 5–8 km depth (Figures 4 and 5), at the same depth range as the Mw 6.2 February earthquake and the subsequent aftershocks. These high velocities are indicative of schist basement, consistent with the interpretation of Wood et al. (1989). Previous passive- and active-source seismic work in the Canterbury region by Reyners and Cowan (1993) and Van Avendonk et al. (2004) has also found Vp greater than 6.0 km/s at 5–10 km depth, but with strong spatial variation. Many of the aftershocks appear to be located close to, or just below, the top of this high-velocity basement.

A possible reason for the lack of correspondence between aftershocks and the inferred fault plane for the Mw 6.2 event is that there may have been very little post-seismic slip on the fault. This explanation would be consistent with the high stress drop of the mainshock (Fry and Gerstenberger 2011, page 833 of this issue), which implies high fault friction. Lack of large-scale post-seismic deformation carries implications for the mechanisms and rate of static stress transfer, and ultimately the longevity and the potential of the sequence to produce temporally clustered aftershocks. This would help to explain the relatively high energy release observed for the September Mw 7.1 mainshock and aftershocks. In this scenario, the lack of continual, shallow viscous deformation would encourage high levels of regional stress build up, followed by large events with relatively long aftershock sequences.

ACKNOWLEDGMENTS We thank GeoNet for providing seismological data, Stacey Martin and GeoNet staff for assisting with phase picking, and Banks Peninsula farmers for hosting temporary seismometers. We used GMT (Wessel and Smith 1998) and ObsPy (Beyreuther et al. 2010) for some figures.

842 Seismological Research Letters Volume 82, Number 6 November/December 2011

A’

0

8

km 4

−43.5°

B’ B

−43.6°

A 172.6°

Mag 2

172.8°

A’

Mag 3

−43.5°

Mag 4 Mag 5

B’ B

−43.6°

A 172.6°

172.8°

▲▲ Figure 3. Top: Epicenters for events occurring between 1 September 2010 and 31 January 2011. The locations of cross-sections AA’ (Figure 4) and BB ’ (Figure 5) are marked. Bottom: Epicenters of events occurring between 1 February 2011 and 30 April 2011, inclusive.

The February Mw 6.2 earthquake is marked as a solid star.

Seismological Research Letters Volume 82, Number 6 November/December 2011 843

10

Distance (kms)

A

A’

Mag 2 Mag 3

Depth (kms)

0

0

Mag 4 Mag 5 Mag 6

10

4

5

km/s

6

▲▲ Figure 4. Aftershock locations for events between 1 February and 31 March 2011, projected onto the vertical plane AA’ of Figure 3; only events within 2 km of the vertical plane are shown. The February Mw 6.2 earthquake is at ~X = 8 km. The solid line shows the projection of the fault plane 1 inferred from geodetic studies (Beavan et al. 2011, page 789 of this issue). Color background shows the P-wave velocity.

20

B

B’

Mag 2 Mag 3

Depth (kms)

0

Distance 10 (kms)

0

Mag 4 Mag 5 Mag 6

10

4

5

6

km/s

▲▲ Figure 5. Aftershock locations for events between 1 February and 30 April 2011, projected onto the cross-section BB ’ of Figure 3—a vertical plane with strike N79.5E degrees; the fault plane 2 of Beavan 2011, page 789 of this issue. Only events within 1.2 km of the plane are shown. Background colors show P-wave velocity; areas that have lower ray path coverage (derivative-weighted-sum less than 50) are grayed out. The February Mw 6.2 earthquake is at ~X = 12 km. Earthquake symbol sizes are as for Figure 4.

844 Seismological Research Letters Volume 82, Number 6 November/December 2011

REFERENCES Avery, H. R., J. B. Berrill, P. F. Coursey, B. L. Deam, M. B. Dewe, C. C. Francois, J. R. Pettinga, and M. D. Yetton (2004). The Canterbury University strong-motion recording project. 13th World Conference on Earthquake Engineering, Vancouver, B.C., Canada, 1–6 August, 2004, paper 1335.[are these published proceedings? can we provide name of publisher & place where published?]

Barnes, P. M. (1995). High-frequency sequences deposited during Quaternary sea-level cycles on a deforming continental shelf, north Canterbury, New Zealand. Sedimentary Geology 97, 131–156.

Barnes, P. M. (1996). Active folding of Pleistocene unconformities on the edge of the Australian-Pacific plate boundary zone, offshore north Canterbury, New Zealand. Tectonics 15 (2), 623–640. Correction to Figure 6 printed in Tectonics 15 (5), 1,110–1,111 [1996].

Barnes, P., C. Castellazzi, A. Gorman and S. Wilcox (2011). Submarine Faulting beneath Pegasus Bay, Offshore Christchurch. Short-term Canterbury Earthquake Recovery Project 2: Offshore Faults.

National Institute of Water & Atmospheric Research Client report WLG2011-28, Wellington, NZ: National Institute of Water and Atmospheric Research.

Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters 82, 789–799.

Beavan, J., S. Samsonov, M. Motagh, L. Wallace, S. Ellis and N. Palmer (2010). The Mw 7.1 Darfield (Canterbury) earthquake: Geodetic observations and preliminary source model. Bulletin of the New Zealand Society for Earthquake Engineering 43, 228–235.

Beyreuther, M., R. Barsch, L. Krischer, T. Megies, Y. Behr, and J. Wassermann (2010). ObsPy: A python toolbox for seismology.

Seismological Research Letters 81 (3), 530–533.

Dorn, C., A. G. Green, R. Jongens, S. Carpentier, A. E. Kaiser, F. Campbell, H. Horstmeyer, J. Campbell, M. Finnemore, and J. Pettinga (2010). High-resolution seismic images of potentially seismogenic structures beneath the northwest Canterbury Plains, New Zealand. Journal of Geophysical Research, Solid Earth 115, B11303; doi:10.1029/2010JB007459.

Du, W., C. H. Thurber, and D. Eberhart-Phillips (2004). Earthquake relocation using cross-correlation time delay estimates verified with the bispectrum method. Bulletin of the Seismological Society of America 94, 856–866.

Eberhart-Phillips, D., M. Reyners, S. Bannister, M. Chadwick, and S. Ellis (2010). Establishing a versatile 3-D seismic velocity model for New Zealand. Seismological Research Letters 81 (6), 992–1,000; doi:10.1785/gssrl.82.6.992.

Fry, B., R. Benites, M. Reyners, C. Holden, A. Kaiser, S. Bannister, M. Gerstenberger, C. Williams, J. Ristau, and J. Beavan (2011). Very strong shaking in New Zealand earthquakes. Eos, Transactions, American Geophysical Union.

Fry, B., and M. Gerstenberger (2011). Large apparent stresses from the Canterbury earthquakes of 2010 and 2011. Seismological Research Letters 82, 833–838.

Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011).

The Darfield (Canterbury, New Zealand) Mw 7.1 earthquake of

September 2010: A preliminary seismological report. Seismological Research Letters 82, 378–386.

Holden, C. (2011). Kinematic source model of the 22 February 2011 Mw 6.2 Christchurch earthquake using strong motion data.

Seismological Research Letters 82, 783–788.

Howard, M., A. Nicol, J. Campbell, and J. R. Pettinga (2005). Holocene paleoearthquakes on the strike-slip Porters Pass Fault, Canterbury, New Zealand. New Zealand Journal of Geology and Geophysics 48 (1), 59–74.

Paige, C., and M. Saunders (1982). LSQR: An algorithm for sparse linear equations and sparse least squares problems. ACM Transactions on Mathematical Software 8, 43–71.

Petersen, T., K. Gledhill, M. Chadwick, N. Gale, and J. Ristau (2011).

The New Zealand National Seismograph Network. Seismological Research Letters 82, 9–20.

Pettinga, J. R., M. D. Yetton, R. J. Van Dissen, and G. Downes (2001).

Earthquake source identification and characterisation for the Canterbury region, South Island, New Zealand. Bulletin of the New Zealand Society for Earthquake Engineering 34, 282–317.

Quigley, M., R. Van Dissen, P. Villamor, N. Litchfield, D. Barrell, K. Furlong, T. Stahl, et al. (2010). Surface rupture of the Greendale fault during the Mw 7.1 Darfield (Canterbury) earthquake, New Zealand: Initial findings. Bulletin of the New Zealand Society for Earthquake Engineering 43, 236–242.

Reyners, M. E., and H. Cowan (1993). The transition from subduction to continental collision: Crustal structure in the North Canterbury region, New Zealand. Geophysical Journal International 115 (3), 1,124–1,136.

Sibson, R., F. Ghisetti, and J. Ristau (2011). Stress control of an evolving strike-slip fault system during the 2010–2011 Canterbury, New Zealand, earthquake sequence. Seismological Research Letters 82, 824–832.

Van Avendonk, H. J. A., W. S. Holbrook, D. Okaya, J. Austin, F. Davey, and T. Stern (2004). Continental crust under compression: A seismic refraction study of SIGHT Transect 1, South Island, New Zealand.

Journal of Geophysical Research 109; doi:10.1029/2003JB002790.

Wessel, P., and W. H. F. Smith (1998). New, improved version of the Generic Mapping Tools released. Eos, Transactions, American Geophysical Union 79, 59.

Wood, R. A., P. B. Andrews, and R. H. Herzer, R. A. Cook, N. de B. Hornibrook, R. H. Hoskins, A. G. Beu, et al. (1989). Cretaceous and Cenozoic Geology of the Chatham Rise Region, South Island, New Zealand. New Zealand Geological Survey Basin Studies 3, 75 pp. Lower Hutt, New Zealand: New Zealand Geological Survey.

Zhang, H., and C. H. Thurber (2003). Double-difference tomography: The method and its application to the Hayward fault, California.

Bulletin of the Seismological Society of America 93, 1,875–1,889.

Zhang, H., C. Thurber, and P. Bedrosian (2009). Joint inversion for Vp, Vs, and Vp/Vs at SAFOD, Parkfield, California. Geochemistry, Geophysics, Geosystems 10 (1), Q11002.

GNS Science 1 Fairway Drive, Avalon Lower Hutt 5040 New Zealand s.bannister@gns.cri.nz

(S. B.)

Seismological Research Letters Volume 82, Number 6 November/December 2011 845

The Character of Accelerations in the Mw 6.2 Christchurch Earthquake B. Fry, R. Benites and A. Kaiser

B. Fry, R. Benites and A. Kaiser GNS Science

INTRODUCTION The Canterbury earthquakes of 2010 and 2011 have produced some of the strongest ground motions ever measured in New Zealand. Many of the highest acceleration recordings arose from seismic stations within the city of Christchurch (population ~377,000). A dense array of strong-motion seismometers was in place prior to the mainshock of 4 September 2010. Subsequent to the mainshock, numerous rapid response accelerometers were installed in the Canterbury Plains, Banks Peninsula, and in the city itself (Gledhill et al. 2011; Cochran et al. 2011). Many of the strongest aftershocks were recorded by this dense amalgamation of permanent and temporary arrays and provide a detailed record of variable ground motion throughout the region during the aftershock sequence.

The most extreme ground motions were recorded during the Mw 6.2 earthquake of 22 February 2011 that struck a few kilometers to the south of Christchurch (Beavan et al. 2011, this issue; Holden et al. 2011, this issue; Bannister et al. 2011, this issue), generating severe damage throughout the city. In fact, damage to the built environment and ground liquefaction was much more widespread in the February event than in the September Mw 7.1 mainshock (Kaiser et al. 2011). This event is one of the best-recorded shallow thrust earthquakes in the near field. Recorded peak ground acceleration (PGA) in February exceeded 2 g near the epicenter and was greater than 0.6 g over much of the central and eastern suburbs (Figure 1). At these near-source stations, vertical accelerations were generally markedly higher than horizontal accelerations. The large accelerations can be reasonably well explained by the combination of

HPSC

CHHC PRPC

HVSC

CCCC

▲▲ Figure 1. Vertical and horizontal acceleration vectors and their location relative to the February 22 epicenter (green star). Green horizontal lines on each waveform show the baseline.

846 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.846

▲▲ Figure 2. A) shows three-component accelerogram from station PRPC (see Figure 1 for location) for the 22 February event, scaled to units g. B) shows their corresponding spectra.

the proximity of the February event to Christchurch and the effects of strong source directivity. However, not all features can be explained by source effects alone. The dense near-source data from the 22 February earthquake have provided us with a valuable opportunity to study the response of the shallow subsurface to extreme ground motions in very fine detail.

The way ground responds to an earthquake is a result of the earthquake rupture process, the path that the waves take between the source and the surface, and the response of the shallow materials below the ground. We know that the top few meters of the ground in Christchurch played an important role in the shaking. This role is evident in low-frequency signals resulting from liquefaction. Many of the poorly consolidated, low shear-wave velocity soils liquefied at shallow depths with less than 0.1 g peak horizontal accelerations and experienced deep liquefaction at around 0.2 to 0.3 g accelera-

tions. The influence of the shallow subsurface is also exhibited by the existence of energetic high-frequency signals resulting from the interaction of the waves with both the water table and unconsolidated soils prior to liquefaction. A marked feature of the strong-motion seismograms recorded at several nearsource sites in Christchurch during the earthquake sequence is the much higher frequency content of the vertical component compared to the corresponding horizontal recordings (Figure 2). We believe that this phenomenon is due to the presence of a shallow water table dramatically attenuating the propagation of high-frequency shear waves. A rigorous numerical demonstration of such water effect would require the calculation of seismic wave propagation in layered media in which one of the layers is a porous, elastic solid containing an incompressible, inviscous fluid (Biot 1956a, Biot 1956b). However, for this work, we simulate such an effect in an indirect way by model-

Seismological Research Letters Volume 82, Number 6 November/December 2011 847

TABLE 1 1-D layered model in the near field of the M 6.2 February 4, 2011 Canterbury earthquake. The fault is embedded in the second layer. H-S denotes the half-space underneath the layering.

Thickness (km)

Vp (km/s)

Vs (km/s)

ρ (g/cm3)

0.3 4.5 5.6 6.24

0.165 2.6 3.2 3.6

1.5 1.8 2.4 2.9

0.02 0.98 9.0 H-S

ing the effect of attenuation of shear waves on the acceleration seismograms. We define a 1-D layered medium representing the Canterbury region close to the fault that ruptured in the Mw 6.2 February 2011 aftershock based on a regional subset of the New Zealand–wide 3-D velocity model (EberhartPhillips et al. 2010) and shallow velocity structure determined by microtremor analysis (Stephenson et al. 2011). The fault is represented by a 9-km-long and 8-km-wide rectangular thrust fault dipping 65° and striking 70° (from north), rupturing with 3.2 km/s rupture velocity and 120° rake. This fault is embedded in the second layer of a four-layered medium of elastic parameters listed in Table 1. The modeling is accomplished with a discrete wavenumber numerical scheme (Bouchon 1979) in which attenuation is introduced by the factor exp(2π*f / Qs) for each plane wave in the layer (Aki and Richards 1980), where f is frequency and Qs is the shear wave quality or attenuating factor.

We apply attenuation to the top 20-m-thick soil layer to test the hypothesis that the observed differences in frequency content between the horizontal and vertical components is due to strong shear wave attenuation in the shallow subsurface. Ideally, modeling the anelastic attenuation would include Biot’s model of fluid-solid interaction in our wave propagation algorithm.

Geli et al. (1987) successfully incorporated Biot’s model in the Aki-Larner (1970) numerical technique to study the response of smooth 2-D basins with water-saturated sediments. Their work prescribes a characteristic frequency ( fc) of about 5 Hz for unconsolidated coarse sands and gravels with a permeability on the order of 10 –8 m2 . This frequency depends on fluid and solid parameters such as permeability, viscosity, density, and pore density. For frequencies lower than fc, the attenuation of shear waves is stronger than for primary waves, and for frequencies above fc, the attenuation of primary P waves increases significantly with frequency due to the rise of a secondary P wave (termed P2) that results from the solid-fluid coupling.

Based on an anelastic representation of attenuation in which velocities are estimated from the fluid and solid parameters, Geli et al. (1987) prescribe the dependency of Q on frequency, showing only weak dependency in the range 1–10 Hz for permeability and porosity in the range of the shallow subsurface of Christchurch (10 –8 and 35%, respectively). We therefore apply only shear wave attenuation (between Qs = 1 and Qs = 10 for 0 Hz–7 Hz) to our range of simulations.

The top panel of Figure 3 shows the three components of displacement and acceleration at a station located 2 km from the fault, when there is no attenuation (Qs = 1,000). Although

(A)

(B)

▲▲ Figure 3. Synthetic wavefield calculated with a discrete wavenumber method at a distance of 2 km from the 22 February rupture.

U is east-west, V is north-south, and W is vertical. A) Results from a simulation with zero attenuation. B) Results from inclusion of a quality factor to represent the attenuating effects of the local groundwater table.

848 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 4. Vertical acceleration waveforms from Figure 1. Waveforms show larger positive accelerations than negative ones. Many of the negative acceleration troughs are also broader than the narrow positive acceleration spikes.

the high acceleration may be unrealistically large (about 5 g), we recall that this is the result of a kinematic model combining the effect of directivity and the presence of the thin (20 m), soft top layer in the near field. The bottom panel of Figure 3 shows the results for the case of Qs = 5. Noticeably, high frequencies have been attenuated in the horizontal components but not in the vertical component. This is compatible with numerous observations of this characteristic recorded during the aftershock sequence (e.g., Figure 2).

VERTICAL COMPONENT ASYMMETRY Another notable characteristic of many of the recordings from the February event and some recordings from other strong local events is the occurrence of asymmetric accelerations and spikes in the vertical direction. In this observational paper, we document an example of a newly discovered phenomenon in the vertical components of the acceleration seismograms that has previously been recognized in a handful of records from strong shallow earthquakes and nuclear explosions. Many accelerograms recorded in the Mw 6.2 earthquake exhibit maximum PGA on the vertical component (Figure 1). The asymmetrical recordings are confined to within ~6–10 km of the epicenter,

suggesting that either very strong near-field motions are necessary to generate them or that they result from high-frequency waves generated during source processes that subsequently attenuate at greater distances. Most of the high-acceleration vertical records are asymmetric with maximum accelerations in the upward direction (>1 g) exceeding accelerations in the downward direction (<1 g). In Figure 4, we show vertical component records from strong-motion recordings within 8 km of the fault plane. We use fault location defined by inversion of geodetic data (Beavan et al. 2011, this issue) to calculate distance to the fault. These records of vertical ground acceleration are dominated by sharp positive acceleration peaks and muted, broad negative acceleration troughs. In all of the three-component records from the stations represented in Figure 1, the vertical components have much higher frequency content than the horizontal accelerations. Typically, the horizontal components carry maximum energy in ~0.5–5 Hz whereas the vertical components are dominated in the broad high-frequency range of ~5–20 Hz. Asymmetric vertical records have long been noticed from single-pulse nuclear explosions. However, asymmetric recordings from multiple-cycle earthquake records was recently attributed to a “trampoline” effect during the Mw 6.9 Iwate-

Seismological Research Letters Volume 82, Number 6 November/December 2011 849

18 Oct Mw 5.0

4 Sep Mw 7.1

13 Jun Mw 5.6

13 Jun Mw 6.0

22 Feb Mw 6.2 ▲▲ Figure 5. Vertical accelerograms from station PRPC for different events sorted from minimum PGA in the top panel to maximum PGA in the bottom panel. Gray region marks twice the minimum acceleration centered about the baseline.

Miyagi Nairiku earthquake of 2008 (Aoi et al., 2008; Yamada et al., 2009). Similar asymmetric recordings from the Mw 6.9 Iwate-Miyagi Nairiku earthquake of 2008 have been attributed to a “trampoline” effect (Aoi et al. 2008; Yamada et al. 2009).

Aoi et al. (2008) attribute the asymmetry to the decoupling of near-surface materials during high-amplitude downward acceleration. This occurs when the tensile forces that arise on an interface or within a granular material from downgoing par-

ticle oscillation as waves pass are larger than its tensile strength.

The result is an approximate free-fall of the material. In this model, the high upward accelerations are caused by the compressional response of the granular media to the stress of the upgoing particle oscillation. Yamada et al. (2009) suggest the large positive accelerations are further enhanced by “slapdown” as free-falling upper soil layers impact/interact with deeper layers that are returning upward during the following earthquake wave cycles. Recently conducted finite element numerical modeling supports the hypothesis that the asymmetry arises due to the difference in response of near-surface layers to compression and tension (Tobita et al. 2010). Figure 5 shows a series of vertical component recordings from station PRPC in Christchurch.

The recorded vertical acceleration from the February event at this station was most similar to asymmetric vertical acceleration recorded in the Iwate-Miyagi event. PRPC appears to have experienced vertical acceleration asymmetry in numerous events over a range of peak accelerations. Recordings from the 13 June 2011 Mw 6.0 and Mw 5.6 events both exhibit slight asymmetry with PGAs as low as ~0.7 g. However, records from the 4 September Mw 7.1 earthquake do not appear asymmetric.

Ground motions recorded at PRPC in this event reach 0.32 g.

This suggests that a threshold for the non-linear effect that causes the asymmetry is between about 0.3 and 0.7 g at this site.

In pure granular material, Tobita et al. (2010) show that the depth of initiation of the non-linear response is proportional to the experienced ground acceleration. The frequency of the response is also likely to be at least partially controlled by the depth of origin of the strong asymmetry. We will focus on station PRPC for a description of the time and frequency of asymmetry. To quantify trace asymmetry, we normalize the difference between the maximum (amax) and minimum (amin) accelerations for time-windowed data (i.e., (amax – abs(amin))/ (max(abs(amax,amin)))). Onset of dominant asymmetry occurs proportionately earlier in the time series for very near-source stations and proportionately later for more distant stations. At station PRPC, onset of dominant asymmetry occurs at about 3 s into the waveform (Figure 6) whereas at station HVSC, within a couple of kilometers of the rupture, onset is within the first second. We analyze the asymmetry of different frequencies by applying the time-windowing to bandpass filtered data. We define frequency bands in the data between 0.2–1 Hz, 1–2 Hz, 2–5 Hz, 5–10 Hz, and 10–30 Hz. Asymmetry at station PRPC is dominated by the 2–5 Hz band (Figure 6). However, we use a window half-width of 0.25 s for all considered bands.

We recognize that relatively more wave cycles will contribute to the time-window for the higher frequency bands than for the lower frequency ones. However, the asymmetry in the 2–5 Hz band is bracketed by the more symmetric 1–2 Hz and 5–10 Hz bands. This supports our suggestion that the 2–5 Hz band of the signal dominates the asymmetry.

CONCLUSIONS The Mw 6.2 Christchurch earthquake generated a wealth of near-field strong-motion data. Analysis of these data suggests

850 Seismological Research Letters Volume 82, Number 6 November/December 2011

PRPC Z

0.8

0.4

Normalized (Max PGA + Min PGA)

0

-0.4

-0.8 0

1

2

3

4

5

6

7

8

9

0.8

10

0.2 - 1 Hz 1 - 2 Hz 2 - 5 Hz 5 - 10 Hz 10 - 30 Hz

0.4

0

-0.4

-0.8 0

1

2

3

4

5

time (s)

6

7

8

9

10

▲▲ Figure 6. A) Normalized sum of maximum and minimum PGA for windows of the vertical recording at PRPC. Windows have a half-width of 0.25 seconds and are sampled every 0.1 s. Bottom) Data windowed as above after band-pass filtering. The band between 2–-5 Hz (dark blue) is dominantly positively asymmetric between about 2.5 and 7.5 seconds (shown as shaded region).

that non-linear effects in the near-surface layers such as those proposed by Aoi et al. (2008) and Yamada et al. (2009) are present in many of the urban recordings. These are manifest in asymmetry in the vertical recordings. We find asymmetric behavior in records from smaller events as well, and bracket initiation of vertical component acceleration asymmetry at between 0.3 and 0.7 g. Furthermore, a multiple-filter approach to analyzing the time signals recorded at one of the eastern city stations suggests that the dominant bandwidth carrying the asymmetric accelerations is between 2 and 5 Hz. Further analysis of these data in conjunction with detailed site response analyses and numerical modeling has the potential to greatly increase our understanding of this phenomenon. We have noted a difference in the frequency content of the recorded vertical and horizontal accelerations from the earthquake. Based on numerical modeling, we propose that the lack of high-frequency energy on the horizontal components can largely be attributed to shear wave attenuation in the shallow, water-saturated sediments. Future work is aimed at fully implementing Biot’s theory into the

modeling, explicitly describing the complete fluid interaction of the water-saturated soils in Christchurch.

Many of the strong-motion recordings have vertical components that are rich in high-frequency energy and horizontal components that are dominated by low-frequency waves. We model attenuation with a discrete wavenumber method as a proxy for this effect. The characteristic difference in energy content appears to be the result of attenuation of high-frequency shear waves in the shallow subsurface.

REFERENCES Aki, K., and K. L. Larner (1970). Surface motion of a layered medium having an irregular interface due to incident plane SH waves.

Journal of Geophysical Research 75, 933–954.

Aki, K., and P. G. Richards (1980). Quantitative Seismology. San Francisco: W. H. Freeman and Company, 556 pp.

Aoi, S., T. Kunugi, and H. Fujiwara (2008). Trampoline effect in extreme ground motion. Science 322 (5902), 727–730, doi:10.1126/science.1163113.

Seismological Research Letters Volume 82, Number 6 November/December 2011 851

Bannister, S., B. Fry, M. Reyners, J. Ristau, and H. Zhang (2011).

Fine-scale relocation of aftershocks of the 22 February Mw 6.2 Christchurch earthquake using double-difference tomography.

Seismological Research Letters, this issue.

Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters, this issue.

Beavan, R. J., S. Samsonov, M. Motagh, L. M. Wallace, S. M. Ellis, and N. Palmer (2010). The Darfield (Canterbury) earthquake: Geodetic observations and preliminary source model. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 228–235.

Biot, M. A. (1956a). Theory of propagation of elastic waves in a fluid saturated porous solid. I. Low frequency range. Journal of the Acoustical Society of America 28, 168–178; doi:10.1121/1.1908239.

Biot, M. A. (1956b). Theory of propagation of elastic waves in a fluid saturated porous solid. II. Higher frequency range. Journal of the Acoustical Society of America 28, 179–191; doi:10.1121/1.1908241.

Bouchon, M. (1979). Discrete wave number representation of elastic wave fields in three-space dimensions. Journal of Geophysical Research 84, 3,609–3,614.

Cochran, E., J. Lawrence, A. Kaiser, B. Fry, A. Chung, and C. Christensen (2011). Comparison between low-cost and traditional MEMS accelerometers: A case study from the M7.1 Darfield, New Zealand aftershock deployment.

Eberhart-Phillips, D., M. E. Reyners, S. C. Bannister, M. P. Chadwick, and S. M. Ellis, (2010). Establishing a versatile 3-D seismic velocity model for New Zealand. Seismological Research Letters 81 (6), 992–1,000; doi:10.1785/gssrl.82.6.992.

Geli, L., P. Bard, and D. Schmitt (1987). Seismic wave propagation in a very permeable water-saturated surface layer. Journal of Geophysical Research 92, 7,931–7,944.

Gledhill, K., J. Ristau, M. E. Reyners, B. Fry, and C. Holden (2011).

The Darfield (Canterbury, New Zealand) Mw7.1 earthquake of September 2010: A preliminary seismological report. Seismological Research Letters 82 (3), 378–386; doi:10.1785/gssrl.82.6.378.

Holden, C. (2011). Kinematic source model of the 22 February 2011 Mw 6.2 Christchurch earthquake using strong motion data.

Seismological Research Letters this issue.

Kaiser, A., C. Holden, J. Beavan, D. Beetham, R. Benites, A. Celentano, D. Collett et al. (2011). The February 2011 Christchurch earthquake: A preliminary report. Submitted to New Zealand Journal of Geology and Geophysics.

Stephenson, B., P. Barker, Z. Bruce, and D. Beetham (2011). Immediate Report on the Use of Microtremors for Assessing Liquefaction Potential in the Christchurch Area. GNS Science Report 2010/30, 26 pp. Lower Hutt, New Zealand: GNS Science.

Tobita, T., I. Susumu, and T. Iwata (2010). Numerical analysis of nearfield asymmetric vertical motion. Bulletin of the Seismological Society of America 100, 1,456–1,469.

Yamada, M., J. Mori, and T. Heaton (2009). The slapdown phase in highacceleration records of large earthquakes. Seismological Research Letters 80, 559–564.

GNS Science 1 Fairway Drive Lower Hutt, New Zealand b.fry@gns.cri.nz

(B. F.)

852 Seismological Research Letters Volume 82, Number 6 November/December 2011

Near-source Strong Ground Motions Observed in the 22 February 2011 Christchurch Earthquake Brendon A. Bradley and Misko Cubrinovski

Brendon A. Bradley and Misko Cubrinovski University of Canterbury

INTRODUCTION On 22 February 2011 at 12:51 p.m. local time, a moment magnitude Mw 6.3 earthquake occurred beneath the city of Christchurch, New Zealand, causing an level of damage and human casualties unparalleled in the country’s history.

Compared to the preceding 4 September 2010 Mw 7.1 Darfield earthquake, which occurred approximately 30 km to the west of Christchurch, the close proximity of the 22 February event led to ground motions of significantly higher amplitude in the densely populated regions of Christchurch. As a result of these significantly larger ground motions, structures in general, and commercial structures in the central business district in particular, were subjected to severe seismic demands and, combined with the event timing , structural collapses accounted for the majority of the 181 casualties (New Zealand Police 2011).

This manuscript provides a preliminary assessment of the near-source ground motions recorded in the Christchurch region. Particular attention is given to the observed spatial distribution of ground motions, which is interpreted based on source, path, and site effects. Comparison is also made of the observed ground motion response spectra with those of the 4 September 2010 Darfield earthquake and those used in seismic design in order to emphasize the amplitude of the ground shaking and also elucidate the importance of local geotechnical and deep geologic structure on surface ground motions.

There are numerous identified faults in the Southern Alps and eastern foothills (Stirling et al. 2007) and several significant earthquakes (i.e., Mw > 6) have occurred in this region in the past 150 years, most notably the Mw 7.1 Darfield earthquake on 04/09/2010 (New Zealand Society for Earthquake Engineeering 2010). The Mw 6.3 Christchurch earthquake occurred at 12:51 p.m. on Tuesday 22 February 2011 beneath Christchurch, New Zealand’s second-largest city, and represents the most significant earthquake in the unfolding seismic sequence in the Canterbury region since the 4 September 2010 Darfield earthquake. The 6.3 event occurred on a previously unrecognized steeply dipping blind fault, which trends northeast to southwest (the location relative to Christchurch is presented in the context of subsequently observed ground motions). Figure 2 illustrates the inferred slip distribution on the fault obtained by Beavan et al. (2011, page 789 of this

TECTONIC AND GEOLOGIC SETTING New Zealand resides on the boundary of the Pacific and Australian plates (Figure 1) and its active tectonics are dominated by: 1) oblique subduction of the Pacific plate beneath the Australian plate along the Hikurangi trough in the North Island; 2) oblique subduction of the Australian plate beneath the Pacific plate along the Puysegur trench in the southwest of the South Island; and 3) oblique, right-lateral slip along numerous crustal faults in the axial tectonic belt, of which the 650-km-long Alpine fault is inferred to accommodate approximately 70–75% of the approximately 40 mm/yr plate motion (DeMets et al. 1994; Sutherland et al. 2006).

doi: 10.1785/gssrl.82.6.853

▲▲ Figure 1. Tectonic setting of New Zealand (courtesy of J. Pettinga).

Seismological Research Letters Volume 82, Number 6 November/December 2011 853

▲▲ Figure 2. Distribution of fault slip inferred in the 22 February 2011 Christchurch earthquake (Beavan et al. 2011, this issue). Arrows indicate the slip vector. The inferred hypocenter is indicated by a star.

issue). It can be seen that slip on the fault occurred obliquely with both significant up-dip and along-strike components (average rake, λ = 146°). The steeply dipping nature of the fault (δ = 69°), as well as the large up-dip component of slip, contributed to the large observed vertical accelerations discussed in the next section. For the purpose of the subsequent engineering analysis of strong ground motion, the Beavan et al. (2011, page 789 of this issue) finite fault model was “trimmed” using the methodology of Somerville et al. (1999), which resulted in the removal of 1 km from the northeast and southwest extents of Figure 2. The resulting “trimmed” fault therefore has dimensions of 15 km along-strike and 8 km down-dip, giving a total area of 120 km2 .

Christchurch is located on the Canterbury Plains, a fan deposit resulting from the numerous rivers flowing eastward from the foothills of the Southern Alps (Brown and Weeber 1992). In the vicinity of Christchurch, the Canterbury Plains are comprised of a complex sequence of gravels interbedded with silt, clay, peat, and shelly sands. The fine sediments form aquicludes and aquitards between the gravel aquifers, and with the nearby coastline to the east, result in the majority of Christchurch having a water table less than 5 m depth, with the majority of the area including, and to the east of, the central business district having a water table less than 1 m from the surface (Brown and Weeber 1992). The postglacial Christchurch Formation created by estuarine, lagoonal, dune, and coastal swamp deposits (containing gravel, sand, silt, clay, shell, and peat) is the predominant surface geology layer in the Christchurch area, which outcrops up to 11 km west of the coast and has a depth of approximately

40 km along the coast itself (Brown and Weeber 1992). At the southeast edge of Christchurch lies the extinct Banks Peninsula volcanic complex.

STRONG MOTION RECORD PROCESSING Volume 1 ground motion records were obtained from GeoNet (http://www.geonet.org.nz/) and processed on a record-byrecord basis. The overall processing methodology adopted is elaborated in Chiou et al. (2008, Figure 4). All ground motions were processed with a low-pass causal Butterworth filter of 50 Hz, and while the corner frequency of the high-pass filter was record-specific, a frequency of less than 0.05 Hz provided physically realistic Fourier spectra amplitudes and integrated displacement histories for all the near-source ground motions.

Owing to the digital nature of all of the instruments, baseline corrections were found to be unnecessary following the above filtering. As a result, the processed ground motions can be considered to provide reliable estimates of peak ground accelerations (PGA) and spectral ordinates over the range 0.01–10 seconds (Douglas and Boore 2010), which are typically of engineering interest. It should be noted that the above procedure does not lend itself to the computation of residual displacements, which may be non-zero for near-source locations. However, as a result of possible instrument tilting, which may be significant at sites where liquefaction occurred, reliable computation of such residual displacements may not be possible (Graizer 2005) and is left for future study.

854 Seismological Research Letters Volume 82, Number 6 November/December 2011

(B)

8 7 6 5 4

2009 NZ database 04/09/2010 22/02/2011 0

10

1

10 Distance, Rrup (km)

2

10

Number of exceedances

Magnitude, Mw

(A)

2

10

2009 NZ database 04/09/2010 22/02/2011 Total

1

10

0

10

0.2 0.4 0.6 0.8 1 1.2 1.4 Peak ground acceleration, PGA (g)

▲▲ Figure 3. Significance of the 22 February 2011 Christchurch and 4 September 2010 Darfield earthquakes in relation to previously recorded ground motions in New Zealand: A) magnitude-distance distribution; B) exceeded values of peak ground acceleration.

While New Zealand can be considered as a region of high seismicity in a global context, prior to the 4 September 2010 Darfield and 22 February 2011 Christchurch earthquakes there was a paucity of high-amplitude recorded strong ground motions, primarily as a result of a sparse instrumentation network before the commencement of GeoNet in 2001. Figure 3 illustrates the magnitude-distance distribution of recorded ground motions from active shallow crustal earthquakes up to 2009 as complied by Zhao and Gerstenberger (2010). Also illustrated in Figure 3A are the ground motions recorded in both the 4 September 2010 Darfield and 22 February 2011 Christchurch earthquakes. The significance of the recorded ground motions in these two earthquakes is even more apparent if the ground motions in Figure 3A are plotted in terms of their geometric mean horizontal PGA. Figure 3B illustrates the number of ground motions exceeding specific values of PGA. It can be seen that up to 2009, the maximum PGA recorded in New Zealand was 0.39 g, with only seven observed ground motions exceeding 0.2 g PGA. Figure 3B also illustrates the exceedance values observed in the Darfield and Christchurch earthquakes.

With the addition of these two events (not to mention records obtained from numerous significant aftershocks, which are not discussed herein) it can be seen that ground motions of up to 1.41 g have now been recorded, with 12 observed ground motions exceeding 0.4 g and 39 exceeding 0.2 g.

OBSERVED NEAR-SOURCE GROUND MOTIONS Table 1 presents a summary of the ground motions in the wider Christchurch region that were recorded within a source-to-site distance of Rrup = 20 km, including: station site class according to the New Zealand loading standard, New Zealand Standards 1170.5 (2004); PGA, peak ground velocity (PGV); 5–95% significant duration, (Ds5–95) (Bommer and Martinez-Pereira 1999) for geometric mean horizontal component; and peak vertical ground acceleration (PGAv).

Figures 4–6 illustrate the spatial distribution of acceleration time histories recorded at the aforementioned strong motion stations in the form of fault-normal, fault-parallel, and vertical components, respectively. The aforementioned “trimmed” finite fault model of Beavan et al. (2011, page 789 of this issue) is also shown. The following sections discuss various aspects of the ground motions observed in Figures 4–6.

Ground Motion on Rock and Soil Sites In interpreting the observed ground motions in Figures 4–6, it is first worth noting that only the Lyttelton Port (LPCC) station to the southeast of Christchurch is located on engineering bedrock (i.e., site class B). Stations HVSC and LPOC located near the edge of the Port Hills rock outcrop are site class C, while all remaining stations are situated on the Christchurch sedimentary basin and are predominantly site class D (the exceptions being HPSC, NNBS, PRPC, and KPOC, which are site class E). Unfortunately at present the site characterization of strong motion stations in the Christchurch region, and New Zealand in general, is relatively poor with the above site classes determined from geological maps and direct surface inspection (N. Perrin, personal communication 2011), and details such as P- and S-wave velocity, SPT, and CPT data not available.

Clearly, obtaining such information is necessary for a rigorous analysis of the observed ground motions, and is the focus of immediate studies. Nonetheless, a wealth of insight can still be obtained from inspection and analysis of the observed ground motions.

A direct comparison of the effect of soil and rock site can be made by comparing the ground motions observed at LPCC and LPOC located at Lyttelton Port approximately 1 km apart.

The LPCC instrument is located on engineering bedrock, and while detailed information of the site conditions at LPOC are presently unavailable, it is said to be a relatively thin (~30 m) colluvium layer comprised primarily of silt and clay (J. Berrill, personal communication 2011). In addition to a comparison of

Seismological Research Letters Volume 82, Number 6 November/December 2011 855

TABLE 1 Strong Motion Stations and Near-source Recordings of the 22 February 2011 Christchurch Earthquake Station Name

Code

Site class*

Rjb† (km)

Rrup‡ (km)

PGA § (g)

PGV|| (cm/s)

Ds5–95 # (s)

Canterbury Aero Club Christchurch Botanic Gardens Christchurch Cathedral College Christchurch Hospital Cashmere High School Hulverstone Dr Pumping Station Heathcote Valley School Kaiapoi North School Lincoln School Lyttelton Port Lyttelton Port Naval Point North New Brighton School Papanui High School Pages Rd Pumping Station Christchurch Resthaven Riccarton High School Rolleston School Shirley Library Styx Mill Transfer Station Templeton School

CACS CBGS CCCC CHHC CMHS HPSC HVSC KPOC LINC LPCC LPOC NNBS PPHS PRPC REHS RHSC ROLC SHLC SMTC TPLC

D D D D D E C E D B C E D E D D D D D D

12.7 4.6 2.6 3.7 1.0 3.8 1.4 17.3 13.5 4.8 4.2 3.7 8.6 2.3 4.6 6.5 19.6 5.0 10.7 12.5

12.8 4.7 2.8 3.8 1.4 3.9 4.0 17.4 13.6 7.1 6.6 3.8 8.6 2.5 4.7 6.5 19.6 5.1 10.8 12.5

0.21 0.50 0.43 0.37 0.37 0.22 1.41 0.20 0.12 0.92 0.34 0.67 0.21 0.63 0.52 0.28 0.18 0.33 0.16 0.11

20.0 46.3 56.3 50.9 44.4 36.7 81.4 18.9 12.7 45.6 69.1 35.1 36.7 72.8 65.4 29.8 8.4 67.8 27.6 11.3

11.8 10.7 9.8 10.3 5.1 10.0 5.7 11.3 12.1 4.0 7.7 2.4 12.8 3.8 10.2 9.9 10.3 7.0 13.6 15.3

PGAv** (g) 0.19 0.35 0.79 0.62 0.85 1.03 2.21 0.06 0.09 0.51 0.39 0.80 0.21 1.88 0.51 0.19 0.08 0.49 0.17 0.16

* As defined by the New Zealand Loadings Standard, NZS1170.5 (2004) † Joyner-Boore distance from surface projection of fault plane to site ‡ Closest distance from fault plane to site § Peak ground acceleration || Peak ground velocity # Significant duration (5–95%) ** Peak vertical ground acceleration. Note that with the exception of PGAv , ground motion parameters are geometric mean horizontal definition

the acceleration time histories in Figures 4–6, Figure 7 illustrates the pseudo-acceleration response spectra of the geometric mean horizontal and vertical ground motion components at the two sites. In regard to horizontal components of ground motion, compared to LPCC, it can be seen that the observed ground motion at the LPOC site has significantly lower highfrequency ground motion amplitude (i.e. PGALPOC = 0.34 g, PGALPCC = 0.92 g), longer predominant period (Figure 7), larger peak ground velocity (i.e., PGVLPOC = 69 cm/s, PGVLPCC = 46 cm/s), and larger significant duration (i.e., Ds, LPOC = 7.7 s, Ds,LPCC = 4.0 s), inferred as the result of nonlinear response of the surficial soils at LPOC. In contrast to the significant difference in horizontal ground motion, it can be seen that there is relatively little difference between the vertical ground motion at LPCC and LPOC, with peak vertical accelerations of 0.51 and 0.39 g, respectively.

Evidence of Liquefaction One of the major causes of damage in the Mw 6.3 Christchurch earthquake resulted from the severity and spatial extent of liq-

uefaction in residential, commercial, and industrial areas. The horizontal components of acceleration depicted in Figures 4 and 5 show evidence of liquefaction phenomena in the central business district and eastern suburbs, which are located in the near-source region beyond the up-dip projection of the fault plane. In the central business district (i.e., REHS, CBGS, CHHC, CCCC), Cashmere (CMHS), and Shirley (SHLC), evidence of liquefaction is inferred based on the manifested reduction in high-frequency content of ground motion following several seconds of S-wave arrivals, and the subsequent acceleration “spikes,” characteristic of strain hardening deformation during cyclic mobility. In the eastern suburbs (i.e., PRPC, HPSC, NNBS), the picture is somewhat more complex. The ground motion at Pages Road (PRPC) also has some of the characteristics discussed above, but in addition exhibits very high accelerations in the fault-normal and vertical directions, which likely result from both surficial soil and source effects, due to its proximity to the up-dip projection of the slip asperity (i.e., Figure 2). The ground motion at North New Brighton (NNBS) exhibits several seconds of cyclic mobility before an

856 Seismological Research Letters Volume 82, Number 6 November/December 2011

Scale

0.5 g 5 seconds

▲▲ Figure 4. Observed fault-normal horizontal acceleration time histories at various locations in the Christchurch region from the 22 February earthquake.

Scale

0.5 g 5 seconds

▲▲ Figure 5. Observed fault-parallel horizontal acceleration time histories at various locations in the Christchurch region from the 22 February earthquake.

Seismological Research Letters Volume 82, Number 6 November/December 2011 857

Scale

0.5 g 5 seconds

Spectral acc, Sa (g)

▲▲ Figure 6. Observed vertical acceleration time histories at various locations in the Christchurch region from the 22 February earthquake.

10

0

10

-1

-2

10 -2 10

tude of ground shaking; 2) a change in surficial soil characterization; and 3) an increase in water table depth as noted previously. Given the observed spatial extent of liquefaction in the 4 September 2010 Darfield earthquake (Cubrinovski et al. 2010), in which the majority of this western region was unaffected by liquefaction despite been subjected to generally stronger shaking than the eastern regions (where liquefaction was prevalent), it can be logically concluded that the character and in situ state of the soils in the western Christchurch region (Brown and Weeber 1992) result in a lower liquefaction susceptibility.

Eq:22/02/2011 Solid:AvgHoriz Dashed:Vert

LPCC LPOC

10

-1

Period, T (s)

10

0

10

1

▲▲ Figure 7. Comparison of geometric mean horizontal and vertical response spectra observed at two stations in Lyttelton Port, one on outcropping rock (LPCC), the other on soil (LPOC).

abrupt reduction in acceleration amplitude resulting in a very short significant duration of 2.4 seconds (Table 1). The ground motion observed at Hulverstone Drive (HPSC) is also of interest due to the relatively small horizontal component acceleration amplitudes compared with what might be expected at such a near-source location (including observed shaking at nearby stations), and relative to its high vertical accelerations.

No significant signs of liquefaction are evident in the ground motions recorded to the west of those discussed above, which could result from three factors: 1) a reduction in ampli-

Basin-generated Surface Waves As previously mentioned, Christchurch is located on a sedimentary fan deposit with the volcanic rock of Banks Peninsula located to the southeast. While specific mechanical and geometrical details of the predominant sedimentary basin layers are presently unknown, previous petroleum exploration has revealed the depth of gravel layers is in excess of 500 m, with basement rock inferred to be at depths in excess of 2.0 km at various locations (Brown and Weeber 1992, Figure 1). Figure 8A provides a schematic illustration of the deep geology of the region along a plane trending southeast to northwest. Figure 8A also illustrates one possible ray path from the Mw 6.3 rupture in which seismic waves propagate up-dip and enter the sedimentary basin through its thickening edge. The large postcritical incidence angles of such waves cause reflections that lead to a waveguide effect in which surface waves propagate across the basin resulting in enhanced long-period ground motion amplitudes and shaking duration (Choi et al. 2005). Figure 8B illustrates the fault-normal, fault-parallel, and geometric mean

858 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

North/ West

Christchurch Mesozoic basement rock

Tertiary Volcanic Rock

Hypocenter

Spectral Acc, Sa (g)

(B) 10

0

10

-1

-2

10 -2 10

Significant amplitude Rayleigh surface waves are also clearly evident in the vertical component of ground motion observed at larger source-to-site distances where body wave amplitudes are smaller (e.g., stations SMTC and CACS in Figure 6).

Station:CHHC Rrup=3.8 km

Z1.0=1000m

Z1.0=300m

Fault Parallel Fault Normal Horiz gm 10

-1

Period, T (s)

10

0

10

1

▲▲ Figure 8. A) Schematic illustration of waveguide effects occurring in the sedimentary basin underlying Christchurch (not to scale); and B) influence of basin depth on pseudo-spectral acceleration ordinates predicted empirically compared with that observed at Christchurch Hospital (CHHC). The prediction shown is for the horizontal geometric mean and dashed lines represent the 16th and 84th percentiles.

horizontal pseudo-response spectra at Christchurch hospital (CHHC), located at a source-to-site distance of Rrup = 3.8 km on the hanging wall. Also shown in Figure 8B is the predicted median, 16th, and 84th percentile response spectra for the site using the Bradley (2010) empirical model for two different values of a proxy for basin depth. The Bradley (2010) model is based on the Chiou and Youngs (2008) model with New Zealand–specific modifications. Basin effects are accounted for in the model through the use of the parameter Z1.0, which represents the depth to sediments with shear wave velocity, Vs = 1.0 km/s. For site class D conditions (a nominal 30-m average shear wave velocity of Vs,30 = 250 m/s) the default value of Z1.0 is on the order of 300 m. Figure 8B illustrates that spectral amplitudes at CHHC for periods greater than 0.3 seconds are underpredicted using this default Z1.0 value. The fact that the thickness of gravels in the Christchurch basin is known to be greater than 500 m implies that Z1.0 would be significantly greater than 500 m. Figure 8B also illustrates the predicted spectral amplitudes, using a value of Z1.0 = 1,000 m, where it can be seen that the empirical prediction of long-period spectral amplitudes is significantly increased compared with those using Z1.0 = 300 m, in line with the observed amplitudes.

The increase in amplitude of horizontal ground motion at long periods illustrated at Christchurch hospital (CHHC) was also observed at numerous other locations in the region.

Near-source Forward Directivity In the near-source region ground motions may exhibit forward directivity effects due to the rupture front and direction of slip being aligned with the direction toward the site of interest.

While the finite fault model of Beavan et al. (2011, page 789 of this issue; see also Figure 2) does not provide information on the temporal evolution of rupture, based on the central location of the inferred hypocenter, the direction of slip is not well aligned with an elliptically inferred rupture front. As a result, it is expected that rupture directivity effects will only be important over a small surface area, relative to other possible rupture scenarios (Aagaard et al. 2004).

Figure 9A illustrates the velocity time history at Pages Road (PRPC), where forward directivity effects can be seen in the fault-normal component. Figure 9B illustrates the velocity time history at Christchurch hospital (CHHC) where a velocity pulse in the fault normal component is not clearly evident, and the large velocity amplitudes are the result of surface waves as previously noted. Figure 9C illustrates the observed and predicted pseudo-acceleration response spectra at CHHC with and without the consideration of directivity effects. The directivity effect was estimated empirically using the model of Shahi and Baker (2011). It can be seen that the predicted effect of forward directivity is relatively small (compared to the basin depth effect in Figure 8B) because of the small propagation distance from the hypocenter along the fault plane toward the site (which gives a low probability of observing a velocity pulse in the model of Shahi and Baker 2011).

The effects of near-source directivity fling step were not examined here, both because on the near-source soil sites static deformations may be the result of ground failure, and also because, as previously noted, the standard record processing adopted removes such long-period signals.

Vertical Ground Motion Figure 6 illustrates that significant vertical ground motion amplitudes were recorded in the near-source region in Christchurch, with peak vertical accelerations exceeding 0.6 g at seven strong motion stations and, in particular, values of 2.21 g and 1.88 g observed at Heathcote Valley (HVSC) and Pages Road (PRPC), respectively. The vertical acceleration time histories at these two sites also exhibit the so-called trampoline effect (Aoi et al. 2008; Yamada et al. 2009) caused by separation of surficial soil layers in tension, limiting peak negative vertical accelerations to approximately –1 g. Such large vertical accelerations can be understood physically, first due to the relatively steep dip of the fault plane (δ = 69°), which results in a large component of fault slip oriented in the vertical direction. Furthermore, at soil sites in sedimentary basins in particular, large vertical accelerations at near-source locations can result from the conversion of inclined SV waves to P waves at the sedimentary basin interface,

Seismological Research Letters Volume 82, Number 6 November/December 2011 859

(A)

100

Station:PRPC ; Rrup =2.5 km, Fault Parallel

50

0

-50

Velocity, v (cm/s)

-100

100

Fault Normal

50

0

-50

-100

100

Vertical

50

0

-50

-100

Velocity, v (cm/s)

(B)

0

5

10

15

20 Time, t (s)

25

30

80 40 0 -40 -80

35

Station:CHHC ; Rrup =3.8 km Fault Parallel

80 40 0 -40 -80 80 40 0 -40 -80 0

40

Fault Normal

Vertical

10

20

30 Time, t (s)

40

Spectral Acc, Sa (g)

(C) 10

0

10

-1

50

60

Station:CHHC Rrup=3.8 km

Z

1.0

=1000m

no directivity Z1.0=1000m

directivity considered

-2

10 -2 10

Fault Parallel Fault Normal Horiz gm 10

-1

Period, T (s)

10

0

10

1

▲▲ Figure 9. A) velocity time history recorded at Pages Road (PRPC); B) velocity time history recorded at Christchurch hospital (CHHC); and C) empirically predicted effect of directivity on spectral amplitudes at Christchurch hospital. The prediction shown is for the horizontal geometric mean and dashed lines represent the 16th and 84th percentiles.

860 Seismological Research Letters Volume 82, Number 6 November/December 2011

3 2

HPSC PRPC

Bozorgnia and Campbell (2004)

CHHC LPOC

1 0 0

Spectral acc, Sa (g)

4

(A)

B C D E Liquefaction Observed

LPCC

Firm Soil Soft Rock

10

0

10

-1

10

5 10 15 Source-to-site distance, R rup (km)

20

▲▲ Figure 10. Observed vertical-to-horizontal peak ground acceleration ratios.

which are subsequently amplified and refracted toward vertical incidence due to the basin P-wave gradient (Silva 1997).

Figure 10 illustrates the ratio of peak vertical acceleration and peak horizontal acceleration at the near-source strong motion sites, as well as the empirical model of Bozorgnia and Campbell (2004). It can be seen that peak vertical-to-horizontal ground acceleration ratios of up to 4.8 were observed. The peak vertical-to-horizontal ground acceleration ratios show a rapid decay with source-to-site distance, and the observed ratios compare favorably with the Bozorgnia and Campbell (2004) empirical model for source-to-site distances beyond 5 km but significantly underpredict the ratios at closer distances. In Figure 10, data are also differentiated by whether liquefaction was observed (as discussed previously). Almost all strong motion records at distances less than 5 km show liquefaction evidence (the exception being Heathcote Valley (HVSC)). At the aforementioned sites (with source-to-site distances less than 5 km), the large peak vertical-to-horizontal ground acceleration ratios observed are interpreted to be the result of both the aforementioned steep fault dip leading to large vertical ground motions as well as significant nonlinear soil behavior (including liquefaction), which generally results in more of a reduction in peak horizontal accelerations than peak vertical accelerations (e.g., Figure 1).

Ground Motion Intensity in the Central Business District (CBD) The Christchurch earthquake caused significant damage to commercial structures in the CBD. At the time of writing, the public access to the majority of the 2 km2 CBD is still prohibited while an estimated 1,000 structures (of various typologies, construction materials, and age) are being demolished.

The complete collapse of the Pine Gould Corporation and Canterbury Television buildings also led to the majority of the 181 casualties (New Zealand Police 2011).

Figures 11A and 11B illustrate the pseudo-acceleration and displacement response spectra of four strong motion sta-

(B)

Eq:22/02/2011 Solid:AvgHoriz Dashed:Vert

CCCC CHHC CBGS REHS NZS1170.5 -2

10

-1

Period, T (s)

10

0

10

1

70

Spectral disp, Sd (cm)

Vertical-to-horizontal PGA ratio

5

60 50 40 30 20 10 0 0

2

4 6 Period, T (s)

8

10

▲▲ Figure 11. Comparison of response spectra from four strongmotion stations located in the Christchurch central business district: A) horizontal and vertical pseudo-acceleration response spectra; and B) horizontal displacement response spectra.

tions (CCCC, CHHC, CBGS, REHS) located in the CBD region. Despite their geographic separation distances (relative to their respective source-to-site distances) it can be seen that the characteristics of the ground motion observed at these locations is relatively similar. This is particularly the case for long-period ground motion amplitudes, which have longer wavelengths and therefore are expected to be more coherent.

On the other hand, at short vibration periods there is more of a discrepancy in seismic intensity due to a shorter wavelength and therefore lower wave coherency, and probably more importantly due to the nonlinear response of significantly different surficial soil layers (Cubrinovski et al. 2011, page 893 of this issue). Figure 11A, in particular, illustrates that the strong long-period ground motion previously discussed with respect to CHHC (i.e., Figure 8B) was observed at all four CBD stations and both A and Figures 11A and 11B illustrate that the seismic demands were above the 475-year return-period-design ground motion for Christchurch site class D as specified by the New Zealand loadings standard, New Zealand Standards 1170.5 (2004). Figure 11B illustrates that for structures whose secant period at peak displacement is in the region of 1.5 or

Seismological Research Letters Volume 82, Number 6 November/December 2011 861

(A)

(B)

-1

-2

10 -2 10

Fault Parallel Fault Normal Horiz gm Vert Prediction 10

-1

Period, T (s)

(C)

Spectral Acc, Sa (g)

10

10

Spectral Acc, Sa (g)

10

10

0

10

1

1

10

0

10

-1

Station:PRPC Rrup=2.5 km

10

(D)

0

-1

-2

10

10

Station:RHSC Rrup=6.5 km

10 -2 10

(E)

Spectral Acc, Sa (g)

10

10

0

Spectral Acc, Sa (g)

10

Fault Parallel Fault Normal Horiz gm Vert Prediction 10

-1

Period, T (s)

10

0

10

-1

10

10

0

10

1

Fault Parallel Fault Normal Horiz gm Vert Prediction -2

10

-1

Period, T (s)

0

-2

-2

10

-1

Period, T (s)

10

0

Station:HVSC Rrup=4 km

10

Fault Parallel Fault Normal Horiz gm Vert Prediction -2

10

-1

Period, T (s)

10

0

10

-1

-2

10

0

10

1

10

1

Station:LPCC Rrup=7.1 km

0

-1

10

Fault Parallel Fault Normal Horiz gm Vert Prediction

(F)

1

10

Station:CACS Rrup=12.8 km

10

Spectral Acc, Sa (g)

Spectral Acc, Sa (g)

Station:CCCC Rrup=2.8 km

1

10 -2 10

Fault Parallel Fault Normal Horiz gm Vert Prediction 10

-1

Period, T (s)

10

0

10

1

▲▲ Figure 12. Horizontal and vertical pseudo-acceleration response spectra observed at various near-source strong-motion stations.

The prediction shown is for the horizontal geometric mean and dashed lines represent the 16th and 84th percentiles.

3.5 seconds, the displacement demands imposed by the ground motion were in the order of two times the seismic design level.

Response Spectra Observed at Various Strong Motion Stations Figure 12 illustrates the horizontal and vertical pseudoacceleration response spectra observed at six locations in Christchurch as well as the empirical prediction for the geometric mean component of Bradley (2010). No attempt is made here to rigorously assess the adequacy of empirical ground models against the observed near-source ground motions, and the purpose of the comparisons is simply to illustrate general features of the response at the strong motion

station that depart from that which is nominally expected.

Figure 12A illustrates the aforementioned strong long-period ground motion at Christchurch Cathedral College (CCCC) due to basin wave propagation. Figure 12B illustrates the forward directivity polarity in the ground motion at Pages Road (PRPC), with significantly higher long-period ground motion in the fault normal component, as well as a vertical pseudospectral acceleration of 6 g for a vibration period of T = 0.1 s.

Figure 12E illustrates that the high-frequency ground motion at Heathcote Valley (HVSC) is significantly above that expected due to its location on a colluvium wedge at the base of the volcanic rock Port Hills. Figure 12F also illustrates the high-frequency ground motion on bedrock at Lyttelton Port

862 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

10

10

0

Spectral acc, Sa (g)

Spectral acc, Sa (g)

10

-1

-2

04/09/2010 22/02/2011 NZS1170.5

10

-1

Period, T (s)

0

10

-1

Spectral acc, Sa (g)

(E)

-2

1

10

0

10

-1

10

10

Period, T (s)

0

10

-1

10

0

10

0

10

-1

10

1

10

Period, T (s)

-1

Period, T (s)

-2

10

0

10

1

10

0

10

1

Station:CACS Solid:AvgHoriz Dashed:Vert

04/09/2010 22/02/2011 NZS1170.5

10

-1

Period, T (s)

(F)

04/09/2010 22/02/2011 NZS1170.5 -1

04/09/2010 22/02/2011 NZS1170.5

10

10

10

0

10

1

Station:LPCC Solid:AvgHoriz Dashed:Vert

Station:HVSC Solid:AvgHoriz Dashed:Vert

-2

-2

Station:PRPC Solid:AvgHoriz Dashed:Vert

(D)

04/09/2010 22/02/2011 NZS1170.5 -1

10

10

1

Station:RHSC Solid:AvgHoriz Dashed:Vert

10

10

0

Spectral acc, Sa (g)

10

10

Spectral acc, Sa (g)

Spectral acc, Sa (g)

(C)

10

(B) 101

Station:CCCC Solid:AvgHoriz Dashed:Vert

10

0

10

-1

10

-2

04/09/2010 22/02/2011 NZS1170.5

10

-1

Period, T (s)

10

0

10

1

▲▲ Figure 13. Comparison of geometric mean horizontal and vertical pseudo-acceleration response spectra observed in the 22 February Christchurch and 4 September Darfield earthquakes at various strong-motion stations.

(LPCC) was significantly above that expected, likely due to low attenuation through the underlying volcanic rock. Note that the Bradley (2010) model accounts for the hanging wall effect (Abrahamson and Somerville 1996), which is not overly significant as a result of the steep fault dip.

COMPARISON WITH GROUND MOTIONS OBSERVED IN THE 2010 DARFIELD EARTHQUAKE AND DESIGN SPECTRA The Mw 6.3 22 February 2011 Christchurch earthquake was the second event in approximately six months to cause significant ground motion shaking in Christchurch, having been

preceded by the 4 September 2010 Darfield earthquake (New Zealand Society for Earthquake Engineeering 2010).

Figure 13 illustrates the geometric mean horizontal and vertical pseudo-acceleration response spectra of ground motions at various strong-motion stations in Christchurch resulting from both the Christchurch and Darfield earthquakes. It can be immediately seen that for the majority of vibration periods of engineering interest the spectral amplitudes are larger for the Christchurch earthquake. The primary exception to the above statement is the spectral amplitudes at long vibration periods (i.e., T > 2 s) due to both the longer duration of shaking and forward directivity effects in the Darfield earthquake. Strong long-period spectral ordinates associated

Seismological Research Letters Volume 82, Number 6 November/December 2011 863

10

0

10

-1

10

(B) Station:PPHS Solid:AvgHoriz Dashed:Vert

-2

Spectral acc, Sa (g)

Spectral acc, Sa (g)

(A)

04/09/2010 22/02/2011 NZS1170.5

10

-1

Period, T (s)

10

0

10

1

10

0

10

-1

10

Station:SMTC Solid:AvgHoriz Dashed:Vert

-2

04/09/2010 22/02/2011 NZS1170.5

10

-1

Period, T (s)

10

0

10

1

▲▲ Figure 14. Similarity of response spectral shapes of horizontal and vertical ground motions observed in the Christchurch and Darfield earthquakes at: A) Papanui (PPHS); and B) Styx Mill (SMTC).

with these phenomena in the Darfield earthquake can be clearly seen at CCCC, CACS, and HVSC stations. Figure 13A illustrates that at Christchurch Cathedral College (CCCC), which is located in the Christchurch CBD, spectral amplitudes in the Christchurch earthquake were approximately twice those of the Darfield earthquake for vibration periods less than T = 1.5 s. It can also be seen that at CCCC station, spectral amplitudes resulting from the Darfield earthquake were notably below the design spectra for T < 2 s. Figure 13C–D also illustrate that spectral amplitudes from the Darfield earthquake were below the design spectra at short periods throughout the majority of Christchurch, with exceptions being Heathcote Valley (HVSC), Lyttelton Port (LPCC), and several western suburbs (i.e., TPLC, ROLC, LINC) not shown here.

Another notable feature illustrated in Figure 13 is the similarity of the response spectral shapes at a given site from these two events. In such an examination it is important to note the markedly different source locations of the two events, with the Christchurch earthquake occurring to the southeast and the Darfield earthquake approximately 30 km west of central Christchurch. Hence, the source and path effects of the ground motion at a single site are expected to be significantly different in both events. For example, Figure 13C and 13D illustrate the similarity of response spectral shapes, for vibration periods less than T = 2 s, of both horizontal and vertical ground motion components at Riccarton (RHSC) and Canterbury Aero Club (CACS), while Figure 14 illustrates the similarities at Papanui (PPHS) and Styx Mill (SMTC). At vibration periods larger than T = 2 s, the aforementioned source effects from the Darfield earthquake become significant, and the response spectral shapes at a given site from these two events deviate.

These observations clearly point to the importance of local site effects on surface ground motions, particularly at high to moderate vibration frequencies, and hence the benefits that can be obtained via site-specific response analysis as opposed to simple soil classification (recall that most of the sites in the Christchurch basin are assigned as site class D (New Zealand

Standards 1170.5 2004)). It should also be noted that the four sites discussed above, while experiencing significant ground motions, are founded on soils that did not exhibit liquefaction.

CONCLUSIONS The 22 February 2011 Mw 6.3 Christchurch earthquake imposed severe ground motion intensities, which were in excess of the current seismic design spectra and those experienced in the 4 September 2010 Darfield earthquake, over the majority of the Christchurch region. The severe ground motion intensities resulted in significant nonlinear soil behavior and severe and widespread liquefaction, which were evident in recorded acceleration time histories.

The deep Christchurch sedimentary basin likely led to a waveguide effect of seismic waves entering through its thickening edge, which resulting in increased ground motion durations and long-period amplitudes over the majority of Christchurch.

Very large vertical accelerations were also recorded at nearsource stations, in part due to the steeply dipping fault plane, which resulted in a large component of slip oriented vertically.

In contrast, forward directivity effects were not significant over a wide region, presumably related to the relatively central location of the inferred hypocenter along-strike and down-dip and the oblique alignment of the slip and rupture front directions.

The similarity of response-spectral shapes of the ground motion observed at a single station resulting from the Christchurch and Darfield earthquakes, for which source and path effects were largely different, also illustrated the significance of site-specific response for short and moderate vibration frequencies.

ACKNOWLEDGMENTS The ground motion records utilized in this manuscript were freely obtained from the GeoNet project. Discussions with John Beavan and John Berrill are greatly appreciated.

864 Seismological Research Letters Volume 82, Number 6 November/December 2011

REFERENCES Aagaard, B. T., J. F. Hall, and T. H. Heaton (2004). Effects of fault dip and slip rake angles on near-source ground motions: Why rupture directivity was minimal in the 1999 Chi-Chi, Taiwan, earthquake.

Bulletin of the Seismological Society of America 94, 155–170.

Abrahamson, N. A., and P. G. Somerville (1996). Effects of the hanging wall and footwall on ground motions recorded during the Northridge earthquake. Bulletin of the Seismological Society of America 86, S93–99.

Aoi, S., T. Kunugi, and H. Fujiwara (2008). Trampoline effect in extreme ground motion. Science 322, 727–730.

Beavan, J., E. J. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters 82, 789–799.

Bommer, J. J., and A. Martinez-Pereira (1999). The effective duration of earthquake strong motion. Journal of Earthquake Engineering 3, 127–172.

Bozorgnia, Y., and K. W. Campbell (2004). The vertical-to-horizontal response spectral ratio and tentative procedures for developing simplified V/H and vertical design spectra. Journal of Earthquake Engineering 8, 175–207.

Bradley, B. A. (2010). NZ-specific Pseudo-spectral Acceleration Ground Motion Prediction Equations Based on Foreign Models. Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand, 324 pp.

Brown, L. J., and J. H. Weeber (1992). Geology of the Christchurch Urban Area. Institute of Geological and Nuclear Sciences map. Lower Hutt, New Zealand: GNS Science.

Chiou, B., R. Darragh, N. Gregor, and W. J. Silva (2008). NGA project strong-motion database. Earthquake Spectra 24, 23–44.

Chiou, B. S. J., and R. R. Youngs (2008). An NGA model for the average horizontal component of peak ground motion and response spectra. Earthquake Spectra 24, 173–215.

Choi, Y., J. P. Stewart, and R. W. Graves (2005). Empirical model for basin effects accounts for basin depth and source location. Bulletin of the Seismological Society of America 95, 1,412–1,427.

Cubrinovski, M., J. D. Bray, M. Taylor, S. Giorgini, B. A. Bradley, L. Wotherspoon, and J. Zupan (2011). Soil liquefaction effects in the central business district during the February 2011 Christchurch earthquake. Seismological Research Letters 82, 893–904.

Cubrinovski, M., R. A. Green, J. Allen, S. A. Ashford, E. Bowman, B. A. Bradley, B. Cox, T. C. Hutchinson, E. Kavazanjian, R. P. Orense, M. Pender, M. Quigley, and L. Wotherspoon (2010). Geotechnical reconnaissance of the 2010 Darfield (Canterbury) earthquake.

Bulletin of the New Zealand Society for Earthquake Engineering 43, 243–320.

DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein (1994). Effect of recent revisions to the geomagnetic time scale on estimates of current plate motion. Geophysical Research Letters 21, 2,191–2,194.

Douglas, J., and D. Boore (2010). High-frequency filtering of strongmotion records. Bulletin of Earthquake Engineering 9(2): 395–409.

Graizer, V. M. (2005). Effect of tilt on strong motion data processing.

Soil Dynamics and Earthquake Engineering 25, 197–204.

New Zealand Police (2011). Christchurch earthquake: List of deceased; http://www.police.govt.nz/list-deceased. Last accessed June 20, 2011.

New Zealand Society for Earthquake Engineering (NZSEE) (2010).

Preliminary observations of the 2010 Darfield (Canterbury) Earthquakes. Special issue, Bulletin of the New Zealand Society for Earthquake Engineering 43, 215–439.

Shahi, S. K., and J. W. Baker (2011). An empirically calibrated framework for including the effects of near-fault directivity in probabilistic seismic hazard analysis. Bulletin of the Seismological Society of America 101, 742–755.

Silva, W. J. (1997). Characteristics of vertical strong ground motions for applications to engineering design. In Proceedings of the FHWA/NCEER Workshop on the National Representation of Seismic Ground Motion for New and Existing Highway Facilities, Burlingame, CA. Technical Report NCEER-97-0010. Buffalo, NY: National Center for Earthquake Engineering Research.

Standards New Zealand Standards (2004). Structural Design Actions, Part 5: Earthquake Actions—New Zealand. Wellington, New Zealand: Standards New Zealand, 82 pp.

Somerville, P. G., K. Ikikura, R. W. Graves, S. Sawada, D. Wald, N. A. Abrahamson, Y. Iwasaki, T. Kagawa, N. Smith, and A. Kowada (1999). Characterizing crustal earthquake slip models for the prediction of strong ground motion. Seismological Research Letters 70, 59–80.

Stirling, M. W., M. Gerstenberger, N. Litchfield, G. H. McVerry, W. D. Smith, J. R. Pettinga, and P. Barnes (2007). Updated Probabilistic Seismic Hazard Assessment for the Canterbury Region. GNS Science Consultancy Report 2007/232, ECan Report Number U06/6, 58 pp.

Sutherland, R., K. Berryman, and R. Norris (2006). Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand. Geological Society of America Bulletin 118, 464–474.

Yamada, M., J. Mori, and T. Heaton (2009). The slapdown phase in highacceleration records of large earthquakes. Seismological Research Letters 80, 559–564.

Zhao, J. X., and M. Gerstenberger (2010). Attenuation Models for Rapid Post Earthquake Assessment in New Zealand. Wellington, New Zealand: Earthquake Commission New Zealand report.

Department of Civil and Natural Resources Engineering University of Canterbury Private Bag 4800 Christchurch, New Zealand brendon.bradley@canterbury.ac.nz

(B A. B.)

Seismological Research Letters Volume 82, Number 6 November/December 2011 865

E

Ground Motion Attenuation during M 7.1 Darfield and M 6.2 Christchurch, New Zealand, Earthquakes and Performance of Global Predictive Models Margaret Segou and Erol Kalkan

Margaret Segou and Erol Kalkan U.S. Geological Survey

Online material: Flat-file for both events

INTRODUCTION The M 7.1 Darfield earthquake occurred 40 km west of Christchurch (New Zealand) on 4 September 2010. Six months after, the city was struck again with an M 6.2 event on 22 February local time (21 February UTC). These events resulted in significant damage to infrastructure in the city and its suburbs. The purpose of this study is to evaluate the performance of global predictive models (GMPEs) using the strong motion data obtained from these two events to improve future seismic hazard assessment and building code provisions for the Canterbury region.

The Canterbury region is located on the boundary between the Pacific and Australian plates; its surface expression is the active right lateral Alpine fault (Berryman et al.

1993). Beneath the North Island and the north South Island, the Pacific plate subducts obliquely under the Australian plate, while at the southwestern part of the South Island, a reverse process takes place. Although New Zealand has experienced several major earthquakes in the past as a result of its complex seismotectonic environment (e.g., M 7.1 1888 North Canterbury, M 7.0 1929 Arthur’s Pass, and M 6.2 1995 Cass), there was no evidence of prior seismic activity in Christchurch and its surroundings before the September event. The Darfield and Christchurch earthquakes occurred along the previously unmapped Greendale fault in the Canterbury basin, which is covered by Quaternary alluvial deposits (Forsyth et al. 2008).

In Figure 1, site conditions of the Canterbury epicentral area are depicted on a VS30 map. This map was determined on the basis of topographic slope calculated from a 1-km grid using the method of Allen and Wald (2007). Also shown are the locations of strong motion stations.

The Darfield event was generated as a result of a complex rupture mechanism; the recordings and geodetic data reveal

that earthquake consists of three sub-events (Barnhart et al.

2011, page 815 of this issue). The first event was due to rupturing of a blind reverse fault with M 6.2, followed by a second event (M 6.9), releasing the largest portion of the energy on the right-lateral Greendale fault. The third sub-event (M 5.7) is due to a reverse fault with a right-lateral component (Holden et al. 2011). The Christchurch earthquake occurred on an oblique thrust fault. The comparison of spectral acceleration values at stations near Christchurch reveals that the second event produced much larger amplitudes of shaking than the Darfield event due to its proximity to the epicenter. Both events resulted in noticeably large amplitudes of the vertical motion, often exceeding horizontal motion in the near-fault area. The vertical motions, showing asymmetric acceleration traces and pulses, reached 1.26 g during the Darfield earthquake and 2.2 g during the Christchurch event.

These events were recorded by more than 100 strong motion stations operated by the Institute of Geological and Nuclear Sciences (http://www.geonet.org.nz/). Using the processed data from these stations, peak ground acceleration (PGA) and 5%-damped spectral acceleration values at 0.3, 1, and 3 s are used for performance evaluation of the global ground motion predictive equations (GMPEs). The selected GMPEs are the Next Generation Attenuation (NGA) models of Abrahamson and Silva (2008), Boore and Atkinson (2008), Campbell and Bozorgnia (2008), and Chiou and Youngs (2008). The Graizer and Kalkan (2007, 2009) model, which is based on the NGA project database, is also included. These GMPEs are abbreviated respectively as AS08, BA08, CB08, CY08, and GK07.

Because they have been used widely for seismic hazard analysis for crustal earthquakes, their performance assessment becomes a critical issue especially for immediate response and recovery planning after major events. The occurrence of aftershocks similar to the Christchurch event will most probably control seismic hazard in the broader area, as confirmed by the recent M 6.0 event on June 13, 2011.

866 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.866

▲▲ Figure 1. Shear-wave velocity (unit = m/s) down to 30 m derived from topographic slope; the locations of strong motion stations are also shown.

PERFORMANCE EVALUATION OF GROUND MOTION PREDICTION EQUATIONS In order to evaluate the relative performance of the GMPEs and their ranking to be used for logic tree weighting in hazard analysis, we used traditional residual analysis and an information theoretic approach. In residual analysis the prediction error for each observation and standard deviation of the errors for each event are computed for each GMPE. Residuals correspond to the difference between the observations and predictions in natural-log space; negative residuals are interpreted as overprediction, whereas positive residuals indicate underestimation of the predictive model. The applied information theoretic approach is based on a log-likelihood value (LLH), which describes the information loss when a GMPE approximates an observation (Scherbaum et al. 2009). The average sample loglikelihood (LLH) value of a GMPE, noted herein as g, over N number of x observations, represented by a log-normal distribution, is calculated as: log ( L( g x ) ) = −

N

1 log ( g ( x i ) ) N∑ i=1

(1)

The negative average log-likelihood value is a measure of distance between the predictions and observations; therefore, a GMPE exhibiting a smaller absolute value of LLH, relative to other GMPEs, corresponds to a better performing model.

For the Darfield event, the relative performance of GMPEs was evaluated for strike slip faulting since the greater amount

of moment release occurred during the second sub-event. For the Christchurch event, however, the evaluation is based on the thrust fault (as discussed before). The hanging wall effects were considered, although their effects are not significant because the causative faults appear to be steep (Bradley and Cubrinovski 2011, page 853 of this issue). The flat-file, listing distance metrics, VS30 for each station, and corresponding observations (PGA and spectral values), is provided for each event as an electronic supplement (http://nsmp.wr.usgs.gov/ekalkan/NZ/index.html).

Using this flat-file, predictions of GMPEs were computed for each event. Figures 2–5 summarize the results for the M 7.1 Darfield (top panels) and M 6.2 Christchurch (bottom panels) earthquakes. The plots shown in row A in each figure represent 16th, 50th (median), and 84th percentile of predictions considering an average VS30 value of 400 m/s. In these plots, observations correspond to the maximum value of two horizontal components. Because the NGA models predict geometric mean of ground motion, their predictions were adjusted for maximum horizontal component by multiplying their predictions with 1.1 for PGA and 1.15, 1.18, and 1.18 for spectral acceleration at 0.3 s, 1 s, and 3 s, respectively. These adjustment factors were adapted from Campbell and Bozorgnia (2008). The GK07 model predicts the maximum of the two horizontal components. It should be also noted that both observations and predictions are plotted against a distance metric specific to the model; for the BA08, the distance metric is the “Joyner-Boore distance” (RJB), defined as the closest distance from the recording station to the surface projection of the fault rupture plane (Boore et al. 1997). For the remaining models, the distance measure is the “closest fault dis-

Seismological Research Letters Volume 82, Number 6 November/December 2011 867

Graizer and Kalkan (2007, 2009)

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

Peak Ground Acceleration, (g)

2

[A]

1 0.5

0.1

0.01

M7.1 Darfield Eq. 0.001 5

σ

= 0.52

[B]

σ

= 0.81

[C]

In[PGA]Actual − In[PGA]GMPE

In[Y]

σ

= 0.58

σ

= 0.52

σ

= 0.63

σ

= 0.52

σ

= 0.97

σ

= 0.75

σ

= 1.14

σ

= 0.91

In[Y]

In[Y]

In[Y]

In[Y]

0

−5 8 LLH

LLH

LLH

LLH

LLH

LLH value

6

4

2

0 5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

250

5

10

20

50

100

250

5

10

20

50

100

250

Distance Measure (km)

Graizer and Kalkan (2007, 2009)

Peak Ground Acceleration, (g)

2

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

[A]

1 0.5

0.1

0.01

M6.2 Christchurch Eq.

0.001

In[PGA]Actual − In[PGA]GMPE

5

σIn[Y] = 0.60

[B]

σIn[Y] = 0.61

σIn[Y] = 0.63

σIn[Y] = 0.77

σIn[Y] = 0.53

σLLH = 1.12

[C]

σLLH = 0.93

σLLH = 1.42

σLLH = 2.82

σLLH = 0.94

0

−5 8

LLH value

6

4

2

0

5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

Distance Measure (km)

250

5

10

20

50

100

250

5

10

20

50

100

250

▲▲ Figure 2. A) Comparison of PGA values recorded from the M 7.1 Darfield (top panels) and M 6.2 Christchurch (bottom panels) earthquakes for 16th, 50th (median), and 84th percentile predictions from five different GMPEs. B) Residuals computed for each GMPE for median prediction; also shown is the trend line to quantify distance bias. C) Average-log likelihood (LLH) values are to determine performance of GMPEs; higher LLH values indicate poorer performance.

868 Seismological Research Letters Volume 82, Number 6 November/December 2011

Graizer and Kalkan (2007, 2009) Spectral Acceleration (0.3 s), (g)

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

[A]

2 1 0.5

0.1

0.01

M7.1 Darfield Eq. 0.001

In[SA(0.3 s)]Actual − In[SA(0.3 s)]GMPE

5

σIn[Y] = 0.59

[B]

σIn[Y] = 0.69

σIn[Y] = 0.63

σIn[Y] = 0.71

σIn[Y] = 0.63

σLLH = 0.87

[C]

σLLH = 1.00

σLLH = 0.86

σLLH = 1.13

σLLH = 1.01

0

−5 8

LLH value

6

4

2

0 5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

250

5

10

20

50

100

250

5

10

20

50

100

250

Distance Measure (km)

Spectral Acceleration (0.3 s), (g)

Graizer and Kalkan (2007, 2009)

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

[A]

2 1 0.5 0.1

0.01

M6.2 Christchurch Eq.

In[SA(0.3 s)]Actual − In[SA(0.3 s)]GMPE

0.001 5

σIn[Y] = 0.63

[B]

σIn[Y] = 0.68

σIn[Y] = 0.72

σIn[Y] = 0.80

σIn[Y] = 0.62

σLLH = 1.03

[C]

σLLH = 1.00

σLLH = 1.70

σLLH = 2.29

σLLH = 0.99

0

−5 8

LLH value

6

4

2

0

5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

Distance Measure (km)

250

5

10

20

50

100

250

5

10

20

50

100

250

▲▲ Figure 3. A) Comparison of 5%-damped spectral acceleration values computed at 0.3 s for the M 7.1 Darfield (top panels) and M 6.2 Christchurch (bottom panels) earthquakes for 16th, 50th (median), and 84th percentile predictions from five different GMPEs. B) Residuals computed for each GMPE for median prediction; also shown is the trend line to quantify distance bias. C) Average-log likelihood (LLH) values are to determine performance of GMPEs; higher LLH values indicate poorer performance.

Seismological Research Letters Volume 82, Number 6 November/December 2011 869

Graizer and Kalkan (2007, 2009)

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

Spectral Acceleration (1 s), (g)

2

[A]

1 0.5

0.1

0.01

M7.1 Darfield Eq. 0.001

In[SA(1 s)]Actual − In[SA(1 s)]GMPE

5

σIn[Y] = 0.61

[B]

σIn[Y] = 0.62

σIn[Y] = 0.60

σIn[Y] = 0.60

σIn[Y] = 0.60

σLLH = 0.53

[C]

σLLH = 0.52

σLLH = 0.46

σLLH = 0.53

σLLH = 0.58

0

−5 4

LLH value

3

2

1

0 5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

250

5

10

20

50

100

250

5

10

20

50

100

250

Distance Measure (km)

Graizer and Kalkan (2007, 2009)

Spectral Acceleration (1 s), (g)

2

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

[A]

1 0.5

0.1

0.01

M6.2 Christchurch Eq.

0.001

In[SA(1 s)]Actual − In[SA(1 s)]GMPE

5

σIn[Y] = 0.60

[B]

σIn[Y] = 0.71

σIn[Y] = 0.73

σIn[Y] = 0.73

σIn[Y] = 0.64

σLLH = 0.68

[C]

σLLH = 0.89

σLLH = 1.13

σLLH = 1.29

σLLH = 0.79

0

−5 4

LLH value

3

2

1

0

5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

Distance Measure (km)

250

5

10

20

50

100

250

5

10

20

50

100

250

▲▲ Figure 4. A) Comparison of 5%-damped spectral acceleration values computed at 1 s for the M 7.1 Darfield (top panels) and M 6.2 Christchurch (bottom panels) earthquakes for 16th, 50th (median), and 84th percentile predictions from five different GMPEs. B) Residuals computed for each GMPE for median prediction; also shown is the trend line to quantify distance bias. C) Average-log likelihood (LLH) values are to determine performance of GMPEs; higher LLH values indicate poorer performance.

870 Seismological Research Letters Volume 82, Number 6 November/December 2011

Graizer and Kalkan (2007, 2009)

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

Spectral Acceleration (3 s), (g)

1

[A]

0.5

0.1

0.01

M7.1 Darfield Eq. 0.001

In[SA(3 s)]Actual − In[SA(3 s)]GMPE

5

σIn[Y] = 0.65

[B]

σIn[Y] = 0.69

σIn[Y] = 0.71

σIn[Y] = 0.68

σIn[Y] = 0.69

σLLH = 0.45

[C]

σLLH = 0.65

σLLH = 0.63

σLLH = 0.77

σLLH = 0.80

0

−5 4

LLH value

3

2

1

0 5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

250

5

10

20

50

100

250

5

10

20

50

100

250

Distance Measure (km)

Graizer and Kalkan (2007, 2009)

Spectral Acceleration (3 s), (g)

1

Abrahamson and Silva (2008)

Boore and Atkinson (2008)

Campbell and Bozorgnia (2008)

Chiou and Youngs (2008)

[A]

0.5

0.1

0.01

M6.2 Christchurch Eq.

0.001

In[SA(3 s)]Actual − In[SA(3 s)]GMPE

5

= 0.57

[B]

σLLH = 0.63

[C]

σ

In[Y]

σ

In[Y]

= 0.65

σ

In[Y]

= 0.75

σ

In[Y]

= 0.67

σ

In[Y]

= 0.61

0

−5 4

σLLH = 0.68

σLLH = 0.83

σLLH = 0.78

σLLH = 0.59

LLH value

3

2

1

0

5

10

20

50

100

250

5

10

20

50

100

250

1

5

10

20

50

100

Distance Measure (km)

250

5

10

20

50

100

250

5

10

20

50

100

250

▲▲ Figure 5. A) Comparison of 5%-damped spectral acceleration values computed at 3 s for the M 7.1 Darfield (top panels) and M 6.2 Christchurch (bottom panels) earthquakes for 16th, 50th (median), and 84th percentile predictions from five different GMPEs. B) Residuals computed for each GMPE for median prediction; also shown is the trend line to quantify distance bias. C) Average-log likelihood (LLH) values are to determine performance of GMPEs; higher LLH values indicate poorer performance.

Seismological Research Letters Volume 82, Number 6 November/December 2011 871

tance” (Rrup) defined as the closest distance to co-seismic rupture plane.

The next set of plots in Figures 2–5 (row B) shows the distance distribution of residuals. Linear fit lines illustrate the distance bias; the trend line passing through zero means that there is no bias in predictions. Unlike the attenuation curves shown in row A, based on average VS30, the residuals are computed based on specific VS30 values at each station estimated from topographic slope (Figure 1) in order to explicitly incorporate the site effects on ground motion estimates. To quantify the quality of fit, the standard errors of predictions (σInY) are computed based on residuals, and these values are given in each panel for each GMPE. The larger σInY indicates a poorer performance of the GMPE. For PGA, all GMPEs indicate an overall good fit to observations up to ~100 km (Figure 2 row A); for distances larger than 100 km, ground motion exhibits faster attenuation, and as a result the observed peak values are lower than expected. This is more pronounced for the Christchurch event. Low ground accelerations recorded at large distances show the effect of the anelastic attenuation due to regional low Q (Zhao and Gerstenberger 2010).

In Figure 2 (row B), residuals for the Darfield event reveal overestimation for distances greater than 70 km for the AB08 and 100 km for the BA08 and CB08 models. On the other hand, the GK07 and CY08 fit better to the observations because their trend lines fitting to residuals do not show a notable distance bias. For the AS08 and CB08, the misfit at larger distances is more evident. In case of PGA, the GK07, BA08, and CY08 models yield the smallest σInY of 0.52 for the Darfield event. For the Christchurch event (Figure 2 bottom panels), the GK07, AS08, BA08, and CY08 present better residual fits than CB08, for which the overestimation begins at 20 km. The smallest σInY is due to the CY08. The same is true for spectral acceleration at 0.3 s as shown in Figure 3 (row B). Finally, Figure 2 (row C) shows the distance distribution of LLH values. In these plots, trend lines identify the consistency of the GMPE in predicting ground motion at various distances; if the slope is close to zero, then GMPE has low distance variability, meaning that it is consistent. Much higher LLH values with increasing distance suggest a poorer fit at far-field, which is observed for all GMPEs for the Christchurch event.

As shown in Figure 3, spectral acceleration at 0.3 s reaches 1 g at 40 km for the Darfield earthquake, and 2 g at 10 km for the Christchurch earthquake. For the Darfield event, the AS08 and CB08 overestimate observations for distances larger than 70 km as shown by the residual plots. The same trend is evident for the Christchurch event (Figure 3 bottom panels).

The BA08 performs better for the Darfield event since there is only a minor overestimation for distances larger than 20 km.

For the Christchurch event (Figure 3 bottom panels), however, the distance trend line of LLH reveals a poorer performance of the BA08 and CB08.

Ground motion estimates are given for spectral acceleration at 1 s in Figure 4, where the observations exceed 1 g in the near field of both earthquakes. For Darfield, all GMPEs present an excellent fit to the observations up to 70 km from the fault.

Beyond 70 km, they slightly overestimate the observations due to faster attenuation of ground motion at far distances. For the Christchurch event, the overestimation is evident for the BA08 and CB08 over 50 km, which resulted in higher LLH values.

In Figure 5 (top panels), comparisons are given for the spectral acceleration at 3 s. For both Darfield and Christchurch events, spectral peaks reach 0.5 g. For the former event, the GK07 is the best fitting model to observations with zero distance bias and with lowest LLH values. The NGA models underestimate long period ground motions up to 100 km, whereas beyond 100 km they tend to overestimate. For the Christchurch earthquake, none of the models provide an excellent fit (Figure 5 bottom panels). The GK07 overestimates observations, as opposed to underestimation of NGA models over a wide distance range.

RANKING GROUND MOTION PREDICTION EQUATIONS In Table 1, the mean (μLLH) and standard deviation (σLLH) of LLH values over the total number of observations for each earthquake and for each GMPE are tabulated; the standard errors (σInY) of predictions are also listed. This table is used for ranking the GMPEs according to these three parameters. For ranking, each parameter is first normalized with the respect to its lowest value due to different GMPEs, and then the arithmetic-mean of normalized values is computed for μ LLH, σLLH, and σInY for each GMPE. The GMPE with the lowest arithmeticmean is ranked as first. This exercise is repeated for each period and for each earthquake, and the results of ranking are given in Table 2. The best performing GMPE has a combined performance value close to unity. The ranking results show that performance of the GK07, BA08, and CY08 are equally the same for different periods and events, while the CB08 and AS08 show relatively poorer performance.

CONCLUDING REMARKS In this study, we examined the performance of global ground motion prediction equations (GMPEs) for the New Zealand M 7.1 Darfield and M 6.2 Christchurch earthquakes, with the objective of improving future seismic hazard assessment and engineering applications for the Canterbury region. These events are characterized by significantly large ground motions at high frequencies, which showed faster attenuation through the crust due to low regional Q. Amplified spectral accelerations at long periods at long distances are attributed to Canterbury’s deep sedimentary basin.

For similar shallow earthquakes in New Zealand, there is an evidence of Moho reflection, which potentially might further amplify long-period ground motions (Zhao and Gerstenberger 2010). Comparison of predictions derived from the five different GMPEs with observations reveal overall good performance of these models, supporting their applicability for the region.

For the purpose of selecting and weighting GMPEs in a logic tree approach for regional seismic hazard analysis, we applied a simple ranking procedure based on the average LLH values

872 Seismological Research Letters Volume 82, Number 6 November/December 2011

TABLE 1 Mean (μLLH) and standard deviation (σLLH) of average log-likelihood (LLH) values, and standard error of predictions (σInY ).

GK07: Graizer and Kalkan (2007, 2009); AS08: Abrahamson and Silva (2008); BA08: Boore and Atkinson (2008); CB08: Campbell and Bozorgnia (2008); CY08: Chiou and Youngs (2008). μLLH GK07

σLLH

σInY

AS08 BA08 CB08

CY08

GK07

AS08 BA08 CB08

CY08

GK07

AS08 BA08 CB08

CY08

0.84 1.04 0.93 1.06

0.80 0.95 0.90 1.09

0.96 1.10 0.91 1.11

0.73 0.94 0.95 1.23

0.80 0.86 0.52 0.45

0.96 0.99 0.52 0.65

0.74 0.85 0.46 0.63

1.13 1.12 0.53 0.76

0.90 1.00 0.58 0.80

0.52 0.59 0.61 0.65

0.58 0.69 0.62 0.69

0.52 0.63 0.60 0.71

0.63 0.71 0.60 0.68

0.52 0.63 0.60 0.69

0.91 1.04 1.11 0.98

1.12 1.02 0.67 0.63

0.92 0.99 0.89 0.67

1.41 1.69 1.12 0.82

1.12 1.03 0.68 0.63

0.93 1.00 0.89 0.68

1.42 1.70 1.13 0.83

2.82 2.29 1.29 0.78

0.94 0.99 0.79 0.59

0.61 0.68 0.71 0.65

0.63 0.72 0.73 0.75

0.77 0.80 0.73 0.67

0.53 0.62 0.64 0.61

0.61 0.68 0.71 0.65

M 7.1 Darfield PGA SA (0.3s) SA (1 s) SA (3 s)

0.77 0.91 0.93 1.02

M 6.2 Christchurch PGA SA (0.3s) SA (1 s) SA (3 s)

0.98 1.01 1.00 1.17

TABLE 2 Combined performance parameters of ground motion prediction equations (GMPEs; GMPE with the lowest performance parameter can be interpreted as better performing one (shown by bold).

Combined Performance Parameter

GK07

AS08

BA08

CB08

CY08

1.05 1.00 1.07 1.00

1.19 1.16 1.07 1.18

1.04 1.04 1.00 1.19

1.35 1.24 1.06 1.28

1.07 1.09 1.11 1.35

1.17 1.04 1.01 1.09

1.08 1.07 1.21 1.10

1.37 1.46 1.39 1.30

2.36 1.85 1.49 1.18

1.00 1.00 1.08 1.02

M 7.1 Darfield PGA SA (0.3s) SA (1 s) SA (3 s) M 6.2 Christchurch PGA SA (0.3s) SA (1 s) SA (3 s)

considering their mean and standard deviation, as well as standard errors (σInY) of predictions. The ranking results show that performance of GK07 (Graizer and Kalkan 2007, 2009), BA08 (Boore and Atkinson 2008), and CY08 (Chiou and Youngs 2008) perform equally well, while the CB08 (Campbell and Bozorgnia 2008) and AS08 (Abrahamson and Silva 2008) show relatively poorer performance.

ACKNOWLEDGMENTS The authors thank Jim Cousins (GNZ) for providing parameters of strong motion recordings, and Volkan Sevilgen for preparing the VS30 map for the Canterbury region. We also wish

to thank David Boore, Tom Hanks, and Vladimir Graizer for their reviews.

DATA AND RESOURCES For NGA models, we used the Fortran code written by David Boore (Kaklamanos et al. 2010); for the Graizer and Kalkan (2007, 2009) ground motion prediction model, Matlab and Fortran codes are available online at http://nsmp.wr.usgs.gov/ ekalkan/PGA07/index.html . The flat-file for both Darfield and Christchurch events is also available online at http://nsmp. wr.usgs.gov/ekalkan/NZ/index.html .

Seismological Research Letters Volume 82, Number 6 November/December 2011 873

REFERENCES Abrahamson, N. A., and W. J. Silva (2008). Summary of the Abrahamson and Silva NGA ground motion relations. Earthquake Spectra 24 (1), 67–98.

Allen, T. I., and D. J. Wald. (2007). Topographic Slope as a Proxy for Seismic Site Conditions (Vs30) and Amplification around the Globe.

USGS Open-File Report 2007-1357, 69 pp.; http://earthquake. usgs.gov/hazards/apps/vs30/.

Barnhart, W. D., M. J. Willis, R. W. Lohman, and A. K. Melkonian (2011). InSAR and optical constraints on fault slip during the 2010–2011 New Zealand earthquake sequence. Seismological Research Letters 82, 815–823.

Berryman, K. R., S. Beanland, A. F. Cooper, H. N. Cutten, R. J. Norris, and P. R. Wood (1993). The Alpine fault, New Zealand: Variation in Quaternary structural style and geomorphic expression. Annales Tectonicae 6, special issue supplement, S126–163.

Boore, D. M., and G. M. Atkinson (2008). Ground motion prediction equations for the average horizontal component of PGA, PGV, and 5%-damped PSA at spectral periods between 0.01 s and 10.0 s.

Earthquake Spectra 24 (1), 99–138.

Boore, D. M., W. B. Joyner, and T. E. Fumal (1997). Equations for estimating horizontal response spectra and peak acceleration from western North American earthquakes: A summary of recent work.

Seismological Research Letters 68, 128–153.

Bradley, B. A., and M. Cubrinovski (2011). Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake. Seismological Research Letters 82, 853–865.

Campbell, K. W., and Y. Bozorgnia (2008). NGA ground motion model for the geometric mean horizontal component of PGA, PGV, PGD, and 5% damped linear elastic response spectra for periods ranging from 0.01 to 10 s. Earthquake Spectra 24 (1), 139–172.

Chiou, B. S. J, and R. R. Youngs (2008). An NGA model for the average horizontal component of peak ground motion and response spectra. Earthquake Spectra 24 (1), 173–215.

Forsyth, P. J., D. J. A. Barrell, and R. Jongens (2008). Geology of the Christchurch Area. Institute of Geological and Nuclear Sciences

1;250,000 geological map 16, 1 sheet + 67 pp. Lower Hutt, New Zealand: GNS Science.

Graizer, V., and E. Kalkan (2007). Ground motion attenuation model for peak horizontal acceleration from shallow crustal earthquakes.

Earthquake Spectra 23, 585–613.

Graizer V., and E. Kalkan (2009). Prediction of response spectral acceleration ordinates based on PGA attenuation. Earthquake Spectra 25 (1), 36–69.

Holden, C., J. Beavan, B. Fry, M. Reyners, J. Ristau, R. Van Dissen, P. Villamor, and M. Quigley (2011). Preliminary source model of the Mw 7.1 Darfield earthquake from geological, geodetic and seismic data. Proceedings of the Ninth Pacific Conference on Earthquake Engineering: Building an Earthquake-Resilient Society, 14–16 April 2011, paper no. 164. Auckland, New Zealand: New Zealand Society for Earthquake Engineering.

Kaklamanos, J., D. M. Boore, E. M. Thomson, and K. W. Campbell (2010). Implementation of the Next Generation Attenuation (NGA) Ground-motion Prediction Equations in Fortran and R. USGS Open-File Report 2010-1296, 47 pp.

Scherbaum, F., E. Delavaud, and C. Riggelsen (2009). Model selection in seismic hazard analysis: An information-theoretic perspective.

Bulletin of the Seismological Society of America 99, 3,234–3,247.

Zhao, J. X., and M. Gerstenberger (2010). Comparison of attenuation characteristics between the data from two distant regions.

Proceedings of the Ninth Pacific Conference on Earthquake Engineering: Building an Earthquake-Resilient Society, 14–16 April 2011, Auckland, New Zealand. Paper no. 008.

874 Seismological Research Letters Volume 82, Number 6 November/December 2011

Earthquake Science Center U.S. Geological Survey 345 Middlefield Road Menlo Park, California 94025 U.S.A. msegkou@usgs.gov

(M.S.)

Strong Ground Motions and Damage Conditions Associated with Seismic Stations in the February 2011 Christchurch, New Zealand, Earthquake Hiroaki Iizuka, Yuki Sakai, and Kazuki Koketsu

Hiroaki Iizuka,1Yuki Sakai,1 and Kazuki Koketsu2

INTRODUCTION The February 2011 Christchurch, New Zealand, earthquake was highly destructive, causing a number of buildings to collapse and killing many people. We examined the properties of strong ground motions in this earthquake using the records released by GeoNet (http://www.geonet.org.nz/). We also investigated the damage around the seismic stations to determine the relationship between structural damage and strong ground motions.

SEISMIC GROUND MOTION INTENSITIES AND ELASTIC RESPONSE SPECTRUM The locations of the seismic stations in our study are shown in Figure 1. Accelerograms and the elastic acceleration response spectra, with a damping factor of 0.05 in the maximum horizontal direction, are shown in Figures 2 and 3, respectively. Peak ground accelerations (PGA) and peak ground velocities (PGV) are shown in Table 1. Ij and I1–2 are also shown in Table 1. Ij is JMA (Japan Meteorological Agency) seismic intensity (Tables 2, 3 and 4). It is publicly used to describe the damaging power of seismic shaking in Japan. I1–2 is also an index like Ij. It was defined by Sakai, Kanno, and Koketsu (2002, 2004) based on elastic responses between 1 and 2 seconds period that were closely related with heavy structural damage (the subscript 1–2 means between 1 and 2 seconds) and represents the damaging power of an earthquake much better than Ij. As shown in Figures 2 and 3, the records of stations REHS, CCCC, and PRPC display pulse waves with a period of 1–2 seconds and have high response in the region of 1–2 seconds, whereas stations HVSC and LPCC with large PGA are dominated by short periods below 1 second, and their responses between 1 and 2 seconds are low. We compared REHS’s spectrum, which shows the highest response in the 1. Graduate School of Systems and Information Engineering, University of Tsukuba, Japan 2. Earthquake Research Institute, University of Tokyo, Japan doi: 10.1785/gssrl.82.6.875

▲▲ Figure 1. Locations of the seismic stations.

1–2-second period, with those at Takatori, Fukiai, and JMA Kobe recorded in the 1995 Kobe earthquake, which devastated the city of Kobe and the surrounding region (Figure 4). REHS’s response in the 1–2-second period is similar to that of JMA Kobe, but it is lower than that of Takatori and Fukiai.

NONLINEAR SEISMIC RESPONSE ANALYSIS We performed a nonlinear seismic response analysis with a single-degree-of-freedom system, accounting for Japanese reinforced concrete (RC) buildings (Kumamoto and Sakai 2007) and wooden houses (Sakai and Iizuka 2009) and compared that data with REHS’s record, which shows the highest elastic response in the period of 1–2 seconds. We adopted the Takeda Model (Figure 5A; Takeda et al. 1970) for RC build-

Seismological Research Letters Volume 82, Number 6 November/December 2011 875

▲▲ Figure 2. Accelerograms.

▲▲ Figure 3. Elastic acceleration response spectrum.

876 Seismological Research Letters Volume 82, Number 6 November/December 2011

ings and the Modified Takeda-Slip Model (Figure 5B; Iizuka and Sakai 2009) for wooden houses as hysteresis models. We set these model parameters, as shown in Table 5, to assume general buildings and houses and an allowable ductility factor of 6, given the heavy damage to buildings (Sakai, Koketsu and Kanno 2002; Sakai and Iizuka 2009).

Figure 6 shows the required strength (base shear coefficient) spectrum of REHS compared with that of Takatori, Fukiai, and JMA Kobe. The required strength figure for REHS is higher than that of the JMA Kobe in the period longer than 0.4 seconds. This shows that the ground motion recorded by REHS causes heavy damage to both RC buildings and wooden houses. The strength of RC buildings based on design standards established in New Zealand in 1965 (Figure 6A, thin line; Davenport 2004) is also shown in Figure 6. These standards are much lower than required by the spectrum recorded by REHS. This difference explains the fact that some RC buildings built before 1976 collapsed in the February 2011 Christchurch, New Zealand, earthquake.

DAMAGE INVESTIGATION AROUND THE SEISMIC STATIONS We carried out damage investigations around the seismic stations shown in Figure 1 to examine the relationship between structural damage and strong ground motions. Our investigations, which took place on March 11 and 12, 2011, encompassed all buildings within 200 m of the stations. We recorded type of structure, number of stories, and level of damage (heavy or not) (Okada and Takai 1999; Architectural Institute of Japan 1980) and calculated a structural damage ratio based on the collected data.

Damage conditions are shown in Table 1. Building distribution and related damage levels around the seismic stations is shown in Figure 7. Because it was sometimes difficult to distinguish wood from masonry houses by appearance, we noted “wood or masonry” in Figure 7. Around REHS, there were many buildings and houses that were heavily damaged, as shown in Figure 8. Roof tiles and exterior materials on several houses were damaged. Around HVSC, there were some buildings with damage to roof tiles or exterior materials, and some masonry structures were heavily damaged. Some houses outside the 200-m study area were also heavily damaged. Around PRPC, many houses displayed damaged roof tiles, but there were no heavily damaged buildings. LPCC seismic station is located in the port and surrounded by the sea, so there are few structures surrounding it. There are some houses located on a cliff to the northeast of the station, but we assumed that their location would have suffered ground motion that differed from that of the seismic station. We were unable to determine an accurate ground position for CCCC because Catholic Cathedral College, where the seismic station is located, had been closed and a restricted zone had been established on its western side. Therefore, we investigated only the area on the north side of the college. Most of the buildings in the area are warehouses or stores, with few residences. We did note, however, that the roof of the one of the college buildings was damaged and there was a heavily damaged store nearby. Around HPSC and SHLC, the vast majority of structures displayed only insignificant damage, and there were no buildings with heavy damage. We also observed many sand boils resulting from liquefaction.

We observed heavily damaged buildings around REHS and CCCC, where the response between 1 and 2 seconds was

TABLE 1 Seismic Intensity of Strong Ground Motions and Damage Conditions around the Seismic Station.

Seismic Station

Ij *

I1–2 †

PGA [cm/s 2]

PGV [cm/s] Damage Situation within 200m

REHS HVSC

6.20 6.42

6.19 5.66

723.0 1860.0

97.7 100.4

PRPC LPCC CCCC HPSC SHLC

6.10 5.77 5.98 5.52 5.76

6.00 5.01 6.04 5.31 5.59

737.0 1070.2 472.5 324.9 363.6

124.2 48.0 68.2 49.9 77.1

Many buildings were heavy damaged.

Destruction of roofing tiles and exterior materials.

Some masonry buildings were broken.

Many buildings were damaged at roofing tiles.

The ground damage of cliff.

Some buildings were heavy damaged.

Destruction of exterior materials Liquefaction.

Crack of RC structure’s pillars.

Liquefaction.

— — —

CHHC 5.79 5.75 460.0 82.7 CMHS 5.77 5.65 415.3 50.2 CBGS 5.83 5.71 653.3 73.5 * Ij : JMA seismic intensity † I1–2 : Seismic intensity based on elastic responses between 1 and 2 second period ‡ It is the reference values because the building was few. § It is the reference values because the damageed building was on the cliff. || It is the reference values because the seismic station position was uncertain.

Number of Buildings

Damage Ratio [%]

98 21

15.3 9.1‡

90 17 23 68 113

0 5.9 § 8.7|| 0 0

— — —

— — —

Seismological Research Letters Volume 82, Number 6 November/December 2011 877

TABLE 2 Explanation of JMA seismic intensity scales (http://www.jma.go.jp/) for human perception and reaction; indoor situations and outdoor situations Seismic Intensity Ij Ij < 0.5 0.5 ≤ Ij < 1.5 1.5 ≤ Ij < 2.5 2.5 ≤ Ij < 3.5 3.5 ≤ Ij < 4.5

4.5 ≤ Ij < 5.0

5.0 ≤ Ij < 5.5

Human Perception and Reaction Imperceptible to people, but recorded by seismometers.

Felt slightly by some people keeping quiet in buildings.

Felt by many people keeping quiet in buildings. Some people may be awoken.

Felt by most people in buildings. Felt by some people walking. Many people are awoken.

Most people are startled. Felt by most people walking. Most people are awoken.

Outdoor Situation

—

—

—

—

Hanging objects such as lamps swing slightly.

—

Dishes in cupboards may rattle.

Electric wires swing slightly.

Hanging objects such as lamps swing significantly, and dishes in cupboards rattle. Unstable ornaments may fall.

Many people are frightened Hanging objects such as lamps and feel the need to hold onto swing violently. Dishes in cupsomething stable. boards and items on bookshelves may fall. Many unstable ornaments fall. Unsecured furniture may move, and unstable furniture may topple over.

Many people find it hard to Dishes in cupboards and items move; walking is difficult with- on bookshelves are more likely out holding onto something to fall. TVs may fall from their stable. stands, and unsecured furniture may topple over.

5.5≤ Ij < 6.0

It is difficult to remain standing.

6.0 ≤ Ij < 6.5

It is impossible to remain standing or move without crawling. People may be thrown through the air.

6.5 ≤ Ij

Indoor Situation

Electric wires swing significantly.

Those driving vehicles may notice the tremor.

In some cases, windows may break and fall. People notice electricity poles moving. Roads may sustain damage.

Windows may break and fall, unreinforced concrete-block walls may collapse, poorly installed vending machines may topple over, automobiles may stop due to the difficulty of continued movement.

Many unsecured furniture moves Wall tiles and windows may sustain and may topple over. Doors may damage and fall. become wedged shut.

Most unsecured furniture moves, Wall tiles and windows are more likely and is more likely to topple over. to break and fall. Most unreinforced concrete-block walls collapse.

Most unsecured furniture moves Wall tiles and windows are even more and topples over, or may even be likely to break and fall. Reinforced thrown through the air. concrete-block walls may collapse.

TABLE 3 Explanation of JMA seismic intensity scales (http://www.jma.go.jp/) for wooden houses Seismic Intensity

High Earthquake Resistance

Low Earthquake Resistance

4.5 ≤ Ij < 5.0 5.0 ≤ Ij < 5.5 5.5 ≤ Ij < 6.0

— — Slight cracks may form in walls.

6.0 ≤ Ij < 6.5

Cracks may form in walls.

Slight cracks may form in walls.

Cracks may form in walls.

Cracks are more likely to form in walls.

Large cracks may form in walls.

Tiles may fall, and buildings may lean or collapse.

Large cracks are more likely to form in walls.

Buildings are more likely to lean or collapse.

Buildings are even more likely to lean or collapse.

6.5 ≤ Ij

Cracks are more likely to form in walls.

Buildings may lean in some cases.

878 Seismological Research Letters Volume 82, Number 6 November/December 2011

high, but there were no buildings damaged around HPSC and SHLC where the 1–2-second response was low. In particular, there were many heavily damaged buildings, similar to what happened in the Kobe earthquake, that corresponded to the result of the nonlinear seismic response analysis around REHS. Thus, our investigation confirmed that the 1–2-second response bore a close relationship to heavy damage to buildings. However, there were no buildings with heavy damage around PRPC, where the 1–2-second response was almost the same as that of CCCC, and there were some houses with heavy damage a little away from HVSC seismic station though it was expected that ground motions there would not cause heavy damage to houses. We surmise that these results arise because the majority of the buildings around the seismic station were ranch houses and thus likely made from masonry, and the period corresponding to damage of such buildings is shorter than 1–2 seconds (Sakai and Nakamura 2004).

▲▲ Figure 4. Elastic acceleration response spectrum compared with the 1995 Kobe earthquake.

▲▲ Figure 5. Hysteresis models: A) Takeda Model and B) Modified Takeda-Slip Model.

TABLE 4 Explanation of JMA seismic intensity scales (http://www.jma.go.jp/) for reinforced-concrete buildings Seismic Intensity

High Earthquake Resistance

Low Earthquake Resistance

5.0 ≤ Ij < 5.5 — Cracks may form in walls, crossbeams and pillars.

5.5 ≤ Ij < 6.0 Cracks may form in walls, crossbeams and pillars. Cracks are more likely to form in walls, crossbeams and pillars.

6.0 ≤ Ij < 6.5 Cracks are more likely to form in walls, crossSlippage and X-shaped cracks may be seen in walls, crossbeams and pillars. beams and pillars. Pillars at ground level or on intermediate floors may disintegrate, and buildings may collapse.

Cracks are even more likely to form in walls, Slippage and X-shaped cracks are more likely to be seen in 6.5 ≤ Ij crossbeams and pillars. walls, crossbeams and pillars.

Ground level or intermediate floors may sustain Pillars at ground level or on intermediate floors are more significant damage. Buildings may lean in some likely to disintegrate, and buildings are more likely to colcases. lapse.

TABLE 5 Parameters of the Hysteresis Models.

Hysteresis Characteristics Model Takeda-Model Modified Takeda-Slip Model

αy

Q c / Qy

α

β

γ

δ

0.25 0.30

0.30 0.30

0.50 0.50

0.01 0.15

— 3.00

— 1.00

Seismological Research Letters Volume 82, Number 6 November/December 2011 879

▲▲ Figure 6. Required strength spectrum compared with the 1995 Kobe earthquake: A) by Takeda Model for RC buildings, and B) by Modified Takeda-Slip Model for wooden houses.

▲▲ Figure 7. Building damage distribution surrounding seismic stations.

880 Seismological Research Letters Volume 82, Number 6 November/December 2011

REFERENCES

▲▲ Figure 8. Heavily damaged wooden house.

CONCLUSIONS We performed a response analysis using strong ground motions and carried out damage investigation around seismic stations located within the area of the February 2011 Christchurch, New Zealand, earthquake. We confirmed the relationship between the 1–2-second response and heavy damage to buildings. However, some results did not correlate with the level of damage to buildings. We believe that these seemingly incongruous occurrences are explained by the popularity of masonry construction in New Zealand, because the period corresponding to damage of masonry buildings popular in New Zealand is shorter than 1–2 seconds.

ACKNOWLEDGMENTS We thank JMA, Osaka Gas, and the Railway Technical Research Institute for providing strong ground motion records of the Kobe earthquake. We wish to express our gratitude to the local people who cooperated with us during our investigations.

Architectural Institute of Japan (AIJ), ed. (1980). Report on the Damage Investigation of the 1978 Miyagiken-oki Earthquake. Tokyo: Architectural Institute of Japan.

Davenport, P. N. (2004). Review of seismic provisions of historic New Zealand loading codes, 2004. New Zealand Society for Earthquake Engineering Conference, 19–21 March 2004, Rotorua, New Zealand. Paper 17, Wellington, NZ: New Zealand Society for Earthquake Engineering.

Iizuka, H., and Y. Sakai (2009). Proposal of hysteresis characteristics model in seismic response analysis using single-degree-of-freedom system for wooden house. Journal of Japan Association for Earthquake Engineering 9 (1), 113–127.

Kumamoto, T., and Y. Sakai (2007). Actual strength distribution of RC buildings considering non-structural members. Summaries of Technical Papers of Annual Meeting Architectural Institute of Japan 2007, B-2 311–312, Tokyo: Architectural Institute of Japan.

Okada, S., and N. Takai (1999). Classifications of structural types and damage patterns of buildings for earthquake field investigation.

Journal of Structural and Construction Engineering (Transactions of AIJ) 52, 65–72.

Sakai, Y., and H. Iizuka (2009). A wooden house cluster model for earthquake damage estimation by nonlinear response analyses. Journal of Japan Association for Earthquake Engineering 9 (1), 32–45.

Sakai, Y., T. Kanno, and K. Koketsu (2002). Method of calculating seismic intensities considering structural damage and human body sense. 11th Earthquake Engineering Symposium, CD-ROM, paper no. 4, Tokyo: 11th Japan Earthquake Engineering Symposium.

Sakai, Y., T. Kanno, and K. Koketsu (2004). Proposal of instrumental seismic intensity scale from response spectra in various period ranges. Journal of Structural and Construction Engineering (Transactions of AIJ) 585, 71–76.

Sakai, Y., K. Koketsu, and T. Kanno (2002). Proposal of the destructive power index of strong ground motion for prediction of building damage ratio. Journal of Structural and Construction Engineering (Transactions of AIJ) 555, 85–91.

Sakai, Y., and Y. Nakamura (2004). Investigation on destructive power indices of strong ground motions using building damage data and strong ground motion records by the 1994 Northridge, California, earthquake. Journal of Structural and Construction Engineering (Transactions of AIJ) 584, 59–63.

Takeda, T., M. A. Sozen, and N. N. Nielsen (1970). Reinforced concrete response to simulated earthquakes. ASCE Journal of the Structural Division 96 (ST12), 2, 557–2,573.

Graduate School of Systems and Information Engineering University of Tsukuba, Japan 1-13-13-205, Sakura, Tsukuba-shi, 305-0003, Japan iizuka_h@edu.esys.tsukuba.ac.jp

(H. I.)

Seismological Research Letters Volume 82, Number 6 November/December 2011 881

Ground Motions versus Geotechnical and Structural Damage in the February 2011 Christchurch Earthquake Eleni Smyrou, Panagiota Tasiopoulou, İhsan Engin Bal, and George Gazetas

Eleni Smyrou,1 Panagiota Tasiopoulou,1 İhsan Engin Bal, 2 and George Gazetas1

INTRODUCTION The Mw = 6.3 earthquake of February 22 was the strongest seismic event in a series of damaging aftershocks in and around Christchurch after the Darfield earthquake on 4 September 2010. The source of the Darfield earthquake was in a sparsely populated area and thus it caused no loss of life. Serious damage was mainly due to extensive liquefaction. By contrast, the Christchurch earthquake was generated on a fault in close proximity to the city, resulting in a death toll of 181 people.

The Canterbury Plains are covered with river gravels that hide any evidence of past fault activity in this region. The newly revealed Greendale fault was therefore completely unknown.

Only a portion of it was revealed on the ground surface during the Darfield earthquake. The second fault (the one that ruptured in February 2011) appears to be a continuation of the first, although no fault structure directly connecting the faults has been recognized. There is a debate among seismologists at this point whether this is a different fault from Greendale one or not (NHRP 2011a; NHRP 2011b; Geonet 2011).

Due to its magnitude, shallow depth and close proximity to the city, the February earthquake proved particularly destructive for the central business district (CBD) of Christchurch, where buildings suffered extensive damage. Thanks to a dense network of strong ground motion stations, a large number of records have been obtained, which provide valuable information on the event and offer the possibility of relating the extent of damage to actual measurements of ground shaking.

Apart from the southern part of the city on the hills and the Lyttelton port area, Christchurch is built on deep estuarine soil, which has been shaped in the last thousands of years by the ever-changing riverbed. Fine sands—the dominant soil type— and the high ground water level contributed to widespread liquefaction in one or both earthquake events. Often accompanied by lateral spreading, liquefaction amplified the level of damage, resulting in the failure of structures in the CBD and surrounding areas, as will be explained below.

1. Soil Mechanics Laboratory, School of Civil Engineering, National Technical University, Athens, Greece 2. Fyfe Europe S.A. Athens, Greece

The older buildings in the city center, many of which are made of unreinforced masonry with timber floors, were mostly built in the late 19th and early 20th century, following English architectural style and construction practice and with no consideration of the high seismicity of the region. However, some of these buildings had been retrofitted in recent years. In contrast, many of the modern buildings in the CBD were designed in accordance with recent seismic codes, although their foundation systems were not always suitable for the adverse effects stemming from liquefaction. Thus, despite the fact that liquefied layers beneath the CBD restricted somewhat the amplitude of already significantly high accelerations, the increased velocities and displacements due to soil softening magnified the demands on long-period structures. Both structural and geotechnical aspects are investigated here in an effort to broadly explain and quantify the observed damage.

THE STRONG MOTION RECORDS Thanks to a dense network of seismographs covering the broader area of Christchurch (Figure 1), a large number of ground motions were recorded during the Christchurch February 2011 earthquake. The CBD area includes four seismic stations: CBGS, CCCC, REHS and CHHC. The first three records are truly free-field motions. CHHC was located near the base of a two-story building and its motion may reflect to some degree the effect of the structure. These ground motions may not have been the strongest ones recorded in terms of PGA values; however, due to certain features, their effect on structures or soils was detrimental.

There is a certain variation in the recorded acceleration time histories (Figure 1). For instance, the range of PGA values varies within a factor of 2, from 0.34 g (CHHC-NS) to 0.72 g (REHS-EW). A dominant common feature in all records is the sign of liquefaction: long-period cycles with reduced acceleration amplitudes occurring after a threshold acceleration has been reached. Soil softening due to excess pore water pressures in combination with sufficient acceleration values has led to amplification of large periods affecting a broad category of structures, as indicated by the acceleration spectra. In particular, the spectral

882 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.882

(A)

(B)

(D)

(C)

(E)

▲▲ Figure 1. A) Map of the broader Christchurch area showing the intersection of the fault plane with the ground surface (from GNS Science), the location of the accelerograph stations, the epicenter of the Christchurch 2011 earthquake, and the location with available soil data. B–E) Acceleration time histories and spectra of four CBD (central business district) seismic stations for NS and EW directions.

Seismological Research Letters Volume 82, Number 6 November/December 2011 883

▲▲ Figure 2. Observed polarity for the records in the CBD in terms of peak ground acceleration, velocity, and displacement. The contours of PGA on the map were computed by interpolation using all records in Christchurch.

amplification at periods exceeding 2 sec is attributed to the fact that once liquefaction has occurred, the overlying soil “crust” oscillates with very low frequencies, causing the bulges observed in the acceleration spectra for periods of about 3 sec (see Youd and Carter 2005 for similar observations from the then-available liquefaction-affected acceleration spectra). In addition to structural damage due to high spectral accelerations, important soilrelated failures have directly affected houses and bridges.

THE POLARITY OF THE RECORDED MOTIONS The two orthogonal components of a record are usually aligned with the north-south and east-west directions (Figure 1) or, ideally, if the faults were known, with fault-parallel and faultnormal directions. Mathematically there is at least one specific angle at which a certain ground motion parameter such as PGA, PGV, or PGD reaches a maximum, indicating the governing direction for that ground motion parameter and revealing a certain polarity of the recorded motion. Polarity plots can be useful in determining the dominant shaking direction of an earthquake and in unveiling any directivity effects (Shabestari and Yamazaki 2003).

A first index of intensity is the value of peak ground acceleration (PGA), the spatial distribution of which is depicted on the map of Figure 2. Additionally, for the records from the four

CBD stations (CCCC, REHS, CBGS, and CHHC) the maximum peak values of ground acceleration, velocity, and displacement are calculated trigonometrically, by varying the angle by 1° between 0° and 180°, resulting in asymmetric plots of positive and negative maxima (in absolute terms). The graphs consistently exhibit distinct polarity in a direction that practically coincides with that of the fault line. Knowing the polarity of shaking may offer information on the rupture mechanism and insight into the dominant damage observed in the CBD area.

TYPICAL SOIL PROFILE, LIQUEFACTION, ANALYSIS The Christchurch urban area, extending from Riccarton in the west to Bexley in the east and reaching Heathcote Valley and the Port Hills in the south, is located on the Canterbury Plains. Its dominant geomorphic feature is the river floodplains. In particular, the Avon (primarily) and Heathcote (secondarily) rivers, originating from various springs in western Christchurch, form endless meanders through the city and the eastern suburbs as they head to the estuary near the sea.

As depicted in Figure 3A, the subsoil in the CBD systematically consists of profiles with random variations in layering in the upper 15–25 m (Cubrinovski et al. 2010; Toshinawa et al. 1997). The volcanic bedrock is located at an approximate

884 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C)

▲▲ Figure 3. A) Typical in-depth soil profile in the CBD. B) Accelerograms and response spectra of the LPCC record used as excitation (applied in outcrop), and at two different depths obtained from the analyses. C) Polarity plots of LPCC record and output of analysis on the ground surface.

depth of 400 m and emerges on the surface at the southern border of the Canterbury Plains, forming the Port Hills of Banks Peninsula. Thick layers of gravel formations overlay the bedrock (Brown and Weeber 1992). The surficial sediments have an average thickness of about 25 m and consist of alternating layers of alluvial sand, silt, and gravel. They have been deposited by overbank flooding (Eidinger et al. 2010)—hence, their loose disposition. In the CBD, especially, sand and non-plastic silt with low content of fines are the dominant soil types (Rees 2010). The latter feature combined with the high ground water level (from 0 to 3 m) below the center of the city explains the sensitivity to liquefaction.

There is significant variability of soil deposits within short distances that can differentiate the ground motion characteristics. For example, Toshinawa et al. (1997) describe the soil profiles of two characteristic sites 1.2 km distant, one consisting of only sandy gravels and sand close to CBGS seismic station (Figure 1), and the other composed of silt and peat deposits to a depth of 7 m close to REHS seismic station. According to Toshinawa et al. (1997), during a 1994 distant earthquake greater amplification was observed at the second site, close to

REHS, in agreement with the records of February 2011 (Figure 1). This seems quite reasonable in cases of strong earthquakes, where the response of such soft, mostly sandy soils is expected to be dominated by the effects of severe liquefaction. However, both sites belong to the same broader classification of soft soils (class D) for structural design purposes in the New Zealand design standards (New Zealand Standards 1170.5 2004).

To investigate the soil response in the CBD urban area while accounting for liquefaction effects, we chose a typical “generic” soil profile (Figure 3A). Soil properties were obtained from boreholes conducted close to the Fitzgerald Bridge, situated at the eastern part of the CBD (see the star on the map in Figure 1). Standard penetration test (SPT) values were obtained from Bradley et al. (2009) and Rees (2010). Shear wave velocity, Vs, values were based on empirical correlations with SPT (Dikmen 2009).

With the “generic” soil profile defined, dynamic effectivestress analyses were conducted in order to capture the excess pore water pressure rise and dissipation, using the finite difference code FLAC (Itasca Consulting Group 2005). Ground motion recorded at station LPCC on the volcanic outcrop

Seismological Research Letters Volume 82, Number 6 November/December 2011 885

at Lyttelton Port was selected as the (outcrop) input motion referred to the base of the gravel formations (Figure 3B). The presumption that this rock motion (the only one on [soft] rock in the area) is a suitable candidate for the base of the CBD is only a crude approximation, because although the LPCC and CBD stations have the same distance from the about 65°-dipping fault, LPCC lies on the hanging wall and the CBD on the footwall of this partly thrust and partly strike-slip fault.

The NS and EW components of the LPPC record excited the soil column in two different one-dimensional wave propagation analyses. The numerical simulation involves the constitutive law of Byrne (1991) for pore pressure generation, which is incorporated in the standard Mohr-Coulomb plasticity model.

In general, as one would expect, the results of the analysis in terms of acceleration time histories and acceleration spectra for the two components (Figure 3B) demonstrate that as the shear waves propagate from the base of volcanic rock, the soil de-amplifies the low-period components of motion and amplifies those of high period. Moreover, the computed response on top of the dense gravel formation indicates that there is no substantial influence of the gravel layer in altering the input motion, other than de-amplifying the values in the high-frequency range (above 5 Hz) and slightly amplifying lower frequencies. In addition, the peak ground acceleration values do not change.

In contrast to the minor effect of gravel on the soil response, the surficial soil layers play a dominant role in defining the ground motion characteristics—hardly a surprise. These layers behave as a filter cutting off the high frequency spikes, while the duration of motion cycles is lengthened. As a result, the peak accelerations have diminished to 0.35 g approximately in both directions. Moreover, in terms of spectral acceleration values, there is considerable spectral amplification to 1 g in the higher period range of up to 1.8 sec. Overall, both components show similar response, with certain disparities in the frequency content, e.g., N-S output is richer in higher periods.

Polarity plots have also been constructed for LPCC motion and the computed ground surface motion. They are portrayed in Figure 3C. Evidently, there is no single (common) dominant direction for all PGA, PGV, and PGD values, contrary to the consistency in polarity of the CBD records (Figure 2). The PGA principal direction is normal (rather than parallel) to the fault. This discrepancy with CBD polarity might be attributed to the fact that Lyttelton is on the hanging wall side while the CBD lies on the footwall. For the “thrust” component of faulting this difference may indeed have an effect, but this is an issue that needs further investigation and is beyond the scope of this paper. The polarity of the output diverges only slightly from the polarity of LPCC. The comparison of polarity plots demonstrates clearly the cut-off of PGA values in all directions and increase of PGV and PGD values. Evidently, the liquefied layers play the role of a seismic isolator, reducing the acceleration amplitude of the wave components propagating through them.

The occurrence of liquefaction is visible in the pattern of recorded ground acceleration time histories and is captured

(with engineering accuracy) in our analysis: pore water pressure increases during shaking, reaching the initial effective overburden stress, σ′νο. At that point onward the soil loses most of its strength and begins to behave as a heavy liquid mass filtering out the high-frequency components, cutting off the acceleration values, and allowing only (long–period) oscillations of the dry cover layer that is “floating” on the top of the liquefied layer.

The ratio of the earthquake-generated (excess) pore water pressure, Δu, normalized by the initial vertical effective overburden stress, σ′νo, ru =

u

σ νo

,

approaches value of 1 at the onset of liquefaction. Values of ru above 0.8 indicate that already large excess pore water pressures have taken place and the soil has softened significantly. Figures 4B and 4C depict the distribution of computed peak values of ru(t) with depth and the time histories of ru(t) at five characteristic depths. In detail, Figure 3B shows that liquefaction did occur from 2.5 m to 17 m (ru > 0.8) throughout the silty sand layer. The dense sand layer experienced some excess pore pressures from flow from the overlying layer, but ru values were too low for liquefaction, as depicted in the time history of ru at 18 m (Figure 4C). In addition, according to the time histories of ru in Figure 4C, liquefaction occurs early, just 3 to 4 sec after the beginning of shaking, which is close to the cut-off of accelerations in the time histories shown in Figure 3B. To validate the analysis, the authors attempted a comparison between real records and numerical results. The record selected for the comparison, CBGS, is depicted on the map of Figure 1. The CBGS station is located in the Botanic Gardens and the recorder is housed in a very light kiosk (Figure 5A). The signs of liquefaction sand boils are visible, although they had been cleaned following the earthquake (the picture was taken by our research team in April 2011 [Tasiopoulou et al. 2011]).

No other facilities exist in the surroundings, ensuring free field conditions. Moreover, the soil profile described by Toshinawa et al. (1997) is appropriate for this location.

As already discussed, LPCC and the four CBD records have different polarity. However, LPCC was the only option in the search for a rock outcrop motion to be used as excitation in our analysis. That is why the comparison of spectra has been conducted in the direction of polarity of the CBGS record. For example, the strong PGA and PGV direction (polarity) for CBGS is approximately S56W and its PGD polarity is S51W. The acceleration time histories of the CBGS recorded and computed motions in the direction of S56W are depicted in Figure 5B. Although these time histories seem to differ, especially in terms of PGA values, a closer look reveals that they have certain common features, better depicted in Figure 5C after filtering out components with frequency above 4 Hz. Notice in particular that the main pulse at 4 sec exists in both time histories.

The response spectral SA, SV, and SD are compared in Figure 5D. The agreement of analysis with reality confirms that the analysis achieves a realistic insight of the mechanisms of soil response during the Christchurch earthquake.

886 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C)

▲▲ Figure 4. A) Typical surficial soil deposit: layers and properties. B) Distribution of computed excess pore water pressure ratio ru with depth. C) Computed time histories of ru at several depths.

BUILDING CATEGORIES AND THE OBSERVED DAMAGE Building Exposure in Christchurch Structures in New Zealand exhibit great variety. Timber and masonry buildings constitute around 80% of the building stock (Uma et al. 2008). Christchurch in particular has many oneor two-story timber and masonry residential buildings outside the CBD and very few modern reinforced concrete (RC) highrise buildings. The building composition in the CBD is different, with medium-rise modern steel and RC structures as well as mid-rise unreinforced masonry (URM) and timber dwellings and office buildings, some of which date from the late 19th and early 20th centuries. The one-story timber houses are commonly found in the suburbs surrounding the city, especially those along the Avon and Heathcote rivers.

It can be said, roughly, that the area outside the CBD consists of relatively low-rise and light structures, while long-period structures are more abundant in the CBD. This might be one of the reasons for the high concentration of damage in the CBD area during the February 2011 earthquake. A preliminary study presented below investigates the spatial distribution of the drift demands of the recorded strong motions for a range of periods.

Observed Damage Most of the casualties in the CBD were due to the collapse of two older mid-rise RC structures, called CTV and PGC, the failure conditions of which are presently being investigated. At

the time of writing the final statistics regarding the building safety evaluation are not yet available. However, as of 18 March 2011, the data by Civil Defence (Kam et al. 2011) referred to 3,621 buildings checked within the CBD, out of which 1,933 were posted red (needs to be demolished), 862 were posted yellow (has serious damage requiring extensive repair), and 826 were posted green (needs minor repair in order to be usable).

More specifically, 19% of the reinforced concrete structures, 14% of the timber, and 7% of the steel buildings checked were evaluated as red, while the equivalent percentages for reinforced and unreinforced masonry structures were 16% and 62%, respectively, reconfirming the poor behavior of URM structures. Insufficient detailing and bad construction techniques, mostly related to non-structural elements, aggravated the damage. Although the aforementioned data have come up before the completion of the second phase of building safety assessment and thus reflect the situation in CBD one month after the earthquake, they offer a representative picture of the extent and severity of damage in the CBD. Reasons behind the Extended Structural Damage The demand imposed by the Christchurch earthquake on different structures is assessed in terms of maximum inter-story drift demands (median values of all simulations done for the maximum values of all possible recording directions considered) in an attempt to broadly correlate ground motion features with the spatial distribution of damage. To this end, some characteristic buildings have been selected as representative of

Seismological Research Letters Volume 82, Number 6 November/December 2011 887

(A)

(D)

(B)

(C)

▲▲ Figure 5. A) The CBGS seismic station, with the remnants of liquefaction sand boils seen as scars on the grass. B) Acceleration time histories of CBGS: record and analysis. C) Comparison of the above acceleration time histories after filtering them at 4 Hz. D) Comparison of 5% damped spectra between CBGS record and analysis.

the building stock in the CBD, representing short-, medium-, and long-period structures. Our goal is not, obviously, to study in detail certain structures but to “reconcile” earthquake damage with ground motions. One- and two-story timber residential houses and two RC frame structures of different height, one six stories and one 17 stories, have been examined “generically” as described below in detail. Another case study could select URM buildings, a fairly representative typology in CBD, which suffered much from out-of-plane wall failures.

The selected buildings are treated as reference structures for their category, while the variability in the structural characteristics within each structural category is assumed to follow a statistical distribution simulated through a Monte-Carlo algorithm. This approach is a necessity since at this stage detailed structural data are not available. The parameters of the statistical distribution, i.e., mean value, coefficient of variation, type of distribution, etc., are either taken from the available literature or estimated using engineering judgment guided by the (macroscopic) visual inspection. The assumed values, as well as

the relative references for each parameter and structural category examined, are summarized in Table 1.

Having created a large number of simulated buildings, we applied the displacement-based assessment procedure established by Priestley et al. (2007) to evaluate the demand on each building. This is then translated to displacement demands for each floor and to inter-story drifts, utilizing the displacement profiles proposed in Priestley et al. (2007). The method is based on the substitute-structure theory, first suggested by Gülkan and Sözen (1974) and Shibata and Sözen (1976), according to which an inelastic multi-degree-of-freedom (MDOF) system can be represented by an equivalent inelastic single-degree-of-freedom system (SDOF). The only aspect of our methodology that, out of necessity, deviates from the Priestley et al. (2007) is that the “yield period” of each structural category is based on literature suggestions rather than an initial estimate of stiffness and the mass of each specific building. The “yield period” refers to the stiffness at the point of yielding, which is the limit beyond which substantial inelastic response begins that eventually may lead to

888 Seismological Research Letters Volume 82, Number 6 November/December 2011

TABLE 1 Key Parameters Used in the Representative Analyses Ranges Used in Monte Carlo Simulations*

Reference for the Key Parameters

1- and 2-story timber Displacement limit states Equivalent viscous damping equation Ratio of the first yield to the base shear coefficient Story height

µ = 8 mm, CoV = 0.15, [N] Deterministic a = 0.5, b = 0.8, [U] a = 2.8 m, b = 3.1 m, [U]

Uma et al. 2008 NZSEE 2006 ATC 1996 Field data

6-story RC frame

Beam depth Beam length Rebar yield strength Yield-period equation Equivalent viscous damping equation

µ = 0.8 m, CoV = 0.15, [N] µ = 7.0 m, CoV = 0.15, [N] µ = 330 MPa, CoV = 0.15, [N] Deterministic Deterministic

Tasiopoulou et al. 2011 Tasiopoulou et al. 2011 Uma et al. 2008 Crowley et al. 2004 Priestley et al. 2007

17-story RC frame

Beam depth Beam length Rebar yield strength Yield-period equation Equivalent viscous damping equation

µ = 0.6 m, CoV = 0.15, [N] µ = 5.0 m, CoV = 0.15, [N] µ = 330 MPa, CoV = 0.15, [N] Deterministic Deterministic

Galloway et al. 2011 Galloway et al. 2011 Uma et al. 2008 Crowley et al. 2004 Priestley et al. 2007

Case Study

Key Parameters

* µ: mean, CoV: coefficient of variation, [N]: Normal distribution, [U]: Uniform distribution, a and b: limits of the uniform distribution.

significant damage. The yield period has been successfully used as a key parameter in performance assessment by Crowley et al.

(2004) and Bal et al. (2010). References for the parameters used for each category of buildings are given in Table 1.

To ensure that the maximum displacement demand is estimated, the components for each record have been rotated in increments of 1° degree from 0° to 180°, thus creating a new set of 180 records and the corresponding response spectra. The contours of the maps presented in the paper (see Figures 6 to 8) have been derived after assessing each simulated building for a total of 180 response spectra. Note that the inter-story drift demands have been calculated only at the position of the recording stations, as shown on the maps in Figures 6 to 8. The values presented between the stations are only the result of linear interpolation among several “anchor” points. Obviously, the interpolation in these figures is bound by the coastline and cannot be extended to Kaiapoi and to Lyttelton Port stations.

Short-period structures are mostly timber buildings.

Such two-story houses are found in the CBD, while one-story houses are outside the CBD and in the suburbs. They are nonengineered buildings, with few if any exceptions; local regulations allow simple timber houses to be constructed without an approved design. Both groups were significantly damaged, but only a few collapsed. However, the damage to such houses due to liquefaction-induced ground differential settlements and horizontal displacements was unprecedented. A generic building has been used in this study as a reference structure. The properties of this generic structure are taken from the work

by Uma et al. (2008), in which the story drift limits are given as 0.3%, 0.6%, 1.2%, and 1.6% for slight, moderate, significant damage, and collapse limit states. Details of the assumed parameters can be found in Table 1.

There are several commercial buildings in the CBD, most of which are mid-rise RC structures designed and built in the 1970s and 1980s when the developed modern design concepts had only partially (at best) been incorporated in codes. A specific building from Kilmore Street (Markham’s Building), shown in Figure 9, is used for generating an ensemble of similar buildings for moderately long-period structures. Despite widespread liquefaction in the area, its pile foundation helped to limit the damage; thus, the results presented below refer to similar buildings founded on stable upper soil layers. The final case study is a real building in Worchester Street, known as Clarendon Tower, which has been reported to have undergone significant but repairable damage in the February earthquake.

It is a regular moment-resisting frame structure, the details of which are given in Galloway et al. (2011).

The spatial distribution of the mean values of inter-story drift demands in Christchurch for two-story timber structures (Figure 6), computed using the approach described above, clearly suggests that there must have been concentration of the inter-story demand in and near the Heathcote Valley where the strongest recorded shaking (HVSC) in terms of PGA and low-period SA and SD took place. On the contrary, damage in the area of the CBD must have been somewhat lighter, apparently due to the smaller low-period SD in the CBD. Such differences can be attributed to the somewhat larger distance from the source and the fact that the soft soils de-amplified the short-period seismic waves. But still, the median inter-story drift demands in the CBD are computed to have been in the order of 1.0%–1.5% for two-story timber structures, a level of demand definitely sufficient to induce substantial structural damage. Indeed, observations from different parts of the CBD on a variety of timber two-story structures confirm this theo-

Seismological Research Letters Volume 82, Number 6 November/December 2011 889

▲▲ Figure 6. Computed median inter-story drift demands (%) on two-story timber structures:maximum of all possible recording directions is considered (coefficient of variation of the results: 31%).

▲▲ Figure 7. Computed median inter-story drift demands (%) on six-story RC frame structures:maximum of all possible recording directions is considered (coefficient of variation of the results: 19%).

▲▲ Figure 8. Computed median inter-story drift demands (%) on 17-story RC frame structures:maximum of all possible recording directions is considered (coefficient of variation of the results: 19%).

890 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 9. Representative buildings for the considered categories: timber residential house (left), mid-rise RC structure (middle), and tall RC structure (right).

retical finding. Analyses on one-story timber structures with similar assumptions showed that the expected drift demands were quite limited, thus explaining the low damage ratio of one-story houses.

The computed spatial distribution of the median inter-story drift demands for six-story RC frames is portrayed in Figure 7, which shows that the highest demands on such structures, in the order of 3.0–3.5% inter-story drifts, are concentrated in the CBD—which explains the damage on mid-rise RC structures during the February 22 earthquake. Interestingly, the computed damage potential for such structures specifically reaches its climax in the CBD. Readers are reminded that both the CTV and PGC buildings, which fatally collapsed from the February shaking, were mid-rise RC structures constructed on soft soil.

Tall RC frame structures in the CBD, though limited in number, also experienced some extent of damage with the most characteristic case being that of the Grand Chancellor Hotel, a 26-story wall-frame structure that was rendered unusable due to large residual displacement. The building we chose to look at, the Clarendon Tower, underwent significant but repairable damage. Nevertheless, the building will be demolished as not meeting insurance standards. The findings illustrated in Figure 8 exhibit 0.6% to 0.8% median inter-story drift demands for similar 17-story structures, a drift level that certainly translates into damage but remains below the unrepairable drift limits in line with field observations (Tasiopoulou et al. 2011).

The inter-story drift demands reach their peak in the CBD as shown in Figure 8, a fact that could arguably be attributed to the characteristic bulges appearing in the response spectra (Figure 1) in the range of long periods. The elastic fundamental period of such structures is estimated around 2 sec, while the yield period (beyond which significant damage arises), is of the order of 5 sec (Crowley et al. 2004). The effective secant period, for example, is expected to elongate up to about 7 sec in the case of an overall ductility equal to 2, as computed with the approximate expression for effective period of Priestley and Kowalsky (2000).

CONCLUSIONS Damaging earthquakes feature large variations in spatial distribution of the strong ground motion parameters, a fact that is mostly attributed to the complexity of source mechanism, radiation pattern, and site conditions. The Mw = 6.3 Christchurch earthquake was a surprising and unusual event which occurred in an unknown fault that had already been awakened by the September 2010 stronger earthquake, and it had a strong thrust component and a steeply dipping plane.

This paper has attempted to identify quantifiable parameters that could provide better insight to seismologists and engineers who try to systematically investigate the reasons behind the structural and soil failures that occurred in the February shaking. The study focuses on connecting the basic features of the recorded strong motions to the nonlinear behavior of the soil layers. Liquefaction, a phenomenon that played a major and devastating role, has been examined through a “generic” downtown soil profile and dynamic effective stress analysis.

The LPCC record was applied as the base excitation, as it was the only available rock outcrop motion. Despite several uncertainties, the output spectra obtained from the liquefaction analyses and the one recorded in the free field in the Botanic Gardens have shown quite a satisfactory match provided that the compared spectra are aligned with the strong direction of the recorded motion. The dominant direction of the CBGS record is consistently almost parallel to the fault plane while the Lyttelton record exhibits more inconsistencies, something that may be related to the effects of the hanging wall and the steep thrust-fault plane. The governing direction of each record has been found by simply turning the record in every possible direction with one-degree intervals and re-recording the strong motion parameters sought—a venerable procedure to uncover the dominant direction of the shaking of a given site.

The paper concludes with an effort to better explain the reasons why some particular structural types showed bad performance in the CBD area. Short, medium, and long period

Seismological Research Letters Volume 82, Number 6 November/December 2011 891

structures have been examined adopting a displacement-based procedure. Results show that the inter-story drift demands in the CBD were particularly damaging for all types of structures but especially catastrophic for mid-rise RC buildings on shallow foundations. This is an important finding that may contribute to understanding why the CTV and PGC buildings collapsed.

ACKNOWLEDGMENTS Financial support for the expedition to the earthquakestricken area and the work outlined in this paper has been provided under the research project “DARE,” funded through the “IDEAS” Programme of the European Research Council (ERC) under contract number ERC-2-9-AdG228254-DARE. The authors would like to thank Professors John Berrill, Misko Cubrinovski, Stefano Pampanin, and Dr. Umut Akgüzel for providing data and assisting the authors during their reconnaissance visit in Christchurch in April 2011.

REFERENCES Applied Technology Council (ATC) (1996). Seismic Evaluation and Retrofit of Concrete Buildings. ATC-40 Report, vols. 1 and 2.

Redwood City, CA: Applied Technology Council.

Bal, İ. E., J. J. Bommer, P. J. Stafford, H. Crowley, and R. Pinho (2010).

The Influence of Geographical Resolution of Urban Exposure Data in an Earthquake Loss Model for Istanbul, Earthquake Spectra 26 (3), 619–634.

Bradley, B. A., M. Cubrinovski, R. P. Dhakal, and G. A. MacRae (2009).

Probabilistic seismic performance assessment of a bridge-foundation-soil system. Soil Dynamics and Earthquake Engineering 30, 395–411.

Brown L. J., and J. H. Weeber (1992). Geology of the Christchurch Urban Area. Institute of Geological and Nuclear Sciences, Scale 1:25,000, Geological Map 1, New Zealand. Lower Hutt, New Zealand: GNS Science.

Byrne, P. (1991). A cyclic shear-volume coupling and pore-pressure model for sand. Proceedings of the Second International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, St. Louis, Missouri, 47–55.

Crowley, H., and R. Pinho (2004). Period-height relationship for existing European reinforced concrete buildings. Journal of Earthquake Engineering 8 (S1), 305–332.

Cubrinovski, M., R. Green, J. Allen, S. Ashford, E. Bowman, B. Bradley, B. Cox, T. Hutchinson, E. Kavazanjian, R. Orense, M. Pender, M. Quigley, T. Wilson, and L. Wotherspoon (2010). Geotechnical reconnaissance of the 2010 Darfield (New Zealand) earthquake.

Bulletin of the New Zealand Society for Earthquake Engineering 43, 243–320.

Dikmen, Ü. (2009). Statistical correlations of shear wave velocity and penetration resistance for soils. Journal of Geophysics and Engineering 6, 61–72.

Eidinger, J., A. Tang, and Thomas O’Rourke (2010). Technical Council on Lifeline Earthquake Engineering (TCLEE), Report of the 4 September 2010 Mw 7.1 Canterbury (Darfield), New Zealand Earthquake. Reston, VA: American Society of Civil Engineers.

Galloway, B. D., H. J. Hare, and D. K. Bull (2011). Performance of multi-storey reinforced concrete buildings in the Darfield earthquake. Proceedings of the Ninth Pacific Conference on Earthquake Engineering—Building an Earthquake-Resilient Society, 14–16 April, 2011, Auckland, New Zealand, paper no. 168.

Geonet (2011). Christchurch badly damaged by magnitude 6.3 earthquake (22 February 2011), http://www.geonet.org.nz.

Gülkan, P., and M. Sözen (1974). Inelastic response of reinforced concrete structures to earthquake motions. ACI Journal 71 (12), 604–610.

Itasca Consulting Group (2005). Fast Lagrangian Analysis of Continua.

Minneapolis, MN: Itasca Consulting Group Inc. Kam, W. Y., U. Akguzel, and S. Pampanin (2011). 4 Weeks on: Preliminary Reconnaissance Report from the Christchurch 22 Feb 2011 6.3Mw Earthquake. Report, New Zealand Society for Earthquake Engineering Library, Wellington, New Zealand.

Natural Hazards Research Platform (NHRP) (2011a). Why the 2011 Christchurch earthquake is considered an aftershock, http://www. naturalhazards.org.nz.

Natural Hazards Research Platform (NHRP) (2011b). Magnitude 6.3 earthquake not on Greendale Fault, http://www.naturalhazards. org.nz.

New Zealand Society for Earthquake Engineering (NZSEE) (2006).

Assessment and Improvement of the Structural Performance of Buildings in Earthquakes, New Zealand Society for Earthquake Engineering.

New Zealand Standards 1170.5 (2004). Structural Design Actions, Part 5: Earthquake Actions—New Zealand. Wellington, New Zealand: Standards New Zealand, 82 pp.

Priestley, M. J. N., G. M. Calvi, and M. J. Kowalsky (2007). Displacementbased Seismic Design of Structures. Pavia, Italy: IUSS Press.

Priestley, M. J. N., and M. J. Kowalsky (2000). Direct displacement-based seismic design of concrete buildings. Bulletin of the New Zealand National Society for Earthquake Engineering 33 (4), 421–444.

Rees, S. D. (2010). Effects of fines on the un-drained behavior of Christchurch sandy soils. PhD thesis, Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand.

Shabestari, K. T., and F. Yamazaki (2003). Near-fault spatial variation in strong ground motion due to rupture directivity and hanging wall effects from the Chi-Chi, Taiwan earthquake. Earthquake Engineering and Structural Dynamics 32, 2,197–2,219.

Shibata, A., and M. Sözen (1976). Substitute structure method for seismic design in reinforced concrete. ASCE Journal of the Structural Division 102 (ST1), 1–8.

Tasiopoulou, P., E. Smyrou, İ. E. Bal, G. Gazetas, and E. Vintzileou (2011).

Geotechnical and Structural Field Observations from Christchurch, New Zealand, Earthquakes. Research Report, National Technical University of Athens, Greece.

Toshinawa, T., J. J. Taber, and J. B. Berrill (1997). Distribution of ground-motion intensity inferred from questionnaire survey, earthquake recordings, and microtremor measurements: A case study in Christchurch, New Zealand, during the 1994 Arthurs Pass earthquake. Bulletin of the Seismological Society of America 87, 356–369.

Uma, S. R., J. Bothara, R. Jury, and A. King (2008). Performance assessment of existing buildings in New Zealand. Proceedings of the New Zealand Society for Earthquake Engineering Conference, Wairakei, New Zealand, 11–13 April, paper no. 45.

Youd, T. L., and B. L. Carter (2005). Influence on soil softening and liquefaction on spectral acceleration. ASCE Journal of Geotechnical and Geoenvironmental Engineering 131 (7), 811–825.

892 Seismological Research Letters Volume 82, Number 6 November/December 2011

Soil Mechanics Laboratory School of Civil Engineering National Technical University Heroon Polytechneiou 9 Zografou Campus Athens 15780 Greece

smiroulena@gmail.com

(E. S.)

Soil Liquefaction Effects in the Central Business District during the February 2011 Christchurch Earthquake Misko Cubrinovski, Jonathan D. Bray, Merrick Taylor, Simona Giorgini, Brendon Bradley, Liam Wotherspoon, and Joshua Zupan

Misko Cubrinovski,1 Jonathan D. Bray, 2 Merrick Taylor,1 Simona Giorgini,1 Brendon Bradley,1 Liam Wotherspoon, 3 and Joshua Zupan2

INTRODUCTION During the period between September 2010 and June 2011, the city of Christchurch was strongly shaken by a series of earthquakes that included the 4 September 2010 (Mw = 7.1), 26 December 2010 (Mw = 4.8), 22 February 2011 (Mw = 6.2), and 13 June 2011 (Mw = 5.3 and Mw = 6.0) earthquakes. The moment magnitude (Mw) values adopted in this paper are taken from GNS Science, New Zealand (http://www.geonet.org.nz); they are 0.1 units higher than the corresponding Mw values reported by the U.S. Geological Survey (http://earthquake. usgs.gov/earthquakes/eqinthenews/2011/usb0001igm/). These earthquakes produced strong ground motions within the central business district (CBD) of Christchurch, which is the central heart of the city just east of Hagley Park and encompasses approximately 200 ha. Some of the recorded ground motions had 5% damped spectral accelerations that surpassed the 475year return-period design motions by a factor of two. Ground shaking caused substantial damage to a large number of buildings and significant ground failure in areas with liquefiable soils. The 22 February earthquake was the most devastating.

It caused 181 fatalities and widespread liquefaction and lateral spreading in the suburbs to the east of the CBD and in areas within the CBD, particularly along the stretch of the Avon River that runs through the city. There were pockets of heavy damage in the CBD, including the collapse of two multistory reinforced concrete buildings, as well as the collapse and partial collapse of many unreinforced masonry structures including the historic Christchurch Cathedral in the center of the CBD. Soil liquefaction in a substantial part of the CBD adversely affected the performance of many multistory buildings, resulting in global and differential settlements, lateral movement of foundations, tilt of buildings, and bearing failures.

The Mw = 6.2, 22 February 2011 earthquake is especially meaningful for earthquake professionals because it occurred just five months after the Mw = 7.1, 4 September 2010 Darfield 1. University of Canterbury, Christchurch, New Zealand 2. University of California, Berkeley, California, U.S.A. 3. University of Auckland, Auckland, New Zealand doi: 10.1785/gssrl .82.6.893

earthquake, the epicenter of which was approximately 40 km from the Christchurch CBD. Whereas the 22 February event killed almost two hundred people, the 4 September event resulted in no deaths. Although the September event caused widespread liquefaction-induced damage in the Christchurch area, it did not cause significant liquefaction-induced damage within the CBD. There is much to learn from comparing the different levels of soil liquefaction, differing magnitudes and seismic source distances, and variable performance of buildings, lifelines, and engineered systems during these two earthquakes. It is rare to have the opportunity to document the effects of one significant earthquake on a modern city with good building codes. It is extremely rare to have the opportunity to learn how the same ground and infrastructure responded to two significant earthquakes.

This paper summarizes the key field observations made following the 22 February 2011 Christchurch earthquake regarding the effects of soil liquefaction on building performance in the CBD. Other papers in this special issue provide information on earthquake ground motions and the geotechnical effects of this event outside the CBD. Additionally, the effects of the 4 September 2010 Darfield earthquake were documented previously (e.g., Cubrinovski et al. 2010). After a brief overview of the CBD, we describe the typical soil conditions in the CBD, followed by a summary of recorded ground motions in the CBD. There are several cases of buildings with different foundation types (e.g., isolated spread footings, spread footings with grade beams, raft foundations, and pile foundations) that performed differently in liquefied ground. Representative cases of building performance on liquefied ground are described to provide insights regarding the effects of soil liquefaction on urban areas with modern construction.

CHRISTCHURCH CENTRAL BUSINESS DISTRICT Christchurch is situated in the middle part of the east coast of the South Island of New Zealand. It has a population of about 350,000 (the second-largest city in New Zealand). Its urban area covers approximately 450 km2 . It is sparsely developed with approximately 150,000 dwellings (predominantly

Seismological Research Letters Volume 82, Number 6 November/December 2011 893

Ground motion recording station with geo-mean peak horizontal ground accelerations from 4 Sept.

2010 event (left) and 22 Feb. 2011 event (right).

Christchurch Cathedral

Figure 2 Section Line

Christchurch CBD

Finite fault model for 22 Feb 2011 Mw = 6.2 event. Refer to Beavan et al. (this issue) and Bradley and Cubrinovski (this issue) for more detail.

▲▲ Figure 1. Christchurch CBD relative to subsurface fault rupture of 22 February 2011 event.

single-story timber-framed houses with a smaller number of two-story houses) spread across a large area with many parks, natural reserves, and recreation grounds. The CBD is the area encompassed by the four avenues, Rolleston to the west, Bealey to the north, Fitzgerald to the east, and Moorhouse to the south. The CBD is more densely developed, including multistory buildings in its central area, a relatively large number of historic masonry buildings dating from the late 19th and early 20th century, residential buildings (typically two- to five-story structures located north of Kilmore Street), and some industrial buildings in the south and southeastern parts of the CBD. In total, about 3,000 buildings of various heights, construction age, and structural systems were within the CBD boundaries before the 2010–2011 earthquake series. Latest estimates indicate that about 1,000 of these buildings will have to be demolished because of excessive earthquake damage. Figure 1 outlines the boundaries of the CBD and the approximate location of the causative fault of the 22 February 2011 earthquake.

LOCAL GEOLOGY The city of Christchurch is located on Holocene deposits of the Canterbury Plains, except for its southern edge, which is located on the slopes of the Port Hills of Banks Peninsula, the eroded remnant of the extinct Lyttelton Volcano, composed of weathered volcanic rock (basalt) and thick deposits of Pleistocene loess. The river floodplain, Pacific coastline, and the Port Hills are the dominant geomorphic features of the Christchurch urban area. The Canterbury Plains are

complex fans deposited by eastward-flowing rivers from the Southern Alps, a NS-trending mountain range, into Pegasus Bay on the Pacific coast. The fan surfaces cover an area 50 km wide × 160 km long. At Christchurch, surface postglacial sediments have a thickness between 15 and 40 m and overlie 300–400-m-thick interlayered formations of gravels and fine to very fine grained sediments, representing deposition during episodic glacial and interglacial periods, and together comprise a series of ground water aquifers (Brown and Weeber 1992).

As shown in Figure 2, the surface sediments are made up of fluvial gravels, sands, and silts (Springston Formation, with a maximum thickness of 20 m to the west of Christchurch) or estuarine, lagoon, beach, dune, and coastal swamp deposits of sand, silt, clay, and peat (Christchurch Formation, with a maximum thickness of 40 m on the coast at New Brighton, east of the CBD). The shallow soil deposits (i.e., depths of up to 15–20 m) vary significantly within short distances, both horizontally and vertically.

Brown and Weeber (1992) describe the original site conditions and development of Christchurch as follows: Originally the site of Christchurch was mainly swamp lying behind beach dune sand; estuaries and lagoons, and gravel, sand and silt of river channel and flood deposits of the coastal Waimakariri River flood plain.

The Waimakariri River regularly flooded Christchurch prior to stopbank construction and river realignment.

Since European settlement in the 1850s, extensive drainage and infilling of swamps has been undertaken.

894 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 2. Representative subsurface cross-section of Christchurch CBD along Hereford Street (modified from I. McCahon, personal communication, 19 July 2011).

Brown and Weeber also state that surface deposits are actively accumulating and that the present-day river channel deposits are excluded from the above-mentioned Christchurch and Springston formations.

Canterbury has an abundant water supply through rivers and streams and rich aquifers. The dominant features of present-day Christchurch are the Avon and Heathcote rivers, which originate from springs in western Christchurch, meander through the city, and feed the estuary at the southeast end of the city. As illustrated in Figure 3, the Avon River meanders through the CBD while the Heathcote River flows south of the CBD. The groundwater table is deepest at the west end of the city (i.e., about 5 m depth), becoming progressively shallower eastward (i.e., within 1.0–1.5 m of the ground surface for most of the city east of the CBD), and approaching the ground surface near the coastline. The water table is generally within 1.5 to 2.0 m of the ground surface within the CBD.

GROUND MOTION CHARACTERISTICS The 4 September 2010 Mw = 7.1 Darfield earthquake was caused by a complex rupture of several fault segments, the largest and nearest to Christchurch being on the Greendale fault about 20 km west of the CBD. A maximum horizontal peak ground acceleration (PGA) of 0.24 g was recorded in the CBD, and the PGA decreased generally with distance downstream along the Avon River. The Mw = 6.2, 22 February 2011 Christchurch earthquake was less than 10 km from the CBD along the southeastern perimeter of the city in the Port Hills (Figure 1). The close proximity of this event caused higher-intensity shaking in the CBD as compared to the Darfield earthquake. Several of the recordings exhibited forward-directivity significant velocity pulses. In the CBD, horizontal PGAs of between 0.37 g

and 0.52 g were recorded. There are four strong motion stations located within or very close to the CBD (Figure 1). The recorded PGAs at these four stations are summarized in Table 1 for the five earthquakes producing highest accelerations (Bradley and Cubrinovski 2011, page 853 this issue).

For the shallow part of a deposit, the variation in the recorded PGA values corresponds closely with variations in the cyclic stress ratio (CSR) for each of these events. Magnitude scaling factors can then be applied to adjust each calculated CSR value to an equivalent value for an Mw = 7.5 event (CSR M7.5) as summarized in Table 1 for the geometric mean horizontal values of the PGA (Bradley and Cubrinovski 2011, page 853 of this issue). The data show that in addition to the high PGAs during the 22 February 2011 earthquake (PGA = 0.37–0.52 g), the CBD buildings were subjected to significant PGAs in the range of 0.16–0.27 g in four additional events. The highest adjusted CSR7.5 values of 0.14–0.20 were obtained for the Mw = 6.2, 22 February 2011 earthquake, which were about 1.6 times the corresponding CSR-values from the Mw = 7.1, 4 September 2010 Darfield earthquake. At many sites in Christchurch liquefaction re-occurred during these earthquakes, which in conjunction with the numerous smaller aftershock records provides invaluable data both in terms of thresholds for liquefaction triggering and CSR levels responsible for producing damaging liquefaction. In the CBD itself, only isolated areas liquefied during multiple events. Instead, widespread liquefaction occurred only in the CBD during the 22 February 2011 earthquake.

LIQUEFACTION IN THE CBD Immediately after the 22 February 2011 earthquake (i.e., from 23 February to 1 March) an extensive drive-through reconnaissance was conducted to map liquefaction and to document the

Seismological Research Letters Volume 82, Number 6 November/December 2011 895

Christchurch CBD

Avon River

Heathcote River

Christchurch Cathedral

▲▲ Figure 3. Liquefaction documentation map of eastern Christchurch from drive-through reconnaissance.

TABLE 1 Geometric mean PGAs and adjusted cyclic stress ratios to Mw = 7.5 earthquake (CSR7.5) for four strong motion stations within/close to CBD, for five earthquakes in the period September 2010–June 2011 Cyclic Stress Ratio CSR7.5*

Geometric Mean PGA (g) Event

CBGS

CCCC

CHHC

REHS

CBGS

CCCC

CHHC

REHS

Magnitude Scaling Factor MSF †

4 SEP 10, Mw = 7.1 26 DEC 10, Mw = 4.8 22 FEB 11, Mw = 6.2 13 JUN 11, Mw = 5.3 13 JUN 11, Mw = 6.0

0.158 0.270 0.501 0.183 0.163

0.224 0.227 0.429 – –

0.173 0.162 0.366 0.199 0.215

0.252 0.245 0.522 0.188 0.264

0.089 0.097 0.199 0.066 0.060

0.127 0.082 0.170 – –

0.098 0.058 0.145 0.072 0.079

0.142 0.088 0.208 0.068 0.097

1.15 1.80 1.63 1.80 1.77

* †

CSR 7.5 = 0.65 (PGA/g)/MSF (at depth of groundwater) MSF = 10 2.24 / Mw 2.56 ≤ 1.8 (corresponding to the lower bound range recommended in Youd et al. 2001, with a cap of 1.8)]

severity of its manifestation across Christchurch. The drivethrough survey aimed at capturing surface evidence of liquefaction as quickly as possible and quantifying its severity in a consistent and systematic manner. The resulting liquefaction documentation map is shown in Figure 3. Three areas of different liquefaction severity are indicated: A) moderate to severe liquefaction (black zone), B) low to moderate liquefaction (dark gray zone), and C) liquefaction predominantly on roads with some on properties (light gray zone). Traces of liquefaction were also observed in other areas. The suburbs to the east of the CBD along the Avon River (Avonside, Dallington, Avondale, Burwood, and Bexley) were most severely affected by liquefaction. About 5,000 residential properties in these suburbs will be abandoned (New Zealand Government 2011). There was also substantial damage in areas of the CBD, and many heavily damaged structures will require retrofit or demolition.

Ten days after the earthquake, after the urban search and rescue efforts had largely finished, the authors initiated a comprehensive ground survey within the CBD to document liquefaction effects in this area. Figure 4 shows a preliminary liquefaction documentation map for the CBD. This paper focuses primarily on key observations of building performance within the principal liquefied zone stretching west-east through the CBD, from Hagley Park to the west, along the Avon River to the northeast boundary of the CBD at the Fitzgerald Bridge.

This zone is of particular interest because many high-rise buildings on shallow foundations and deep foundations were affected by the liquefaction in different ways.

Even though the map shown in Figure 4 distinguishes the zone most significantly affected by liquefaction, the severity of liquefaction within this zone was not uniform. The manifestation of liquefaction was primarily of moderate intensity with

896 Seismological Research Letters Volume 82, Number 6 November/December 2011

Location of structures illustrated in subsequent figures.

Geomorphic feature

Moderate to severe liquefaction zone indicated with black shading.

Avon River Christchurch Cathedral

▲▲ Figure 4. Preliminary liquefaction map indicating zones of weakness and locations of buildings discussed in the paper.

(A)

(B)

▲▲ Figure 5. Representative areas of: A) moderate liquefaction (7 March 2011; S43.52791 E172.63653), and B) severe liquefaction within the CBD principal liquefaction zone (4 March 2011; S43.52604 E172.63839).

relatively extensive areas and volumes of sediment ejecta (Figure 5). There were also areas of low manifestation or only traces of liquefaction, but also pockets of severe liquefaction with very pronounced ground distortion, fissures, large settlements, and substantial lateral ground movements. This non-uniformity in liquefaction manifestation reflects the complex and highly variable soil conditions even within the CBD principal liquefaction zone. Survey maps of Christchurch dating back to the

time of early European settlement (1850s) show a network of streams and swamps scattered across this area (Archives New Zealand 2011).

The north extent of the zone, which is shown by the thick solid line in Figure 4, is a clearly defined geomorphic boundary running east-west that was delineated by a slight change in elevation of about 1 m to 1.5 m over an approximately 2 m to 10 m wide zone before the earthquakes. After the 22 February event,

Seismological Research Letters Volume 82, Number 6 November/December 2011 897

(A)

(B)

Liquefaction induced sediment ejecta.

▲▲ Figure 6. Apartment complex: A) looking south from northern building showing tilt of southern building, and B) looking north at liquefaction feature at edge of southern building (7 March 2011; S43.52434 E172.64432).

it was further characterized by ground fissuring or distortion associated with localized spreading, as well as gentle slumping of the ground surface on the downslope side. Ground cracks, fissures, and a distorted pavement surface marked this feature, which runs continuously through properties and affected a number of buildings causing cracks in both the foundations and superstructures. Liquefaction and associated ground deformation were pronounced and extensive on the downslope side between the identified geomorphic feature and the Avon River, but noticeably absent on the slightly higher elevation to the north (upslope side away from the river). This feature is thought to delineate the extent of a geologically recent river meander loop characterized by deposition of loose sand deposits under low-velocity conditions. A similar geomorphic feature was observed delineating the boundary between liquefaction damage and unaffected ground within a current meander loop of the river to the east of this area (Oxford Terrace between Barbadoes Street and Fitzgerald Avenue).

EFFECTS OF LIQUEFACTION IN THE CBD Some of the most important observations of the effects of soil liquefaction on structures in the CBD of Christchurch are described in this paper. These are the types of case histories that are required to glean important findings from and to add to the empirical database of the seismic performance of buildings at sites that have liquefied. There are several important cases of buildings with different foundation types (e.g., isolated spread footings, spread footings with grade beams, raft foundations, and pile foundations) that performed differently in liquefied ground, as we will describe in the following sections of this paper.

Preliminary geotechnical zoning based on existing data indicates several different areas within the CBD that are dominated either by gravelly layers, thick liquefiable sands or sandysilt mixtures, and peat in the top 8–10 m of the deposits. The soil profiles and thicknesses of these layers are highly variable even within a single zone, thus imposing difficult foundation conditions and sometimes resulting in unconventional or hybrid types of foundations being adopted for buildings. The gravelly soils, even though relatively more competent foundation soils, typically show medium standard penetration test (SPT) N values of about 15 to 25 blow counts, whereas the liquefiable loose sands and silt-sand mixtures have low resistance of less than N = 12 or cone penetration test (CPT) qc values less than 3–6 MPa. Another influencing feature in the design of deep foundations is the presence of gravel aquifers, with artesian water pressures, at a depth of approximately 22 m, which has imposed additional restrictions in terms of the cost and use of deep foundations. Additionally, the upward gradient has potentially adversely affected the liquefaction resistance of the overlying soils by increasing the pore water pressures in these soils during the 2010–2011 earthquakes.

Ground Failure Effects on Nearly Identical Structures—East Salisbury Area A mini-complex of three nearly identical buildings (with one small but important difference) is shown in Figure 6. The buildings are three-story structures with a garage at the ground floor, constructed on shallow foundations. This case clearly illustrates the impact of liquefaction, as the nearly identical structures have been built across the EW-trending geomorphic feature identified previously in Figure 4, with one building

898 Seismological Research Letters Volume 82, Number 6 November/December 2011

Shading indicates area over which pronounced grade change occurs

Duplex homes; center structure is shown in Figure 8

Apartment buildings shown in Figure 6

Tilted structure shown in Figure 6b

N

▲▲ Figure 7. Location of geomorphic feature in area of apartment and duplex complexes north of Salisbury Street in CBD. Darkened band is the area of pronounced grade change.

located on the higher level to the north suffering no damage, and the buildings located below the crest suffering progressively higher amounts of damage. This geomorphic feature, which is expressed here by a significant change in grade of the pavement between the northern and middle buildings, is shown in Figure 7. The northern building that sits on the higher ground showed no evidence of cracking and distortion of the pavement surface.

Conversely, large sediment ejecta were found along the perimeter of the southern building indicating severe liquefaction in its foundation soils (Figure 6B). Liquefaction features were also observed near the middle building, but the resulting distress of this building was significantly less than that of the southern building. The southern building had a shortened end wall with a column at its southwest corner, which appeared to produce additional settlement at the location of the column’s concentrated load. It suffered differential settlement of about 40 cm and more than 3 degrees of tilt toward the west-southwest, which is visible in Figure 6A. Adjacent to these buildings is another complex of three identical but structurally different buildings from the former set. Their locations relative to the abovementioned geomorphic feature is identical, but these buildings are two-story duplexes that are apparently supported on different foundations. Figure 8A shows the middle building with clear evidence of pavement distortion, cracking, and settlement of the surrounding ground. The settlement of the building was likely not significant, but the ground settled about 20 cm, exposing the top of the foundation at the southwest corner (Figure 8B).

Another apartment complex, constructed on a single level basement that extends almost the full length of the complex and provides off-street parking for the development, lies to

the west of the two case histories discussed previously. It also crosses the geomorphic feature. Noticeable settlement of the ground at the southern end of the complex of the order of 15–20 cm occurred and compression features in the pavement suggest that it displaced laterally toward the street. The concrete basement floor and structure appeared to have undergone negligible distortion, which indicates an overall rigid response despite the differential ground movements across the site.

Punching Settlement—Madras-SalisburyPeterborough Area Several buildings with shallow foundations located within the liquefied zone underwent punching settlements with some undergoing significant differential settlements and bearing capacity failures. An example of such performance is shown in Figure 9 for a two-story industrial building located 200 m southwest of the buildings discussed previously. There are clear marks of the mud-water ejecta on the walls of the building, indicating about 25-cm-thick layer of water and ejected soils due to the severe liquefaction. Note the continuous sand ejecta around the perimeter of the footing and signs of punching shear failure mechanism in Figure 9. At the front entrance of the building large ground distortion and sinkholes were created due to excessive pore water pressure and upward flow of water.

Settlement of the building around its perimeter was evident and appeared substantially larger than that of the surrounding soil, that was unaffected by the building. The building settled approximately 25 cm relative to a fence at its southeast corner and settled 10–20 cm relative to the ground at its northwest corner. The ground floor at the entrance was uplifted and blis-

Seismological Research Letters Volume 82, Number 6 November/December 2011 899

(A)

(B)

Focus area of Figure 8b

Pavement level prior to 22 February 2011 event

Exposed foundation

▲▲ Figure 8. Duplex housing complex: A) looking north at center building, and B) close-up of ground settlement next to center building (16 March 2011; S43.52399 E172.64417).

Structure shown in Figure 11

Structure shown in Figure 12

Observed liquefaction features ▲▲ Figure 9. Two-story building that underwent liquefactioninduced punching movements (7 March 2011; S43.52506 E172.64176).

▲▲ Figure 10. Relatively narrow liquefaction-induced feature that traverses parking lot northeast of the intersection of Madras and Armagh streets (24 March 2011; S43.52842 E172.64308).

tered, which is consistent with the pronounced settlement beneath the walls along the perimeter of the building.

The building shown in Figure 11 is a three-story structure on shallow foundations that settled substantially at its front, resulting in large differential settlements that tilted the building about 2 degrees. The building was also uniformly displaced laterally approximately 15 cm toward the area of significant liquefaction near the front of the building (i.e., to the north).

There was a large volume of sand ejecta at the front part of the building with ground tension cracks propagating east of the building and in the rear car-park that were consistent with the lateral movement of the building toward the north.

The building shown in Figure 12 is immediately across the street to the north. It is a six-story building on isolated footings with tie beams and perimeter grade beam. The isolated footings are 2.4 m × 2.4 m and 0.6 m deep. Figure 12 shows the view of the building looking toward the west and indicates total settlements measured relative to the building to its north, which did

Differential Settlement and Sliding—Armagh-Madras Area Farther to the south, at the intersection of Madras and Armagh streets, several buildings were affected by severe liquefaction that induced significant differential settlements or lateral movements. At this location, the liquefaction was manifested by a well-defined, narrow zone of surface cracks, fissures, and depression of the ground surface about 50 m wide, as well as water and sand ejecta (Figure 10). This zone stretches from the Avon River to the north toward the buildings affected by this liquefaction feature, shown in the background of Figure 10 to the south. Traces of liquefaction were evident further to the south of these buildings.

900 Seismological Research Letters Volume 82, Number 6 November/December 2011

1.8 deg

Ground cracking due to lateral displacements

15 cm

▲▲ Figure 11. Liquefaction-induced differential settlement and sliding of building in the CBD (24 March 2011; S43.52878 E172.64252).

N Liquefaction-induced sediment ejecta

29 cm

18

9

6

5

4

0

▲▲ Figure 12. Building undergoing significant liquefaction-induced differential settlement due to part of it being founded on the liquefaction feature in this area (24 March 2011; S43.52878 E172.64252).

not appear to settle. Starting from its northern edge and proceeding south, the differential settlement is 1 cm for the first span, 1 cm for the second span, then 3 cm, 9 cm, and 11 cm, respectively, for the final three spans. This results in an overall differential settlement across the structure of 25 cm, with 20 cm of it occurring across the two southernmost spans. A strong tie beam 0.6 m wide × 1.2 m deep was used between the footings for the first two northernmost spans, whereas the tie beams between the footings for the remaining spans were only 0.3 m wide × 0.45 m deep. This foundation detail, together with the fact that the observations of liquefaction were most severe at the southeast corner of the building and diminished across the footprint of the building toward the north, led to substantial differential settlements and pronounced structural distortion and cracking. Both buildings were considered

uneconomical to repair after the 22 February 2011 earthquake.

The building shown in Figure 11 has been demolished and the building shown in Figure 12 will be demolished.

Performance of Adjacent Structures—Town Hall Area The Christchurch Town Hall for Performing Arts, designed by Sir Miles Warren and Maurice Mahoney and opened in 1972, is located within the northwest quadrant of the CBD, with the meandering Avon River to its immediate south. It is a complex facility comprising a main auditorium (seating 2,500) with adjoining entrance lobby, ticketing, and café areas. Further extensions provide a second, smaller auditorium, the James Hay Theatre (seating 1,000), and a variety of function rooms and a restaurant. The structures are supported on shallow foundations except for the kitchen facility, which was added later.

Seismological Research Letters Volume 82, Number 6 November/December 2011 901

(A)

(B)

Crack

(C)

▲▲ Figure 13. Town Hall auditorium and adjacent dining facility undergoing significant liquefaction-induced differential settlement and lateral movements (24 March 2011; S43.52727 E172.63521).

(A)

(B) (C)

▲▲ Figure 14. Building in area of significant liquefaction that displays negligible to minor differential settlement or punching settlement (24 March 2011; S43.526508 E172.634646).

Air bridges connect the complex to the Crowne Plaza, a major hotel, and to the Christchurch Convention Centre (opened 1997) to the north. Tiled paved steps lead from the southern side of the complex down to the river’s edge, with fountains and views across to Victoria Park.

The facility suffered extensive damage caused primarily by liquefaction-induced ground failure. Differential settlements, caused by punching shear beneath the building’s main internal columns that surround the auditorium and carry the largest dead loads to shallow foundations and a second ring of exterior columns (Figure 13A) that are connected to the inner ring via beams (Figure 13B), caused distortion to the structure. The cracked beam shown in Figure 13B underwent an angular distortion of 1/70 across its span. The seating for the auditorium has been tilted and dragged backward due to the settlement of the surrounding columns. Additionally, the floor of the auditorium is now domed due to differential uplift relative to the columns. The air bridge connecting the main auditorium to the Christchurch Convention Centre to the north (away from river) has separated from the building.

With no significant deformations of the ground as the obvious source of this lengthening between the two buildings, the explanation appears to be that distortions to the auditorium

structure have pulled the outer walls in toward the building, creating this separation. The entire complex appears to have moved laterally toward the river (albeit by a barely perceptible amount on the northern side) with parts of the complex closest to the river undergoing increasingly larger movements (Figure 13C). These sections have settled and moved laterally toward the river more than the remainder of the building, leading to significant structural deformations where the extension and original structures are joined.

Contrary to the liquefaction-induced punching settlement of buildings into the surrounding ground that was observed at the Town Hall and in other parts of the CBD, the seven-story building shown in Figure 14A did not punch significantly into the liquefied ground nor undergo significant differential settlement. As shown in Figure 14B there were significant amounts of sand ejecta observed in this area.

However, there was no obvious evidence of significant differential ground or building movement (Figure 14C). The differential settlement measured between adjacent columns was typically negligible, but differential settlements of up to 3.5 cm were measured at a few locations. This building is across the street and slightly to the west of the Town Hall. It is a case

902 Seismological Research Letters Volume 82, Number 6 November/December 2011

17 cm 30 cm

30 cm

Foundation beam

▲▲ Figure 15. Building on pile foundations in area of severe liquefaction showing large settlement of the surrounding soils relative to the foundation beams (4-6 March 2011; S43.526575 E172.638668).

of liquefaction without significant differential settlement and building damage.

Contrasting Performance of a Pile-Supported Structure—Kilmore Area Several pile-supported structures were identified in areas of severe liquefaction. Although significant ground failure occurred and the ground surrounding the structures settled, the buildings supported on piles typically suffered less damage. However, there are cases where pile-supported structures were damaged in areas that underwent lateral spreading near the Avon River. In other cases, such as the building shown in Figure 15, which is located approximately 200 m to the east from the Town Hall, the ground-floor garage pavement was heavily damaged in combination with surrounding ground deformation and disruption of buried utilities. The settlement of the surrounding soils was substantial, with about 30 cm of ground settlement on the north side of the building and up to 17 cm on its south side. The first-story structural frame of the building that was supported by the pile foundation with strong tie-beams did not show significant damage from these liquefaction-induced ground settlements.

Across from this building to the north is a seven-story reinforced concrete building on shallow spread footing foundations that suffered damage to the columns at the ground level.

This building tilted toward the southeast as a result of approximately 10-cm differential settlement caused by the more severe and extensive liquefaction at the south-southeast part of the site. It is interesting to note that in the vicinity of this building, the site contained areas that liquefied during the 4 September 2010 earthquake. Following the extensive liquefaction in the 22 February 2011 event, there was also significant liquefaction in some areas during the 13 June 2011 earthquakes.

Having all of this in mind, these two buildings provide invaluable information on the performance of shallow founda-

tions and pile foundations in an area of moderate to severe liquefaction that induced uneven ground settlements. Extensive field investigations are planned to document the ground conditions in detail at these sites.

Presence of Shallow Gravelly Soils—Victoria Square Near Victoria Square, the liquefied zone was composed predominantly of relatively deep loose sand deposits that transitioned relatively sharply into a zone where gravelly soil layers reach close to the ground surface. Shallow foundations (spread footings and rafts) for many of the high-rise buildings in this latter area are supported on these competent gravelly soils.

However, the ground conditions are quite complex in the transition zone, which resulted in permanent lateral movements, settlements, and tilt of buildings either on shallow foundations or hybrid foundation systems, as illustrated in Figure 16.

Immediately to the north of these buildings, the liquefaction was severe with massive sand ejecta; however, approximately 100 m and further to the south where the gravels predominate, there was neither evidence of liquefaction on the ground surface nor visible distress of the pavement surface. Again, it appears that the ground and foundation conditions have played a key role in the performance of these buildings, which therefore have been selected for in-depth field investigations.

Lateral Spreading—Avon River Liquefaction-induced lateral spreading was evident within the CBD along the Avon River in the liquefied zone, and the horizontal stretching of the ground adversely affected several buildings. Detailed measurements by ground surveying conducted at about 10 transects on the Avon River within the CBD after the 22 February earthquake indicated that at several locations the maximum spreading displacements at the banks of the Avon River reached about 50–70 cm, whereas at most of the other locations the spreading was on the order of 10 cm to 20

Seismological Research Letters Volume 82, Number 6 November/December 2011 903

documented observations, critically important case histories of soil-foundation-structure-interaction can be developed. When completed, these well-documented case histories of building performance in liquefied ground can be used to evaluate and calibrate computational software with advanced geotechnical soil models and provide empirical data for developing design procedures for evaluating the effects of liquefaction on building performance.

ACKNOWLEDGMENTS The primary support for the New Zealand GEER team members was provided by the Earthquake Commission New Zealand (EQC) and University of Canterbury. The primary support for the U.S. GEER team members was provided by grants from the U.S. National Science Foundation (NSF) as part of the Geotechnical Extreme Events Reconnaissance (GEER) Association activity through CMMI-0825734 and CMMI1137977. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation, EQC, or the host institutions of the authors.

We would also like to acknowledge the assistance of all New Zealand and U.S. GEER team members who participated in the reconnaissance of these events. Their contributions are noted at the GEER Web site (http://www.geerassociation.org/). ▲▲ Figure 16. Buildings on shallow and hybrid foundations in transition area from moderate liquefaction to low/no liquefaction; arrows indicate direction of tilt of the buildings (7 March 2011; S43.52878 E172.63528).

cm. There were many smaller buildings suffering serious damage to the foundations due to spreading as well as clear signs of the effects of spreading on some larger buildings both at the foundations and through the superstructure.

CONCLUSIONS Documenting and learning from observations after designlevel earthquakes are vital to advancing the state-of-practice in earthquake engineering. Surveying the re-occurrence of liquefaction, documenting cases of liquefaction-induced ground movements, and evaluating the effects of liquefaction on buildings and lifelines provide invaluable information that will serve as benchmarks to the profession’s understanding of the effects of earthquakes. The series of earthquakes that shook Christchurch in 2010 and 2011 provides insights and data more valuable than that which can be developed through experiments due to the problems of model scaling. These earthquakes, in particular, represent important earthquake scenarios worldwide. Each of the documented building responses in the CBD provides critical insights regarding the performance of structures and foundations sited on ground that could potentially liquefy. Site investigations are planned to document fully the ground conditions at these sites, so that with these

REFERENCES Archives New Zealand (2011). Black Map of Christchurch, March 1850. http://archives.govt.nz/gallery/v/Online+Regional+Exhibitions/ Last Chregionalofficegallery/sss/Black+Map+of+Christchurch/.

accessed July 18, 2011.

Bradley, B. A., and M. Cubrinovski (2011). Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake. Seismological Research Letters 82,853–865.

Brown, L. J., and J. H. Weeber (1992). Geology of the Christchurch Urban Area. Institute of Geological and Nuclear Sciences. Lower Hutt, New Zealand: GNS Science.

Cubrinovski, M., R. Green, J. Allen, S. Ashford, E. Bowman, B. Bradley, B. Cox, T. Hutchinson, E. Kavazanjian, R. Orense, M. Pender, M. Quigley, and L. Wotherspoon (2010). Geotechnical reconnaissance of the 2010 Darfield (Canterbury) earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 243–320.

New Zealand Government (2011). http://www.beehive.govt.nz/release/ govt-outlines-next-steps-people-canterbury. Last accessed 18 July 2011.

Youd, T. L., I. M. Idriss, R. D. Andrus, I. Arango, G. Castro, J. T. Christian, R. Dobry, et al. (2001). Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. ASCE Journal of Geotechnical & Geoenvironmental Engineering 127 (10), 817–833.

Department of Civil and Natural Resources Engineering University of Canterbury Private Bag 4800 Christchurch 8140 New Zealand

904 Seismological Research Letters Volume 82, Number 6 November/December 2011

misko.cubrinovski@canterbury.ac.nz

(M. C.)

Comparison of Liquefaction Features Observed during the 2010 and 2011 Canterbury Earthquakes R. P. Orense, T. Kiyota, S. Yamada, M. Cubrinovski, Y. Hosono, M. Okamura, and S. Yasuda

R. P. Orense,1 T. Kiyota, 2 S. Yamada, 3 M. Cubrinovski,4 Y. Hosono,5 M. Okamura,6 and S. Yasuda7

INTRODUCTION

alties were reported (New Zealand Police; http://www.police.

On 4 September 2010, a magnitude M = 7.1 earthquake struck the Canterbury region on the South Island of New Zealand.

The epicenter of the earthquake was located near Darfield, about 40 km west of the central business district (CBD) of the city of Christchurch and at a depth of about 10 km. Extensive damage was inflicted on lifelines and residential houses due to widespread liquefaction and lateral spreading in areas close to major streams, rivers, and wetlands throughout the city of Christchurch and the town of Kaiapoi. In the months following the Darfield M 7.1 earthquake, numerous aftershocks were felt across the city.

Almost six months after the Darfield mainshock, on 22 February 2011, the Canterbury region was hit by a magnitude M = 6.3 earthquake. The epicenter was located near Lyttelton, only 6 km to the southeast of the Christchurch CBD and at a depth of 5 km. In spite of its smaller magnitude, this earthquake resulted in more damage to pipeline networks, transport facilities, residential houses/properties, and multistory buildings in the CBD than the September 2010 event, mainly because of the short distance to the city and the shallower depth.

Although there were no casualties after the 2010 Darfield earthquake, which is sort of a miracle considering the magnitude of the earthquake, the 2011 Christchurch earthquake resulted in a significant number of casualties due to the collapse of multistory buildings and unreinforced masonry structures in the Christchurch city center. As of 1 June 2011, 181 casu1. Department of Civil and Environmental Engineering, University of Auckland, New Zealand 2. Institute of Industrial Science, University of Tokyo, Japan 3. Department of Civil Engineering, University of Tokyo, Japan 4. Department of Civil and Natural Resources Engineering, University of Canterbury, New Zealand 5. Department of Architecture and Civil Engineering, Toyohashi University of Technology, Japan 6. Department of Civil and Environmental Engineering, Ehime University, Japan 7. Department of Civil and Environmental Engineering, Tokyo Denki University, Tokyo, Japan doi: 10.1785/gssrl.82.6.905

govt.nz/list-deceased).

While it is extremely regrettable that the 2011 Christchurch earthquake resulted in significant casualties, engineers and seismologists now have a hard-to-find opportunity to learn the response of ground and structures to two large-scale earthquakes that occurred less than six months apart. From a geotechnical engineering point of view, it is interesting to look at the widespread liquefaction in natural sediments, re-liquefaction of ground occurring over a short period of time, and further damage to earth structures that had been damaged as a result of the first earthquake.

Following the two earthquake events, detailed geotechnical investigations were conducted by the authors as part of the Japanese Geotechnical Society (JGS) earthquake reconnaissance teams. The reconnaissance was a collaboration between the society’s New Zealand-based members and researchers dispatched from Japan for this purpose. The first visit was made 12–15 September 2010, while the second one was 27 February– 3March 2011. This paper attempts to present a comparison of the two events based on the observations made by the authors following these reconnaissance trips, with emphasis on the geotechnical implications of liquefaction-observed damage in the affected areas.

It is worth mentioning that a series of aftershocks, the largest of which were M 5.6 and M 6.3, rattled the city on 13 June 2011. These aftershocks again caused extensive liquefaction in many parts of Christchurch. As we write this paper, reconnaissance work is underway to shed more light on the damage caused by re-liquefaction.

GEOLOGIC SETTING The Canterbury Plains, about 180 km long and of varying width, are New Zealand’s largest areas of flat land. They have been formed by the overlapping fans of glacier-fed rivers issuing from the Southern Alps, the mountain range of the South Island. The plains are often described as fertile, but the soils are variable. Most are derived from the greywacke of the mountains or from loess (fine sediment blown from riverbeds). In

Seismological Research Letters Volume 82, Number 6 November/December 2011 905

43°21′35″

2010 EQ

2011 EQ 0

Kaiapoi 27 km

Waimakariri River Avon River Bexley

City Centre

Heathcote River 2011 EQ Epicentre

Port Hills

Banks Peninsula 172°26′23″

43°39′18″ 172°55′40″

▲▲ Figure 1. Map of Christchurch region highlighting relevant geographical information.

addition, clay and volcanic rock are present near Christchurch from the Port Hills slopes of Banks Peninsula.

The city of Christchurch is located at the east coast of the Canterbury Plains adjacent to an extinct volcanic complex forming Banks Peninsula. A map of Christchurch CBD and its environs is shown in Figure 1. Most of the city was mainly swamp behind sand dunes, and estuaries and lagoons that have now been drained (Brown et al. 1995). The surface geology of the greater Christchurch area consists of predominantly recent Holocene (<11,700 years old) alluvial gravel, sand, and silt of the Springston Formation, while Christchurch Formation sediments have been mapped along the eastern margin of the city (Brown and Weeber 1992). The Springston Formation alluvial deposits, with maximum thickness of 20 m to the west of Christchurch, consist of river flood channels, which contain alluvial gravel as the main component, and overbank deposits of sand and silt, which are generally in a loose state and susceptible to liquefaction. The Christchurch Formation units, with maximum thickness of 40 m at the New Brighton coast east of CBD, are described as fixed and semi- fixed dunes and beach sands, and are often denser and less prone to liquefaction.

Below these formations lies the 300–400-m-thick interlayered Riccarton Gravel Formation. Bedrock below most of the city is generally at a depth of many hundreds of meters. A simplified geology of Canterbury Plains, highlighting the soil profile in Christchurch, is shown in Figure 2.

The two main rivers, Avon and Heathcote, which originate from springs in western Christchurch, meander through

the city and act as its main drainage system. The groundwater table is generally between 2 to 3 m below the ground surface in the west and 0 to 2 m below the surface in the central and eastern areas of Christchurch.

COMPARISON OF STRONG-MOTION RECORDS The New Zealand Earthquake Commission (EQC) has provided funding for the country’s real-time seismic hazard monitoring and data collection through GeoNet. The system, currently managed by GNS Science, consists of more than 100 seismographs and 180 strong-motion sensors, which monitor thousands of small shakes and many large quakes (http://www. gns.cri.nz/Home/Our-Science/Natural-Hazards/Alert-Nation/ Earthquake-Monitoring). In addition, when the September 2010

earthquake occurred, the Canterbury Regional Strong Ground Motion Network (CanNet) had installed 36 of the planned 60 free-field instruments in the Canterbury Plains in anticipation of an M 8+ rupture of the Alpine fault (Berrill et al. 2011).

Locations of seismic stations within the city of Christchurch are shown in Figure 3.

2010 Darfield Earthquake The 2010 Darfield earthquake was caused by a rupture of a previously unrecognized strike-slip fault, now well-known as the Greendale fault. During the earthquake, a series of strong-motion accelerographs was triggered, and motions were recorded at several stations. Based on the GeoNet strong-

906 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 2. A) Simplified geology of Christchurch region. B) Simplified soil strata along cross-section A-A’ (modified from Brown and Weeber 1992).

43°27′36″

SWS Dallington - 1

SHLC

REHS CBGS CHHC

CCCC

CMHS

SWS Dallington - 2

HPSC

PRPC

SWS Bexley - 1 SWS Bexley - 2

HVSC LPCC

43°37′01″ 172°30′10″

172°47′41″

▲▲ Figure 3. Location of strong-motion sites near the city of Christchurch (after GeoNet Web site, http://magma.geonet.org.nz/delta/ app?service=page/Home). Also shown are the locations of Swedish weight sounding (SWS) tests.

Seismological Research Letters Volume 82, Number 6 November/December 2011 907

TABLE 1 Comparison of peak ground accelerations recorded at strong-motion sites near the city during the 2010 Darfield earthquake and 2011 Christchurch earthquake. Data are from GeoNet strong motion FTP site. The unit of acceleration is g (1 g = 9.80 m/s 2).

2011 Christchurch Earthquake

2010 Darfield Earthquake

Seismic Stations HVSC LPCC CCCC CMHS PRPC CHHC REHS CBGS HPSC SHLC

Site Name

Ep. Dist. (km)

Vert

Hor-1 Hor-2

Heathcote Valley Primary School Lyttelton Port Company Chch Cathedral College ChCh Cashmere High School Pages Road Pumping Station Christchurch Hospital Christchurch Resthaven Christchurch Botanic Gardens Hulverstone Dr Pumping Station Shirley Library

43 45 38 36 41 36 37 36 43 39

0.28 0.16 0.16 0.25 0.31 0.16 0.21 0.11 0.13 0.12

0.56 0.33 0.23 0.25 0.20 0.20 0.24

0.15 0.16 0.18

0.62 0.23 0.20 0.24 0.23 0.15 0.25 0.17 0.11 0.18

Max Hor

Ep. Dist. (km)

Vert

Hor-1 Hor-2

Max Hor

0.66 0.37 0.24 0.26 0.23 0.20 0.33 0.18 0.16 0.19

1 4 6 6 6 8 8 9 9 9

1.47 0.41 0.69 0.80 1.63 0.51 0.53 0.27 0.86 0.50

1.46 0.78 0.48 0.35 0.66 0.34 0.71 0.53 0.15 0.31

1.50 1.00 0.49 0.42 0.73 0.46 0.73 0.64 0.25 0.34

1.19 0.88 0.37 0.38 0.59 0.36 0.37 0.43 0.24 0.34

Note: Ep. Dist – Epicentral distance; Vert – vertical acceleration; Hor-1 and Hor-2 – horizontal components of acceleration; Max. Hor – calculated maximum resultant acceleration of horizontal components. Unit of acceleration is g (1 g = 980 cm/s 2 ). Source: GeoNet 2011.

motion FTP site, the maximum recorded acceleration was on the order of 0.95 g near the earthquake epicenter (GeoNet 2010); however, no serious damage was reported in the area.

In the city of Christchurch, the recorded peak ground accelerations (PGA) were on the order of 0.15–0.30 g, as shown in Table 1. The seismic stations indicated in the table correspond to those shown in Figure 3. Figure 4 shows a typical acceleration record obtained during the 2010 earthquake. Note that the duration of significant shaking at Christchurch Hospital (CHHC), located at the southwest edge of the CBD, is on the order of about 25–30 sec.

2011 Christchurch Earthquake The 2011 Christchurch earthquake occurred more than five months after the 2010 Darfield earthquake, with an epicenter located on an unmapped fault different from the Greendale fault. Yet it is considered an aftershock because it was caused by a fault rupture within the zone of aftershocks that followed the September 2010 mainshock (National Hazards Research Platform 2011). Because the M 6.3 aftershock was much closer to the Christchurch CBD than the M 7.1 mainshock, the ground accelerations experienced in the CBD as a result of the 2011 earthquake were three to four times greater than during 2010 event (see Table 1); in the eastern suburbs, they were about five times greater. The vertical PGA recorded was 1.47 g at Heathcote Valley primary school (about midway between the CBD and the epicenter) while in the CBD the PGA was 0.5–0.7 g and in the eastern suburbs the maximum recorded vertical PGA was 1.63 g (GeoNet 2011). A feature of this earthquake was the very strong vertical component of PGA, which in general was greater than the horizontal components. Figure 4 also illustrates the time histories of acceleration recorded at Christchurch Hospital on 22 February 2011. Because of the

shorter distance to the epicenter, the acceleration records in this earthquake have a higher frequency and shorter duration time, as well as larger amplitude, in comparison with the ones recorded on 4 September 2010.

COMPARISON OF OBSERVED LIQUEFACTION DAMAGE Although structural failure of commercial buildings led to the greatest casualties in the M 6.3 Christchurch earthquake, by far the most significant damage to residential buildings and lifelines in both Canterbury earthquakes was the result of liquefaction and associated ground deformations. Liquefaction occurred in areas that are known to have high potential for liquefaction—former river channels, abandoned meanders, wetlands, and ponds. Immediately following some of the largest aftershocks from the M 7.1 earthquake, liquefaction reoccurred in some of these areas. During the M 6.3 earthquake, liquefaction was more widespread and vents continued to surge during the aftershocks immediately following this event. The impact of sand boils and cracks caused by lateral spreading was that parts of the eastern suburbs were inundated with sand and silt—in places there were layers of ejected soil that were many tens of centimeters thick.

As mentioned earlier, two large aftershocks, measuring M 5.6 and M 6.3, shook the city on 13 June 2011 and caused extensive re-liquefaction in many parts of the city. Streets were again flooded with water and ejected sands, reminiscent of what happened immediately after the 22 February 2011 earthquake. Such reoccurrence of liquefaction indicates that the soil deposits in the area were still loose even after the intense shaking they had been subjected to over the last nine months.

908 Seismological Research Letters Volume 82, Number 6 November/December 2011

600 400 200 0 -200 -400 -600 600 400 200 0 -200 -400 -600 600 400 200 0 -200 -400 -600 0

(B)

Christchurch Hospital, 4 Sep 2010, Mainshock

N01W (Horizontal 1)

S89W (Horizontal 2)

Vertical 10

20

30

Time (sec)

40

50

60

Acceleration (Gal)

Acceleration (Gal)

(A)

600 400 200 0 -200 -400 -600 600 400 200 0 -200 -400 -600 600 400 200 0 -200 -400 -600 0

Christchurch Hospital, 22 Feb 2011, Mainshock

N01W (Horizontal 1)

S89W (Horizontal 2)

Vertical 10

20

30

Time (sec)

40

50

60

▲▲ Figure 4. Acceleration time histories recorded at Christchurch hospital during the 2010 Darfield earthquake and 2011 Christchurch earthquake (data from GeoNet strong motion FTP Web site). Note: 1 g = 980 Gal.

43°31′18″

43°32′16″ 172°36′43″

172°39′14″

▲▲ Figure 5. Location of Avon River as well as wetlands and streams in 1850 superposed to the present-day map of the Christchurch CBD.

Eastern Suburbs Liquefaction and lateral spreading were extensive in areas adjacent to the Avon River, which follows a meandering course through Christchurch from its source in the west through the CBD, then toward the east passing through Avonside, Dallington, Avondale, and Aranui, and finally flowing to the Pacific Ocean via the Avon-Heathcote estuary.

Figure 5 shows the locations of Avon River and other streams based on a 1850 map of the city superposed on the present map of Christchurch CBD. The meandering nature of the Avon is conspicuous as it flows from the west toward the east. Also, it can be seen that several wetlands and streams crisscrossed the future city center, some of which were later artificially reclaimed as the city grew. The locations of these former

river channels had a significant effect on the damage observed following the M 6.3 earthquake. Details of liquefactioninduced damage observed in the central business district are presented by Cubrinovski et al. (2011, page 893 of this issue).

After the 2010 Darfield earthquake, Swedish weight sounding (SWS) tests were performed by the JGS-University of Canterbury reconnaissance teams at numerous locations affected by liquefaction and lateral spreading. The SWS test is a simple manually operated penetration test under a dead-load of 100 kg in which the number of half-rotations required for a 25-cm penetration of a rod (screw point) is recorded (Japanese Standards Association 1995). As a result of the SWS test, the corresponding standard penetration test (SPT) N-value can be obtained through the following empirical equation (Inada 1960).

Seismological Research Letters Volume 82, Number 6 November/December 2011 909

0

1

1

1

1

2

2

2

2

3

3

3

3

4

4

4

4

5

5

Depth (m) Depth (m)

0

Depth (m) Depth (m)

0

Depth (m) Depth (m)

Depth (m) Depth (m)

0

5

5

6

6

6

6

7

7

7

7

8

8

8

8

9 10

9

Bexley (e) Spot No.5 0 10 20 30 Conversion N-value

10

40

Converted N-value

Bexley (d) Spot No.4 0 10 20 30 Conversion N-value

40

Converted N-value

9 10

Dallington (b) Spot No.2 0 10 20 30 Conversion N-value

Converted N-value

40

9 10

Dallington (c) Spot No.3 0 10 20 30 Conversion N-value

40

Converted N-value

▲▲ Figure 6. Converted SPT N-value profiles from Swedish weight sounding tests conducted by the University of Canterbury and JGS reconnaissance teams in September 2010.

N = 0.002WSW + 0.067 N SW ,

(1)

where WSW (kg) is the weight less than 100 kg and NSW is the number of half-rotations for every meter of penetration. WSW is counted when penetration occurs with dead-load less than 100 kg. Note that this equation is applicable to gravel, sand, and sandy soils.

Typical results of SWS tests in Christchurch are shown in Figure 6. The locations of the test sites are indicated in Figure 3. It can be seen from the strength-depth profiles that in these areas, layers of about 5 m or thicker exist with high potential to liquefy (very loose silt/sand layer with SPT N-value < 5). The presence of loose sandy deposits in many areas in Christchurch has also been confirmed through dynamic cone penetrometer (DCP) tests and spectral analysis of surface waves (SASW) tests conducted by the Geotechnical Extreme Events Reconnaissance (GEER) team (Green et al. 2011, page 927 of this issue).

Immediately following the two earthquakes, reconnaissance work was performed to investigate the extent and features of the damage. Figures 7A and 7B show the distribution of liquefaction observed in the suburbs east of the CBD following the September 2010 and February 2011 earthquakes, respectively. These maps, which were constructed from on-foot investigations and drive-through surveys with the help of the University of Canterbury reconnaissance team, may be incomplete due to the limited time spent by the team in the area following each earthquake. As mentioned earlier, the shorter distance to the city and the shallower depth of the February

2011 earthquake resulted in more significant and more widespread damage in these areas than the September 2010 earthquake. The circle and square data points plotted in the figures correspond to the maximum distance from the Avon River at which lateral spreading was observed in the north and south banks, respectively, based on ground inspection. It is worth noting that while major liquefied sites in the September 2010 earthquake were concentrated along the Avon River, liquefaction was observed in the 2011 earthquake across a wider area, i.e., not only in the eastern suburbs but in the north and in the CBD as well.

Considering the short time interval between the two large earthquakes, the 2011 earthquake induced additional damage to many facilities that suffered liquefaction-induced damage after the 2010 earthquake and had not been repaired. Figures 8A and 8B show the condition of a river embankment adjacent to the Avon Rowing Club (east bank of Avon River) after the 2010 earthquake and 2011 earthquake, respectively. The width of crack openings on the shoulder and the settlement of the crown became larger due to the re-liquefaction of the foundation ground of the embankment. Extensive re-liquefaction was observed in the entire Porritt Park in 2011, where almost half of the green grassy area was covered by sand boils, similar to that observed after the 2010 earthquake, as shown in Figure 9.

The southern portion of the Bexley suburb was formerly a swamp and formed part of the Bexley wetlands. It had been reclaimed in the late 1990s by filling the area, and the subdivision was built over it. Interviews with homeowners indicate that the area was fairly new, with some houses built as recent as five years ago (Orense et al. 2011). The September 2010 earth-

910 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 7. Distribution of liquefaction-induced damage in the eastern suburbs: A) September 2010 earthquake; B) February 2011 earthquake.

Seismological Research Letters Volume 82, Number 6 November/December 2011 911

(A)

(B)

▲▲ Figure 8. Damage to a river embankment near the Avon Rowing Club: A) 2010 earthquake; and B) 2011 earthquake.

(A)

(B)

▲▲ Figure 9. Cracks observed in Porritt Park: A) 2010 earthquake; and B) 2011 earthquake.

quake triggered liquefaction of the loose uncompacted fill, resulting in ground settlement and lateral spreading. Ejected sands filled up the whole neighborhood, as thick as 30 cm in some areas (Figure 10A). Following the 2011 earthquake, Bexley was again one of the worst-hit areas in terms of liquefaction-induced damage. Massive amount of sands were again ejected and deposited around houses (Figure 10B). The massive sand boils ejected from underground caused differential ground settlements, resulting in the tilting of many houses.

Sand boils were also observed in the swamps of Bexley wetlands, indicating that the ground below the swamp also underwent liquefaction.

Kaiapoi The township of Kaiapoi is located in the northeastern end of the Canterbury Plains, about 20 km north of Christchurch (Figure 1). The Kaiapoi River, which cuts through the center of the town, joins the Waimakariri River on the eastern edge of the town and flows to the sea. In terms of liquefaction during the 2010 earthquake, Kaiapoi was probably the worst-hit area,

with many residential houses, several commercial buildings, and other infrastructure facilities suffering damage due to lateral spreading, ground subsidence, and differential settlement.

Investigations of old maps by Wotherspoon et al. (2010) showed that that much of the most significant liquefaction damage in and around Kaiapoi during the 2010 Darfield event occurred in areas where river channels had been reclaimed or in old channels that have had flow diverted away. The highly modified nature of the Waimakariri River and its proximity to Kaiapoi meant that some of these reclaimed areas overlapped regions that have since been developed as the town has grown, as shown in Figure 11.

Following the 2011 earthquake, re-liquefaction occurred in Kaiapoi; however, because of the farther epicentral distance, the impact of liquefaction was minor compared to that observed in September 2010. The peak ground accelerations recorded in Kaiapoi during the 2010 and 2011 earthquakes were 0.36 g and 0.20 g, respectively. A comparison of the distribution of liquefaction in Kaiapoi during the two events showed smaller liquefaction zones in 2011. Areas where re-liquefaction

912 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 10. Damage to a residential house due to liquefaction in Seabreeze Close, Bexley: A) 2010 earthquake; and B) 2011 earthquake.

43°22′11″

43°24′52″ 172°37′56″

172°42′32″

▲▲ Figure 11. Map of river channels in 1865 superposed on the present-day map of Kaiapoi (courtesy of L. Wotherspoon).

was observed include some areas adjacent to the stopbanks (levees) in northern Kaiapoi and in the filled-up sections of Courtenay Drive in southern Kaiapoi.

Figure 12 shows a residential house that suffered severe damage due to lateral spreading following the September 2010 earthquake. This house was standing on ground that moved toward the Waimakariri River, resulting in tilting of the house

and formation of a 1.6-m-wide crack between the house and the adjacent ground. Following the 2011 earthquake, the JGS team revisited the same house. Sand boils were observed only in the ground cracks adjacent to the house, with the width of the crack increasing to 1.9 m. It is unknown, however, whether the increase in the crack opening was caused by the 2011 earthquake alone. Creep deformation in this area due to the

Seismological Research Letters Volume 82, Number 6 November/December 2011 913

(A)

(B)

▲▲ Figure 12. A) Damage to a residential house in south Kaiapoi that underwent foundation failure due to lateral spreading and liquefaction in the 2010 Darfield earthquake. B) Additional damage after the 2011 Christchurch earthquake.

(A)

(B)

▲▲ Figure 13. A) Liquefaction at a park adjacent to Courtenay Lake, south Kaiapoi, following the September 2010 earthquake. B) Re-liquefaction during the February 2011 earthquake resulted in sand boils being ejected through existing cracks, but ground distortion was minor.

aftershocks of the 2010 Darfield earthquake has been reported (Cubrinovski and Orense 2010). Therefore, there is a possibility that the width of the crack was more than 1.6 m before the February 2011 earthquake and the impact of the earthquake to this area was minor. No other remarkable additional damage to residential houses/properties was observed in south Kaiapoi.

After February 2011, most of the sand boils in areas close to the waterways were observed at existing/repaired cracks caused by the 2010 earthquake. Aside from the lower intensity of ground shaking in Kaiapoi, it is possible that the excess pore water pressure generated by the earthquake motion could have been dissipated easily through the existing cracks and therefore this earthquake did not induce significant ground deformation (Figure 13).

On the other hand, a more pronounced liquefaction was observed in residential houses/properties in north Kaiapoi, although relatively minor compared to that after the 2010 Darfield earthquake. Figure 14 compares the settlement of a two-story house due to liquefaction. The slope in front of the garage was originally uphill, but it became downhill after the 2010 Darfield earthquake as a result of more than 50 cm of ground subsidence. This house suffered an additional 15 cm subsidence following the 2011 earthquake. The narrowing gap between the roof and the head of a member of the reconnaissance team can be recognized from the figure. Although the team members appearing in the photos are different, their heights are almost the same.

914 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 14. A) Subsidence of a two-story house adjacent to the stopbank in north Kaiapoi caused by the 2010 earthquake. B) Additional subsidence of the same house after the 2011 earthquake.

LIQUEFACTION OBSERVED ADJACENT TO HEATHCOTE RIVER Heathcote River, located in the southern boundary of Christchurch, meanders around the base of the Port Hills from west to southeast. It drains into the Avon-Heathcote estuary before draining into Pegasus Bay. Earlier studies have indicated that aside from parts of the eastern suburbs, the areas around the Heathcote River are underlain by loose saturated sand and silt, which have high potential to liquefy (Christchurch City Council 2005).

Following the September 2010 earthquake, a quick drivethrough investigation was conducted along the Heathcote River, specifically targeting areas that were denoted as having high potential for liquefaction-induced damage. However, there was very little evidence of ground distortion and liquefaction in this area, with only a few sand boils found in a period of about two hours of drive-through and on-foot surveys (Cubrinovski et al. 2010).

On the other hand, after the February 2011 earthquake, significant ground distortions due to liquefaction were observed adjacent to Heathcote River. In flat areas with shallow ground-water tables (e.g., St. Martins, Opawa, and Woolston), a number of structures such as stopbanks, bridges, and residential properties suffered severe damage due to liquefaction. Figure 15 shows the distribution of liquefaction along the Heathcote River observed during a walk-through investigation conducted two weeks after the 2011 earthquake. Again, the circular and square dots in the figure correspond to the maximum distance from the Heathcote River at which lateral spreading was observed. It is clear from the figure that severe liquefaction occurred at limited areas along the Heathcote River—considerably smaller than the ones observed adjacent to Avon River (see Figure 7B).

The lower areas along Wilson Road in St. Martins may be the worst-hit areas near the Heathcote River. Figure 16 shows the St. Martins library, a brick building whose collapse was caused by the differential subsidence of the foundation ground due to liquefaction. Ejected sands, with thickness on the order of 20 cm, were deposited around the right half of the foundation. Additionally, a tilted power pole can be seen in the right side of the figure, indicating that liquefaction occurred at shallow depth in this area.

Heathcote River winds along the foot of Port Hills, and therefore the topography around the river is full of ups and downs. From geological information, loess deposits are present in the subsurface at the base of Port Hills (Brown and Weeber 1992). No liquefaction was observed at the ground that is considered to be loess.

Figure 17 shows a trench in a residential property under construction in Eastern Terrace just beside the river. The trench depth was greater than 2 m and yet no groundwater was observed. Soil samples were collected from the trench at depths of 1 and 2 m from the ground surface, and their grain sizes were analyzed. Figure 18 shows the grain size distribution curves of soils taken from the trench in comparison with those of soils collected in other parts of Christchurch and Kaiapoi. The open dots correspond to the ejected sand collected from sites adjacent to Avon River and Kaiapoi, while the solid dots represent soils collected at a slope in Port Hills and near the Heathcote River where liquefaction was not observed. It can be seen that the sand boils have similar grain size distributions, regardless of the location where they were collected (in Kaiapoi or adjacent to the Avon or Heathcote rivers). They have fines content less than 25%. On the other hand, the subsurface soil adjacent to the Heathcote River, which did not liquefy, contained >90% fines with clay content >20%. Therefore, it is possible that the presence of unliquefiable soil at subsurface is a major reason

Seismological Research Letters Volume 82, Number 6 November/December 2011 915

▲▲ Figure 15. Distribution of liquefaction-induced damage adjacent to the Heathcote River after the February 2011 earthquake.

▲▲ Figure 16. Damage to a brick structure in St. Martins (near the Heathcote River) induced by soil liquefaction.

916 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 17. A trench in a residential property in Heathcote. Groundwater table was not evident even up to a depth of 2 m below the ground surface.

Percent finer by weight (%)

100

80

60

40

Huntsbury(Port Hills) Trench 2m (Eastern Tce) Trench 1m (Eastern Tce) Porritt Park Seabreeze(Bexley) North Kaiapoi Wilsons st(St. Martins)

20

0 0.001

0.01

0.1

1

10

Grain size (mm) ▲▲ Figure 18. Comparison of grain size distribution curves of ejected sands from different sites in Christchurch and soils from unliquefied subsurface sites near the Heathcote River.

why the extent of liquefaction along the Heathcote River was minor compared to that near the Avon River.

CONCLUDING REMARKS Although the M 7.1 Darfield earthquake caused liquefaction in Christchurch and adjacent areas, the M 6.3 Christchurch earthquake induced more widespread liquefaction and caused more serious damage to infrastructure. Liquefaction and re-

liquefaction were observed in areas with high potential to liquefy, such as natural deposits close to major streams, rivers, and wetlands as well as loose or uncompacted fill. Experiences from case histories all over the world have highlighted the effect of liquefaction on buildings and buried structures, but the scale of damage experienced in Christchurch following the 2010 and 2011 events was unprecedented and may be the greatest ever observed in an urban area. Moreover, the short time interval between the two large earthquakes presented a very rare oppor-

Seismological Research Letters Volume 82, Number 6 November/December 2011 917

tunity to investigate the liquefaction mechanism in natural deposits. Finally, the re-liquefaction experienced by the city as a result of the recent aftershocks on June 2011 highlights the high susceptibility of soil deposits in Christchurch to liquefaction and presents a very challenging problem not only to the local residents but to the geotechnical engineering profession as well.

ACKNOWLEDGMENTS The authors would like to acknowledge the other members of the NZ-JGS reconnaissance team: Kohji Tokimatsu (Tokyo Institute of Technology, Japan), Ryosuke Uzuoka (Tokushima University, Japan), and Hirofumi Toyota (Nagaoka University of Technology, Japan). The insights provided by Michael Pender, Tam Larkin, and Liam Wotherspoon, all of the University of Auckland, as well as the assistance of many postgraduate students from the University of Auckland and University of Canterbury, are gratefully acknowledged. Finally, we acknowledge the New Zealand GeoNet project and its sponsors EQC, GNS Science, and Land Information New Zealand for providing data used in this paper.

REFERENCES Berill, J., H. Avery, M. Dewe, A. Chanerley, N. Alexander, C. Dyer, C. Holden, and B. Fry (2011). The Canterbury Accelerograph Network (CanNet) and some results from the September 2010 M 7.1 Darfield earthquake. In Proceedings of the Ninth Pacific Conference on Earthquake Engineering, paper no. 181 (CD-ROM).

Auckland: New Zealand Society for Earthquake Engineering Brown, L. J., R. D. Beetham, B. R. Paterson, and J. H. Weeber (1995).

Geology of Christchurch, New Zealand. Environmental & Engineering Geoscience 1 (4), 427–488.

Brown, L. J., and J. H. Weeber (1992). Geology of the Christchurch Urban Area. Lower Hutt, New Zealand: Institute of Geological and Nuclear Sciences.

Christchurch City Council (CCC) (2005). 3.4.5 Earthquake Risk.

City Plan Online; http://www.cityplan.ccc.govt.nz (updated 14 November 2005).

Cubrinovski, M., J. D. Bray, M. Taylor, S. Giorgini, B. Bradley, L. Wotherspoon, and J. Zupan (2011). Soil liquefaction effects in the

central business district during the February 2011 Christchurch earthquake. Seismological Research Letters 82, 893–904.

Cubrinovski, M., R. Green, J. Allen, S. Ashford, E. Bowman, B. Bradley, B. Cox, T. Hutchinson, E. Kavazanjian, R. Orense, M. Pender, M. Quigley, and L. Wotherspoon (2010). Geotechnical reconnaissance of the 2010 Darfield (Canterbury) earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 243–320.

Cubrinovski, M., and R. Orense (2010). 2010 Darfield (New Zealand) earthquake—Impacts of liquefaction and lateral spreading. Bulletin of the International Society for Soil Mechanics and Geotechnical Engineering 4 (4), 15–23.

GeoNet (2010). Strong motion FTP site; ftp://ftp.geonet.org.nz/strong/ processed/Proc/2010/09_Darfield_mainshock_extended_pass_ band/.

GeoNet (2011). Strong motion FTP site; ftp://ftp.geonet.org.nz/strong/ processed/Proc/2011/02_Christchurch_mainshock_extended_ pass_band/.

Green, R. A., C. Wood, B. Cox, M. Cubrinovski, L. Wotherspoon, B. Bradley, T. Algie, J. Allen, A. Bradshaw, and G. Rix (2011).

Use of DCP and SASW tests to evaluate liquefaction potential: Predictions vs. observations during the recent New Zealand earthquakes. Seismological Research Letters 82, 927–938.

Inada, M. (1960). Interpretation of Swedish weight sounding. Tsuchi-toKiso [monthly magazine of the Japanese Geotechnical Society] 8 (1), 13–18 (in Japanese).

Japanese Standards Association (JSA) (1975). Japanese Industrial Standards: Method of Swedish Weight Sounding—JIS A 1221 (1975), 1995 revision.

Natural Hazards Research Platform (NHRP) (2011). Why the 2011 Christchurch Earthquake is Considered an Aftershock; http://www. naturalhazards.org.nz.

Orense, R., M. Pender, L. Wotherspoon, and M. Cubrinovski (2011).

Geotechnical aspects of the 2010 Darfield (New Zealand) earthquake. Invited lecture, Eighth International Conference on Urban Earthquake Engineering, Tokyo (Japan) (7–8 March 2011).

Wotherspoon, L. M., M. J. Pender, and R. P. Orense (2010). Relationship between observed liquefaction at Kaiapoi following the 2010 Darfield earthquake and old channels of the Waimakariri River.

Submitted to Engineering Geology.

Department of Civil and Environmental Engineering University of Auckland Private Bag 92019 Auckland 1142 New Zealand

918 Seismological Research Letters Volume 82, Number 6 November/December 2011

r.orense@auckland.ac.nz

(R. P. O.)

Ambient Noise Measurements following the 2011 Christchurch Earthquake: Relationships with Previous Microzonation Studies, Liquefaction, and Nonlinearity Marco Mucciarelli

Marco Mucciarelli Basilicata University

INTRODUCTION Following the Christchurch 2011 earthquake, the Basilicata University (Potenza, Italy) organized a field trip to New Zealand mainly to examine structural engineering issues but also to investigate the similarity between this event and the L’Aquila 2009 quake that struck central Italy. In both cases an event with magnitude slightly above 6 occurred on a blind fault underlying an area inhabited by a population of the order of hundreds of thousands, killing a few hundred people and severely damaging the city center, and in both cases a site amplification study was available before the event. At the same time there were striking differences between the two earthquakes in maximum recorded acceleration, the nonlinear behavior of soils, and the occurrence of liquefaction.

It was also an opportunity to look at some issues related to the use of microtremor measurements, in particular: 1. to verify if the soil frequencies estimated more than 15 years ago by Toshinawa et al. (1997) are a persisting feature or if there were changes following the strong motions in 2010 and 2011; 2. to verify the usefulness of the soil vulnerability index proposed by Nakamura (1996) as a proxy of liquefaction susceptibility; and 3. to compare the strong-motion recordings with elastic limit soil behavior derived from ambient noise, looking for hints of hardening nonlinearity as proposed by Bonilla et al. (2005) and similarity with the observations in L’Aquila (Puglia et al. 2011).

PREVIOUS STUDIES AND DATA COLLECTION In 1994 the Arthurs Pass Earthquake (Ml = 6.6) occurred about 100 km northwest of Christchurch. The macroseismic intensity was estimated for the city together with local site amplifications inferred from seismic recordings and microtremors (Toshinawa et al. 1997). The authors found a satisfactory correlation among the results of the different techniques and prepared a microzonation map. doi: 10.1785/gssrl.82.6.919

As for horizontal-to-vertical spectral ratio (HVSR) analysis of microtremors, Toshinawa et al. (1997) collected three sets of 40-sec-long samples at each site on a 1 by 1 km grid. They found that the H/V spectral ratio of microtremors was well correlated to the ground motion characteristics during earthquakes recorded at a seismic array deployed within the city and also correlated with the local geology. The outcropping lithology of the Christchurch area (Brown and Weeber 1992) is composed of: 1. Volcanic rock, in the southern part of the city.

2. Holocene marine dunes, in the vicinity of the coast.

3. Alluvial sand and silt deposits from the estuarine area to the center of the city, where swamps and lagoons were drained to reclaim land.

4. Alluvial gravel area, in the westernmost part of the city.

5. Transition area, with alternating deposits, located between the estuarine and gravel areas.

The soil fundamental frequency had higher values in the western gravel area, starting from 5 Hz and decreasing down to 1 Hz proceeding eastward in the transition area and in the sand and silt alluvium beneath the city center. The volcanic rock at the south returned a flat response and no clear peak was identified in the dune area near the ocean coastline.

During our field trip, we devoted three days to microtremor measurements. It was possible to perform 43 measurements, as listed in Table 1. We also collected 12 recordings as close as possible to accelerometric stations that recorded the February 2011 event, while the others were taken in the most damaged areas but with an effort to obtain good spatial coverage (Figure 1). The data were sampled at 128 Hz using a digital threecomponent tromometer (Micromed Tromino) for an acquisition length of at least 12 minutes. The data were then filtered, tapered, transformed in frequency domain, smoothed with a triangular filter (n = 5), and finally averaged between horizontal components and among 20-sec subsets using the geometric mean. Almost all the recordings returned HVSR peaks that passed the ensemble of tests proposed by the SESAME project (Chatelain et al. 2008).

Seismological Research Letters Volume 82, Number 6 November/December 2011 919

TABLE 1 List of the HVSR measurements, their geographic coordinates, the frequency of the highest HVSR peak and its amplitude. The last column is the Kg coefficient as introduced by Nakamura (1996).

Site CACS RSMC N13 N26 N27 PPHS N28 N25 CBGS CHHC CMHS N3 N2 N1 N12 RHES N30 N32 N31 N14 N24 N4-N5 CCCC N29 N15 N10 N16 SHLC N11 PRPC N17 N6 HPSC N18 N21 HVSC N9 N8 N19 N7 N23 N20 N22

Lon

Lat

172.5275768 172.5628001 172.5705568 172.5777560 172.5818618 172.6064083 172.6125849 172.6167294 172.6199070 172.6234361 172.6273183 172.6306775 172.6306950 172.6313143 172.6346267 172.6354074 172.6355683 172.6355845 172.6413124 172.6416583 172.6429684 172.6458412 172.6461493 172.6473579 172.6519704 172.6596168 172.6642247 172.6651345 172.6752989 172.6823280 172.6874653 172.6879189 172.7019825 172.7049053 172.7071643 172.7082133 172.7228573 172.7233046 172.7291321 172.7313212 172.7360934 172.7433333 172.7604590

–43.4826609 –43.5355676 –43.5437258 –43.5108726 –43.5213617 –43.4936198 –43.5122464 –43.5493591 –43.5292657 –43.5368474 –43.5675024 –43.5275045 –43.5285730 –43.5311002 –43.5806096 –43.5235211 –43.5310515 –43.5284343 –43.5309137 –43.5213395 –43.5476508 –43.5257455 –43.5373261 –43.4943542 –43.5319409 –43.5500570 –43.5319177 –43.5040313 –43.5624248 –43.5281150 –43.5401185 –43.5015612 –43.5024466 –43.5361145 –43.5575285 –43.5807977 –43.6046841 –43.6033945 –43.5239186 –43.5077893 –43.5611169 –43.5401993 –43.5769734

Fmax Amax 5.88 0.94 2.00 3.88 3.19 2.06 1.81 0.50 1.38 1.44 3.75 2.06 2.13 1.46 0.81 2.05 1.50 1.50 1.75 2.25 0.69 1.75 1.50 1.56 1.63 1.13 1.38 1.50 2.63 1.25 1.63 1.80 1.25 1.94 1.55 3.56 7.44 6.06 0.38 0.31 2.00 0.50 1.25

2.8 3.2 5.0 3.7 5.9 3.2 3.1 4.0 3.5 4.5 4.5 5.0 4.0 3.8 6.0 3.9 5.0 6.0 5.0 2.5 4.3 4.9 5.1 3.7 4.4 2.2 2.5 4.0 3.4 2.8 2.2 1.5 2.8 5.6 4.2 6.2 3.4 1.7 2.2 4.5 3.8 9.8

Kg 1.3 10.9 12.5 3.5 2.5 16.9 5.7 19.2 11.6 8.5 5.4 9.8 11.7 11.0 17.8 17.6 10.1 16.7 20.6 11.1 9.1 10.6 16.0 16.7 8.4 17.1 3.5 4.2 6.1 9.2 4.8 2.7 1.8 4.0 20.2 5.0 5.2 1.9 7.6 15.6 10.1 28.9 76.8

RESULTS We compared our results with the results of Toshinawa et al.

(1997) by preparing two maps (Figure 2 and Figure 3) that show the frequency of the highest peak in the HVSR curve and the relevant amplitude (Fmax and A max). It is worth noting that this highest peak does not always coincide with the soil fundamental frequency but is closer to the parameters chosen by Toshinawa et al. (1997), for a reason that is evident if one compares the frequency maps. As mentioned before, Toshinawa et al. did not find any resonance peak in the coastal dune area, while our measurements return values below 1 Hz. It is probable that the older instrumentation, coupled with a shorter-duration sampling time, could not detect low frequency soil resonance due to a deeper structure. This low frequency is also often visible moving westward, providing a peak whose amplitude is generally lower than those in the range above 1 Hz. The peaks at frequency >1 Hz appear to be in good agreement with those reported by Toshinawa et al. (1997), passing from 1 Hz below the city center up to 5–6 Hz in the westernmost part of Christchurch. Our results present quite a different picture when the amplitude is taken into account. While the Toshinawa et al. (1997) values never exceed 6 and below the city center HVSR maximum amplitudes are in the range of 2 to 3, our measurements top a value of 10 and under the city center (or central business district, CBD, as it is often called) the HVSR amplitudes range from 3.5 to 7. It is well known that HVSR amplitude could be rather sensitive to processing techniques, and a more precise comparison was not possible due to lack of further information from Toshinawa et al. (1997).

More detail on our results is given in Figures 4 and 5.

Figure 4 shows the clear change in fundamental frequency moving along longitude, with an increase moving westward.

The second plot (Figure 5) shows a site with HVSR that has a single, low-frequency peak while another site has a similar peak plus another one at higher frequency. It is known that HVSR has a problem in showing modes higher than the fundamental one (Parolai and Richwalski 2004), so the two peaks are most likely related to a complex stratigraphic situation with two resonant strata separated by a velocity inversion (Di Giacomo et al. 2005).

The second check we performed was the reliability of the soil damage index or vulnerability index introduced by Nakamura (1996) for use in estimating earthquake damage on the ground surface. Its distribution should delineate weak areas on the ground. It is given as: Kg = Ag2/ Fg , where Ag is the amplitude corresponding to soil resonance frequency Fg .

This coefficient was used in studies aimed at mapping the soil susceptibility to liquefaction prior to an earthquake (see, e.g., Beroya et al. 2009 for Laoag City, Philippines), but to our knowledge was never tested after an earthquake. The fact that the soil frequencies measured after the 2011 quake are in good agree-

920 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 1. Location of the noise measurements taken in Christchurch overlaid on a satellite view from Google Maps. Accelerometric station CACS and the relevant noise measurement are located northwest of Christchurch airport, out of the upper left corner of the image.

▲▲ Figure 2. Map of the frequency of the higher peak in HVSR curves.`

Seismological Research Letters Volume 82, Number 6 November/December 2011 921

▲▲ Figure 3. Map of the amplitude of the higher peak in HVSR curves.

▲▲ Figure 4. Shift in soil fundamental frequency moving from the easternmost site N10 to the westernmost site N27.

922 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 5. Site N20 showing HVSR as a single, low frequency peak and site N25 with a similar peak plus another one at higher frequency.

ment with those determined 17 years earlier provides an opportunity to test the Kg coefficient on soil that has not changed its characteristics as assessed with microtremor measurements.

The map of this coefficient is provided in Figure 6. The dotted area in the figure indicates the extent of the most severe liquefaction episodes as reported by Cubrinovski and Taylor (2011). The lack of correlation is evident in the most affected area, where both high and low values of Kg are present, and the same happens in those zones with no evidence of liquefaction. More detailed analyses are needed, however, before ruling out the Kg parameter, since the original formulation is based on the idea that just one peak is visible in the HVSR curve, corresponding to the fundamental frequency. As stated above, our measurement sometimes returned two peaks corresponding to two resonant strata at different depths, so it would be important to have more data about the depth of the layer where liquefaction occurred.

Our third activity was about the possible difference in soil behavior between the elastic, weak-motion domain and the strong-motion signals. In this paper I report preliminary results, since a comprehensive study on all the recording stations is still underway.

To check for variation of soil fundamental frequency that could be taken as a possible indication of nonlinear behavior, our research group has recently developed a technique that was

first applied to the recordings of the L’Aquila, 2009 earthquake (Puglia et al. 2011). The basic idea is the comparison between the soil fundamental frequency estimated using the HVSR technique and the time-frequency behavior during strong motion estimated using the S-transform technique (Stockwell et al.1996). This transform is particularly useful for analyzing a system that changes its dynamic characteristics over time since it provides information about the local spectrum of a generic signal overcoming the limitations derived from the assumptions of the stationarity of a signal (as is the case for the shorttime Fourier transform).

Using the same frequency scale it is possible to compare the S-transform with the HVSR to verify whether the fundamental frequency obtained from the noise recording remains constant during the S-waves phase, until the coda-waves and to the end of the signal. The S-transform is normalized to the maximum of each time step. The normalized S-transform allows us to better identify the site fundamental frequency, while the standard S-transform is more useful for identifying the instant-by-instant frequency content of the signal, its change over time, and the related energy.

The example given here is for station CBGS at Christchurch Botanical Gardens (Figure 7). The dashed red line highlights the cracks in the ground and the sand left by liquefaction, still clearly visible two months after the February 2011 quake. The acceler-

Seismological Research Letters Volume 82, Number 6 November/December 2011 923

▲▲ Figure 6. Map of the Kg parameter and its relation to liquefaction.

▲▲ Figure 7. The building hosting the accelerometric station CBGS at Christchurch Botanical Gardens. The dashed red line highlights the cracks in the ground and the sand left by liquefaction. There are a few centimeters of settlement to the left of this line.

924 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 8. Comparison between normalized S-transform and HVSR at CBGS (see text for details).

ometer is located inside the building, directly on the concrete foundation slab visible outside the door and under the bench. In addition to cracks and liquefaction, a settlement of the station occurred with a few centimeters displacement. This place is the ideal candidate for strong nonlinear effects, which are indeed visible in Figure 8. Figure 8 includes information derived from accelerometric and noise recordings, showing the accelerometric recording on the top left, the S-transform on the bottom left, and the HVSR on the bottom right panel; most of the energy of the largest horizontal component of motion is at frequencies lower than the fundamental one determined by HVSR. It is also worth noting that between 15 to 20 s, the time-domain trace and the S-transform show high-frequency acceleration evidence of hardening nonlinearity of the kind first described

by Bonilla et al. (2005), due to hysteretic dilatant behavior of non-cohesive, partially saturated soils (for more details on accelerometric recordings and liquefaction studies, see Bradley and Cubrinovski (2011, page 853 of this issue) and Smyrou et al.

(2011, page 882 of this issue).

CONCLUSIONS During a quick field survey in Christchurch after the February 2011 earthquake it was possible to collect microtremor mea-

surements close to accelerometric stations that recorded the event in the most damaged areas and with overall good spatial coverage. This allowed us to compare current measurements with the microzonation map produced by Toshinawa et al.

(1997), and in particular with their map of HVSR frequencies and peak values. There is a general agreement in the frequency map, with frequency decreasing from the western part of the city moving toward the estuarine area. The main difference with the previous study is that we were able to identify frequencies below 1 Hz that returned the maximum HVSR amplitude in the ocean coastline area but were also visible in other parts of the city. Another difference is that the HVSR amplitude map always provided higher values in this study with respect to Toshinawa et al. (1997).

Christchurch was affected by severe and widespread soil liquefaction. The fact that the soil frequency remained stable with respect to measurements performed before the ongoing earthquake sequence prompted us to check the reliability of the soil vulnerability index Kg introduced by Nakamura (1996) for estimating earthquake damage on the ground surface and previously used to map the soil susceptibility to liquefaction but never tested after an earthquake. No clear correlation has been found in this study between Kg and the occurrence of liquefaction. Possible explanations for this are (1) the Nakamura

Seismological Research Letters Volume 82, Number 6 November/December 2011 925

(1996) formulation is based on just the fundamental frequency peak, while HVSR in Christchurch often returns two peaks corresponding to resonant strata at different depths; and (2) the Kg parameter is dependent on the square of HVSR amplitude, which is quite unstable as discussed before.

Finally, the same technique based on S-transform, which after the L’Aquila 2009 earthquake did not point out significant evidence of nonlinearity, here shows clear signs of energy at frequencies lower than the fundamental one in the elastic domain (softening nonlinearity) in the coda of accelerograms from CBGS; at the same time it is possible to recognize hints of hardening nonlinearity due to hysteretic dilatant behavior of soils.

Future research will include a second, more detailed mapping of soil frequency using HVSR and comparison between elastic and nonlinear behavior at all the accelerometric stations, including the recordings of the September 2010 Darfield earthquake and the June 2011 Christchurch earthquake.

ACKNOWLEDGMENTS Many thanks are due to the staff of Canterbury University (Christchurch) who helped with logistical assistance, insightful field trips, and stimulating discussions, and in particular to Stefano Pampanin, Misko Cubrinovski, Tobias Smith, Weng Kam, and Umut Akguzel. Thanks to Rocco Ditommaso for the S-transform calculations. The paper was prepared during a stay at GFZ–Helmholtz Zentrum, Potsdam, and benefited from comments from colleagues after a seminar presentation.

REFERENCES Beroya, M. A. A., A. Aydin, R. Tiglao, and M. Lasala (2009). Use of microtremor in liquefaction hazard mapping. Engineering Geology 107, 140–153.

Bonilla, L. F., R. J. Archuleta, and D. Lavallée (2005). Histeretic and dilatant behavior of cohesionless soils and their effects on nonlinear site response: Field data observation and modeling. Bulletin of the Seismological Society of America 95, 2,373– 2,395.

Bradley, B. A., and M. Cubrinovski (2011). Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake. Seismological Research Letters 82, 853–865.

Brown, L. J., and J. H. Weeber (1992). Geology of the Christchurch Urban Area. Institute of Geological and Nuclear Sciences, Map 1, 1 sheet + 104 pp. Lower Hutt, New Zealand: GNS Science.

Chatelain, J.-L., B. Guillier, F. Cara, A.-M. Duval, K. Atakan, and the SESAME Working Group (2008). Evaluation of the influence of experimental conditions on H/V results from ambient noise recordings. Bulletin of Earthquake Engineering 6 (1), 33–74.

Cubrinovski, M., and M. Taylor (2011). Liquefaction Map V.1.0 22 Feb. 2001 earthquake, http://db.nzsee.org.nz:8080/en/web/ chch_2011/geotechnical/-/blogs/liquefaction-map-drive-throughreconnaissance. Last accessed 19 September 2011.

Di Giacomo, D., M. R. Gallipoli, M. Mucciarelli, S. Parolai, and S. M. Richwalski (2005). Analysis and modeling of HVSR in the presence of a velocity inversion: The case of Venosa, Italy. Bulletin of the Seismological Society of America 95, 2,364–2,372.

Nakamura, Y. (1996). Real-time information systems for hazard mitigation. In Proceedings of the 10th World Conference in Earthquake Engineering, paper # 2134.

Parolai, S., and S. M. Richwalski (2004). The importance of converted waves in comparing H/V and RSM site response estimates. Bulletin of the Seismological Society of America 94 (1), 304–313.

Puglia, R., R. Ditommaso, F. Pacor, M. Mucciarelli, L. Luzi, and M. Bianca (2011). Frequency variation in site response as observed from strong motion data of the L’Aquila, 2009 seismic sequence.

Bulletin of Earthquake Engineering 9 (3), 869–892; doi:10.1007/ s10518-011-9266-2.

Smyrou, E., P. Tasiopoulou, İ. E. Bal, and G. Gazetas (2011). Ground motions versus geotechnical and structural damage in the February 2011 Christchurch earthquake. Seismological Research Letters 82, 882–892.

Stockwell, R. G., L. Mansinha, and R. P. Lowe (1996). Localization of the complex spectrum: The S transform. IEEE Transactions on Signal Processing 44, 998–1,001.

Toshinawa, T., J. J. Taber, and J. B. Berrill (1997). Distribution of ground-motion intensity inferred from questionnaire survey, earthquake recordings, and microtremor measurements—A case study in Christchurch, New Zealand, during the 1994 Arthurs Pass earthquake, Bulletin of the Seismological Society of America 87 (2), 356–369.

Department of Structural Engineering, Geotechnical Engineering, Engineering Geology Basilicata University Viale dell’Ateneo Lucano, 10 85100 Potenza Italy

926 Seismological Research Letters Volume 82, Number 6 November/December 2011

marco.mucciarelli@unibas.it

Use of DCP and SASW Tests to Evaluate Liquefaction Potential: Predictions vs.

Observations during the Recent New Zealand Earthquakes Russell A. Green, Clint Wood, Brady Cox, Misko Cubrinovski, Liam Wotherspoon, Brendon Bradley, Thomas Algie, John Allen, Aaron Bradshaw, and Glenn Rix

Russell A. Green,1 Clint Wood, 2 Brady Cox, 2 Misko Cubrinovski, 3 Liam Wotherspoon,4 Brendon Bradley, 3 Thomas Algie,5 John Allen,6 Aaron Bradshaw,7 and Glenn Rix8

INTRODUCTION Following both the 4 September 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquakes, Geotechnical Extreme Events Reconnaissance (GEER) team members from the United States and New Zealand visited the affected areas to assess geotechnical related damage (e.g., Allen et al. 2010a, b). As shown in Figure 1, liquefaction was pervasive in large portions of the region after both earthquakes. The widespread liquefaction caused extensive damage to residential properties, water and wastewater networks, high-rise buildings, and bridges. For example, nearly 15,000 residential houses and properties were severely damaged from liquefaction and lateral spreading. More than 50% of these houses were damaged beyond economic repair. Also, portions of the central business district (CBD) were severely damaged by liquefaction during the Christchurch earthquake. It is estimated that approximately 30% of the buildings in the CBD were damaged beyond repair, although not all of the damage resulted from liquefaction.

Among the field tests performed by the GEER team were the dynamic cone penetrometer (DCP) test (Sowers and Hedges 1966) and spectral analysis of surface waves (SASW) test (Stokoe et al. 1994). Both of these tests can provide information about the liquefaction susceptibility of soil and are relatively portable, making them suitable for rapid post-earthquake reconnaissance field studies. The objective of this paper is to 1. Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia U.S.A. 2. University of Arkansas, Fayetteville, Arkansas U.S.A. 3. University of Canterbury, Christchurch, New Zealand 4. University of Auckland, Auckland, New Zealand 5. Partners in Performance, Sydney, Australia 6. TRI Environmental, Inc., Duluth Minnesota, U.S.A. 7. University of Rhode Island, Kingston, Rhode Island, U.S.A. 8. Georgia Tech, Atlanta, Georgia, U.S.A. doi: 10.1785/gssrl .82.6.927

provide an overview of DCP and SASW tests performed across the Christchurch region and to summarize the comparison of the observed versus predicted liquefaction occurrence during both the Darfield and Christchurch earthquakes.

BACKGROUND At 4:35 a.m. on 4 September 2010 NZ Standard Time, the previously unmapped Greendale fault ruptured, producing the Mw 7.1 Darfield earthquake. The epicenter for this event was approximately 40 km west of the center of Christchurch, but the closest distance from the fault rupture to the western suburbs of Christchurch (e.g., Hornby) was only about 10 km (e.g., Allen et al. 2010a). As shown in Figure 2, representative geometric means of the recorded horizontal peak ground accelerations (PGAs) were 0.71 g in the epicentral region, 0.20 g in the CBD, 0.32 g in Kaiapoi (north of Christchurch), and 0.27 g in Lyttelton (south of Christchurch) (e.g., Allen et al. 2010b).

The Mw 6.2 Christchurch earthquake occurred at 12:51 p.m. on 22 February 2011 NZ Standard Time. As with the Darfield earthquake, the Christchurch earthquake occurred on a previously unmapped fault, the Port Hills fault, located in the Port Hills south of Christchurch. The distance from the epicenter to the center of Christchurch was about 8 km, but the rupture plane was directly beneath some of the southern neighborhoods of Christchurch (e.g., Heathcote Valley) and Lyttelton.

As shown in Figure 2, representative geometric means of the recorded PGAs were 1.31 g in the epicentral region, 0.42 g in the CBD, 0.20 in Kaiapoi (north of Christchurch), and 0.11 g in Templeton (west of Christchurch).

Much of Christchurch and its environs were originally swampland, beach dune sand, estuaries, and lagoons that were drained as part of European settlement (Brown et al. 1995).

Consequently, in large areas the near-surface soil stratigraphy is characterized by inter-bedded, loose Holocene aged silt, sand, and gravel that are highly susceptible to liquefaction

Seismological Research Letters Volume 82, Number 6 November/December 2011 927

▲▲ Figure 1. Aerial image of the Canterbury Plains region. Black bordered areas are those that liquefied during the 4 September 2010 Mw 7.1 Darfield earthquake, while the white shaded areas are those that liquefied during the 22 February 2011 Mw 6.2 Christchurch earthquake.

(Environment Canterbury [ECan] 2004). This is especially the case in the eastern portion of Christchurch where the ground water table is only one to two meters below the ground surface. Unfortunately, the intense shaking of the Darfield and Christchurch earthquakes proved correct ECan’s (2004) findings regarding the high liquefaction susceptibility of these soils.

As shown in Figure 1, widespread liquefaction occurred in the eastern part of Christchurch and in Kaiapoi during both the Darfield and Christchurch earthquakes (also see Orense et al. 2011, page 905 of this issue). However, the Christchurch earthquake caused more widespread liquefaction in highly developed areas than did the Darfield earthquake due to the relative close proximity of the fault rupture. For example, liquefaction occurred in large portions of the CBD during the Christchurch earthquake that did not liquefy during the Darfield earthquake (Cubrinovski et al. 2011, page 893 of this issue). The areas most severely affected by liquefaction were the suburbs along the Avon River to the east of the CBD (Avonside, Dallington, Avondale, Burwood, and Bexley). The soils in these suburbs are predominantly loose fluvial deposits of clean fine sands and sands with non-plastic silts, with the top 5–6 m in a very loose state (Gerstenberger et al. 2011). Also, the town of Kaiapoi was significantly impacted by liquefaction

during both events, especially portions of the town that were built on abandoned river channels and fill (Wotherspoon et al.

2011).

Figure 3 illustrates the typical manifestation of liquefaction on the streets of Christchurch. The severity of the liquefaction led to large settlements of many houses including differential settlements that caused foundation and structural damage. The largest damage to land and built environment was caused by liquefaction-induced lateral spreading along the Avon River, streams, and wetlands in the eastern Christchurch suburbs. As mentioned above, lateral spreading displacements ranged anywhere from a few tens of centimeters up to 2 m at the river banks and extended inland as far as 200–300 m from the waterway, severely damaging roads, pipe networks, and residential properties within the affected zone (Robinson et al. 2011).

IN-SITU TESTS Following both the Darfield and Christchurch earthquakes, GEER team members from the United States and New Zealand performed in-situ tests using the DCP and SASW. Both tests are portable and provide information about the subsurface properties, making them suitable for immediate

928 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 2. Aerial image of Christchurch and its environs. Bullets are the location of the strong motion instrument stations, with the geometric mean of the peak horizontal accelerations listed for the Mw 7.1 Darfield earthquake (first number) and the Mw 6.2 Christchurch earthquake (second number).

post-earthquake reconnaissance investigations. Of particular interest to the team were the properties of the soils that liquefied in either, or both, the Darfield and Christchurch earthquakes. In the following, the DCP and SASW equipment, tests performed, and data reduction are described in more detail.

Dynamic Cone Penetrometer (DCP) The dynamic cone penetrometer (DCP) used for this study was designed by Professor George Sowers (Sowers and Hedges 1966) and is shown in Figure 4. This system utilizes a 6.8-kg mass (15-lb drop weight) on an E-rod slide drive to penetrate an oversized 45° apex angle cone. The cone is oversized to reduce rod friction behind the tip. At sites that liquefied, the DCP tests were performed in hand-augered holes that were bored to the top of the layer that liquefied, as determined by comparing the liquefaction ejecta to the auger tailings. At the sites tested that did not liquefy, the augered holes were bored to the top of the potentially liquefiable layer (i.e., sand layer below the ground water table), if such a layer was found. The augered holes minimized rod friction and allowed collection of samples of the liquefiable soil. Experience has shown that

the DCP can be used effectively in augered holes to depths up to 4.6 to 6.1 m.

The DCP tests consist of counting the number of drops of the 6.8-kg mass that is required to advance the cone ~4.5 cm (1.75 inches), with the number of drops, or blow count, referred to as the DCP N-value or NDCPT. NDCPT is approximately equal to the standard penetration test (SPT) blow count up to an N-value of about 10 (Sowers and Hedges 1966; Green et al. forthcoming). However, beyond an N-value of 10, the relationship becomes non-linear. Figure 5A shows the relationship between SPT and DCP N-values that was used in this study, which is a slightly modified version of the one proposed by Sowers and Hedges (1966). The modifications to the Sowers and Hedges (1966) relationship are specific to the soils in the Canterbury region and are based on comparing the NDCPT values to SPT N-values, cone penetration test (CPT) tip resistance, and shear wave velocity measurements made near the DCP test sites.

Following the procedure outlined in Olson et al. (forthcoming), the SPT equivalent N-values (NSPTequiv) values were normalized for effective overburden stress and hammer energy using the following relationship:

Seismological Research Letters Volume 82, Number 6 November/December 2011 929

(A)

(B)

(C)

▲▲ Figure 3. Typical manifestation of liquefaction during the Mw 6.2 Christchurch earthquake (similar manifestation occurred during the Mw 7.1 Darfield earthquake): A) A street in Hoon Hay after initial clean up (piling up) of sand and silt ejecta (25 February 2011); B) piles of sand ejecta from residential properties at Burwood (26 February 2011); and C) massive sand boils in recently developed residential areas of Burwood; this area did not liquefy in the 4 September quake (26 February 2011). All photos by M. Cubrinovski.

930 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 4. DCP test being performed adjacent to a house in Bexley after the 4 September 2010 Mw 7.1 Darfield earthquake.

Photo by R. Green on 15 September 2010.

P 0.5 ER , (1) N1,60−SPTequiv ≈ N SPTequiv ( N DCPT ) ⋅ ⎛ a ⎞ ⎝ σ vo ′ ⎠ 60% where NSPT equiv(NDCPT) is the functional relationship between NSPT and NDCPT shown in Figure 5A, Pa is atmospheric pressure (i.e., 101.3 kPa), σ′vo is initial vertical effective stress (in the same units as Pa), and ER is energy ratio. This relationship uses the effective stress and hammer energy normalization schemes outlined in Youd et al. (2001).

Although the energy ratio for the system was not measured, the DCP hammer is similar to the donut hammer used for the SPT. Skempton (1986) and Seed et al. (1984) suggested that the energy ratio for an SPT donut hammer system ranges from about 30 to 60%. However, because the DCP system does

▲▲ Figure 5. A) Relationship between DCP test and SPT N-values for an energy ratio of 60%, and B) comparison of NDCPT and the equivalent N1,60cs (N1,60cs-SPTequiv) for a site in an eastern neighborhood of Bexley.

not have pulleys, a cathead, etc., we anticipate that the energy ratio for the DCP is likely to be near the upper end of this range.

Therefore, we assumed an ER = 60% for our calculations. In addition to the effective stress and hammer energy corrections, the NSPT equiv values were also corrected for fines content following the procedure proposed in Youd et al. (2001). Figure 5B shows a plot of NDCPT and N1,60cs–SPTequiv for a test site in the eastern Christchurch neighborhood of Bexley, which

Seismological Research Letters Volume 82, Number 6 November/December 2011 931

▲▲ Figure 6. Aerial image of Christchurch and its environs. Superimposed on the image are locations where DCP tests were performed after either the Darfield or the Christchurch earthquake.

experienced severe liquefaction during both the Darfield and Christchurch earthquakes.

In total, 30 DCP tests were performed across Christchurch and its environs after the Darfield and Christchurch earthquakes. Figure 6 shows the locations of these test sites. In addition to the Darfield and Christchurch earthquakes, the DCP has been used on several other recent post-earthquake investigations to evaluate deposits that liquefied (e.g., the 2008 Mw 6.3 Olfus, Iceland, earthquake; the 2010 Mw 7.0 Haiti earthquake; the 2010 Mw 8.8 Maule, Chile, earthquake; and the 2011 Mw 5.8 Central Virginia, U.S.A. earthquake).

Spectral Analysis of Surface Waves (SASW) The spectral analysis of surface waves (SASW) method is used to determine the shear wave velocity (VS) profile at sites tested.

The SASW method is widely accepted and has been used to characterize the subsurface shear stiffness of soil and rock sites for the past 20-plus years (e.g., Nazarian and Stokoe 1984; Stokoe et al. 1994, 2003, 2004; Cox and Wood 2010, 2011; Wong et al. 2011). In particular, the SASW method has often been applied in geotechnical earthquake engineering to characterize materials for near-surface site response analyses (e.g., Rosenblad et al. 2001; Wong and Silva 2006) and soil liquefaction analyses (e.g., Andrus and Stokoe 2001). The SASW test is

a non-intrusive, active source seismic method that utilizes the dispersive nature of Rayleigh-type surface waves propagating through a layered material to infer the subsurface VS profile of a site.

The SASW field measurements in this study were made using three 4.5-Hz geophones, a “pocket-portable” dynamic signal analyzer, and a sledge hammer. Figure 7 shows the test setup at a site in south Kaiapoi. The geophones were model GSC-11Ds manufactured by Geo Space Technologies, while the analyzer was a Quattro system manufactured by Data Physics Corporation. The Quattro is a USB-powered, fourinput channel, two-output channel dynamic signal analyzer with 205-kHz simultaneous sampling rate, 24-bit ADC, 110-dB dynamic range, and 100-dB anti-alias filters. It is controlled with a flexible, Windows-based software package (Data Physics Signal Calc) that has measurement capabilities in both the time and frequency domains. The compact, highly portable nature of this setup is ideal for earthquake reconnaissance efforts where shallow VS profiles are desired. At most locations, receiver spacings of approximately 0.61, 1.22, 2.44, 4.88, 6.10, and 12.20 m were used to collect surface wave data. These tests took less than 45 minutes per location and typically enabled VS profiles to be generated down to 6.1–9.1 m below the surface.

In total, 36 SASW tests were performed across Christchurch

932 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 7. SASW setup at a site in south Kaiapoi. Photo by B. Cox on 12 Sept 2010.

and its environs after the Darfield and Christchurch earthquakes. Figure 8 shows the locations of the SASW test sites.

Spectral analysis was used to separate the measured surface waves by frequency and wavelength to determine the experimental (“field”) dispersion curve for the sites via phase unwrapping. An effective/superposed-mode inversion that takes into account ground motions induced by fundamental and higher-mode surface waves as well as body waves (i.e., a full wavefield solution) was then used to match theoretically the field dispersion curve with a one-dimensional (1D) layered system of varying layer stiffnesses and thicknesses (Roesset et al.

1991; Joh 1996). The 1D VS profile that generated a dispersion curve that best matched the field dispersion curve was selected as the site profile. Per Youd et al. (2001), the VS profiles were then normalized for effective overburden stress using the following relationship: P 0.25 VS1 = VS ⎛ a ⎞ , (2) ⎝ σ vo ′ ⎠

where VS1 is the shear wave velocity normalized to 1 atm effective stress, Pa is atmospheric pressure (i.e., 101.3 kPa), and σ′vo is initial vertical effective stress (in the same units as Pa).

Figure 9 shows a plot of VS and VS1 for a test site in the eastern Christchurch neighborhood of Bexley, which experienced severe liquefaction during both the Darfield and Christchurch earthquakes. Also plotted in this figure is the empirically determined upper-bound VS1 for liquefiable soils (i.e., soils having VS1 > V*S1 will not liquefy regardless of the intensity of shaking imposed on them).

ESTIMATION OF PGAs AT DCP AND SASW TEST SITES As discussed in the next section, the in-situ test data described above correlates to the ability of the soil to resist liquefaction (i.e., capacity). However, to evaluate liquefaction potential, both the soil’s ability to resist liquefaction and the demand imposed on the soil by the earthquake needs to be known. For the approach used herein to evaluate liquefaction potential (i.e., stress-based simplified procedure), the amplitude of cyclic

Seismological Research Letters Volume 82, Number 6 November/December 2011 933

▲▲ Figure 8. Aerial image of Christchurch and its environs. Superimposed on the image are locations where SASW tests were performed after either the Darfield or the Christchurch earthquake.

loading correlates to the PGA at the ground surface and the duration correlates to earthquake magnitude. Accordingly, the PGAs at the sites where DCP and SASW tests were performed needed to be estimated for both the Darfield and the Christchurch earthquakes. As outlined below, the PGAs recorded at the strong motion stations (refer to Figure 2) were used to compute the conditional PGA distribution at the test sites.

The PGA at a strong motion station i can be expressed as: ln PGA i = ln PGA i(Site, Rup) + η + εi ,

▲▲ Figure 9. Measured (VS ) and corrected (VS1) shear wave velocity profiles for a test site in the eastern Christchurch neighborhood of Bexley. Also shown is the theoretical limiting upperbound value of VS1 for liquefaction triggering (V*S1) for soil having FC = 9%.

(3)

where ln(PGA i) is the natural logarithm of the observed PGA at station i; ln PGA i (Site, Rup) is the predicted median natural logarithm of PGA at the same station by an empirical ground motion prediction equation (GMPE), which is a function of the site and earthquake rupture; η is the inter-event residual; and εi is the intra-event residual. Based on Equation 3, empirical GMPEs provide the distribution (unconditional) of PGA shaking as: ln(PGAi ) ~ N ( ln PGA i , σ η2 + σε2 ) ,

934 Seismological Research Letters Volume 82, Number 6 November/December 2011

(4)

where X ~ N(μ X, σX 2) is shorthand notation for X having a normal distribution with mean μ X and variance σX 2 .

By definition, all recorded PGAs from a single earthquake have the same inter-event residual, η. On the other hand, the intra-event residual, εi, varies from site to site, but is correlated spatially due to similarities of path and site effects among various locations. Accordingly, use can be made of recorded PGAs at strong motion stations (e.g., Figure 2) to compute a conditional distribution of PGAs at the DCP and SASW test sites.

First, we used the empirical GMPE proposed by Bradley (2010) to compute the unconditional distribution of PGAs at the strong motion stations. A mixed-effects regression was then used to determine the inter-event residual, η, and the intra-event residuals, εi ’s, for each strong motion station (Abrahamson and Youngs 1992; Pinheiro et al. 2008).

Second, the covariance matrix of intra-event residuals was computed by accounting for the spatial correlation between all of the strong motion stations and a test site of interest. The joint distribution of intra-event residuals at a test site of interest and the strong motion stations is given as: ε site

ε SMstation

σ 2εsite 0 , 0 Σ21

=N

Σ12 Σ 22

,

(5)

where X ~ N(μ X, Σ) is shorthand notation for X having a multivariate normal distribution with mean μ X and covariance matrix Σ (i.e., as before, but in vector form); and σ2εsite is the variance in the intra-event residual at the site of interest.

In Equation 5, the covariance matrix has been expressed in a partitioned fashion to elucidate the subsequent computation of the conditional distribution of εsite. The individual elements of the covariance matrix were computed from: Σ (i, j) = ρi,j σεi σεj ,

(6)

where ρi,j is the spatial correlation of intra-event residuals between the two locations i and j; and σεi and σεj are the standard deviations of the intra-event residual at locations i and j.

Based on the joint distribution of intra-event residuals given by Equation 5, the conditional distribution of εsite was computed from Johnson et al. (2007):

[ ε site ε SMstation] = N ( Σ12

(

= N με site

Σ 221 ε SMstation , σε2site Σ 12 Σ 221 Σ21 )

)

2 ε SMstation , σε site ε SMstation

(7)

Using the conditional distribution of the intra-event residual at a test site of interest given by Equation 7 and substituting into Equation 4, the conditional distribution of the PGA i was computed from:

[ ln PGA site ln PGA SMstation ] =

(

N ln PGAsite + η + μ ε site ε SMstation , σ 2ε site

ε SMstation

)

(8)

It should be noted that in cases where the test site of interest was located far from any strong motion station, the conditional distribution was similar to the unconditional distribution, and for test sites of interest located very close to a strong motion station the conditional distribution approached the value observed at the strong motion station.

To estimate the PGAs at the DCP and SASW test sites, the unconditional PGAs were estimated using the empirical GMPE proposed by Bradley (2010) and the conditional PGAs were estimated following the approach outlined above wherein the spatial correlation model of Goda and Hong (2008) was used.

LIQUEFACTION EVALUATION Using the PGAs determined as described above, the cyclic stress ratios (CSRs) at the DCP test sites, for both the Darfield and Christchurch earthquakes, were calculated following the methodology outlined in Youd et al. (2001). The average of the recommended range of magnitude scaling factors (MSFs) proposed in Youd et al. (2001) was used to compute CSR M7.5 at the sites.

As outlined previously, equivalent SPT N1,60 values were determined from the NDCPT values using Equation 1. These values were then corrected for fines content (FC) using the procedure proposed in Youd et al. (2001). For many of the sites, samples of the liquefiable soil were collected and analyzed in the laboratory to determine the FC. However, for sites where no samples were collected, FC = 12% was assumed, which is representative of the approximate fines content of soils at the sites sampled. Once the N1,60cs-SPTequiv were determined, the correlation proposed by Youd et al. (2001) was used to estimate the cyclic resistance ratio (CRR) for an Mw 7.5 event (i.e., CRR M7.5). Comparisons of the computed CSR M7.5 for both the Darfield and Christchurch earthquakes and CRR M7.5 for a test site in the eastern Christchurch suburb of Bexley are shown in Figure 10A. As shown in this figure, liquefaction is predicted to have occurred during both earthquakes (i.e., CSR M7.5 > CRR M7.5). However, the factor of safety against liquefaction (FS) is lower for the Christchurch earthquake than the Darfield earthquake, where FS = CRRM7.5/CSRM7.5. The lower factor of safety indicates increased severity of liquefaction. These predictions are consistent with field observations in Bexley made shortly after the two earthquakes (i.e., liquefaction occurred during both earthquakes, but was more severe during the Christchurch earthquake).

To compare the predicted versus observed liquefaction at all the DCP test sites, each of the DCP logs was analyzed for quality, and critical depths for liquefaction/thickness of the critical layers were selected. Logs where refusal was met within ~0.3 to 0.5 m of the start of the test were removed from the database, where refusal was taken as NDCPT > ~35 for more than two 4.5-cm drives. The reason for this is that too little of the profile was tested in these cases to make a meaningful interpretation. The thicknesses of the critical layers were selected based on how liquefaction manifested at the ground surface.

Seismological Research Letters Volume 82, Number 6 November/December 2011 935

(A)

(A)

(B) (B)

▲▲ Figure 10. Comparison of CSRM7.5 for the Darfield and Christchurch earthquakes with CRR M7.5 for a site in Bexley (FC = 9%): A) profiles for DCP test; and B) profiles for SASW test.

▲▲ Figure 11. Comparison of predicted versus observed liquefaction: A) DCP test; and B) SASW test.

In general, the selected critical layer thickness was thinnest for cases of lateral spreading with no ejecta, intermediate for lateral spreading with ejecta, and thickest for large sand boils with no associated lateral spreading. For example, the profile shown in Figure 10A laterally spread (see Figure 4) and there was a significant amount of ejecta that vented to the ground surface nearby. Using this information, and trends in the NDCPT, shown in Figure 5B, the selected critical layer was ~2 m thick, as indicated in Figure 10A. Once the critical layers were determined for each test site, the N1,60cs-SPTequiv values, CSR M7.5, and CRR M7.5 were averaged over these depths. The results were plotted along with Youd et al. (2001) SPT CRR M7.5 curve in Figure 11A. A similar procedure as that outlined above was used to compute the CSR M7.5 for the SASW test sites. However, the MSF proposed by Andrus and Stokoe (2000) was used instead

of the average of the recommended range proposed by Youd et al. (2001). The reason for using slightly different MSFs was to be consistent with how the respective cyclic resistance ratio curves were developed from the observational data. Using the computed VS1, the CRR M7.5 for the test site profiles were calculated following the Andrus and Stokoe (2000) procedure; this procedure is also outlined in Youd et al. (2001).

Comparisons of the computed CSR M7.5 for both the Darfield and Christchurch earthquakes and CRR M7.5 for a test site in the eastern Christchurch suburb of Bexley are shown in Figure 10B. Consistent with the DCP test results, liquefaction is predicted to occur at this site during both the Darfield and Christchurch earthquakes, with the liquefaction predicted to be more severe during the Christchurch earthquake. Again, these predictions are in line with the post-earthquake observations.

936 Seismological Research Letters Volume 82, Number 6 November/December 2011

Using the same critical layers as selected for DCP test liquefaction evaluations, VS1, CSR M7.5, and CRR M7.5 were averaged over the critical depths for each test site profile. The results were plotted along with the Andrus and Stokoe (2000) CRR M7.5 curves in Figure 11B.

DISCUSSION As shown in Figure 11, the liquefaction predictions made using both the DCP and SASW test data reasonably match field observations. This is particularly significant for the DCP data because a correlation was first required to convert the measured NDCPT to SPT N-values (shown in Figure 5A), and undoubtedly, this correlation is inherently uncertain. Also, the DCP was only able to test down to a depth of ~6 m at a maximum and usually less than about 4.5 m. Below this depth, NDCPT became large because of the presence of a dense layer and/or because of the increase in effective confining stress. Because the DCP is manually operated, performing tests beyond ~5 m depths becomes very laborious even in relatively loose sand deposits. The SASW test was able to test to deeper depths that the DCP, but was still limited to depths of ~6 to 9 m with the sledge hammer source. These depth limits are true shortcomings of both tests because at a few DCP and SASW test sites, available cone penetration test (CPT) soundings indicated the presence of potentially liquefiable layers deeper in the profiles.

As a result, our selected critical layer may only be one of multiple critical layers in the profile and may not be the most critical.

Also from Figure 11, it can be noted that most of the DCP and SASW tests were performed at sites that liquefied, with a paucity of data from sites that did not liquefy. The reason for this is the manifestation of liquefaction at the ground surface is a definite indication that liquefiable soils are present. Several no-liquefaction sites were investigated, especially ones adjacent to sites that liquefied. However, in the majority of these cases we were not able to find a sandy stratum below the ground water table in the upper ~5 m of these sites using the handauger. As a result, DCP tests were not performed at these sites, and the sites were not included in the DCP database.

CONCLUSIONS The U.S. and New Zealand members of the GEER team performed DCP and SASW tests after the 4 September 2010 Mw 7.1 Darfield earthquake and the 22 February 2011 Mw 6.2 Christchurch earthquake. Both tests are relatively portable, making them suitable for rapid, post-earthquake investigations.

Of particular interest to the team were characterizing sites that liquefied during either one or both of the earthquakes. Using the in-situ test data in combination with estimated PGAs, the liquefaction potential at the test sites was evaluated and compared with post-earthquake observations. Despite some shortcomings of the tests, they did a relatively good job in correctly predicting the occurrence/non-occurrence of liquefaction, proving the value of these tests for rapid, post-earthquake investigations.

ACKNOWLEDGMENTS The primary support for the US GEER Team members was provided by grants from the U.S. National Science Foundation (NSF) as part of the Geotechnical Extreme Event Reconnaissance (GEER) Association activity through CMMI00323914 and NSF RAPID grant CMMI-1137977. Also, Dr. Wotherspoon’s position at the University of Auckland is funded by the Earthquake Commission (EQC). However, any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the EQC.

REFERENCES Abrahamson, N. A. and R. R. Youngs (1992). A stable algorithm for regression analyses using the random effects model. Bulletin of the Seismological Society of America 82 (1), 505–510.

Allen, J., S. Ashford, E. Bowman, B. Bradley, B. Cox, M. Cubrinovski, R. Green, T. Hutchinson, E. Kavazanjian, R. Orense, M. Pender, M. Quigley, and L. Wotherspoon (2010a). Geotechnical reconnaissance of the 2010 Darfield (Canterbury) earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 243–320.

Allen, J., S. Ashford, E. Bowman, B. Bradley, B. Cox, M. Cubrinovski, R. Green, T. Hutchinson, E. Kavazanjian, R. Orense, M. Pender, M. Quigley, and L. Wotherspoon (2010b). Geotechnical Reconnaissance of the 2010 Darfield (New Zealand) Earthquake. GEER Association Report No. GEER-024, ed. R. A. Green and M. Cubrinovski.

Andrus, R. D., and K. H. Stokoe II (2000). Liquefaction resistance of soils from shear wave velocity. ASCE Journal of Geotechnical & Geoenvironmental Engineering 126 (11), 1,015–1,025.

Bradley, B. A. (2010). NZ-specific Pseudo-spectral Acceleration Ground Motion Prediction Equations Based on Foreign Models. Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand, 324 pp.

Brown L. J., R. D. Beetham, B. R. Paterson, and J. H. Weeber (1995).

Geology of Christchurch, New Zealand. Environmental & Engineering Geoscience 1 (4), 427–488.

Cox, B. R., and C. M. Wood (2010). A comparison of linear-array surface wave methods at a soft soil site in the Mississippi Embayment. In GeoFlorida 2010: Advances in Analysis, Modeling, and Design, ed.

D. O. Fratta et al., 1,369–1,378. Reston, VA: American Society of Civil Engineers.

Cox, B. R., and C. M. Wood (2011). Surface wave benchmarking exercise: Methodologies, results and uncertainties. In GeoRisk 2011: Geotechnical Risk Assessment and Management, ed. C. H. Juang et al., 845–852. Reston, VA: American Society of Civil Engineers.

Cubrinovski, M., J. D. Bray, M. Taylor, S. Giorgini, B. Bradley, L. Wotherspoon, and J. Zupan (2011). Soil liquefaction effects in the central business district during the February 2011 Christchurch earthquake. Seismological Research Letters 82, 893–904.

Environment Canterbury (ECan) (2004). Solid Facts on Christchurch Liquefaction. Environment Canterbury, Christchurch, New Zealand; http://ecan.govt.nz/publications/General/solid-factschristchurch-liquefaction.pdf.

Gerstenberger, M., M. Cubrinovski, G. McVerry, M. Stirling, D. Rhoades, B. Bradley, R. Langridge, T. Webb, B. Peng, J. Pettinga, K. Berryman, and H. Brackley (2011). Probabilistic Assessment of Liquefaction Potential for Christchurch in the Next 50 Years. GNS Science Report 2011/15, 30 pp.

Goda, K., and H. P. Hong (2008). Spatial correlation of peak ground motions and response spectra. Bulletin of the Seismological Society of America 98 (1), 354–465.

Seismological Research Letters Volume 82, Number 6 November/December 2011 937

Green, R. A., S. M. Olson, B. R. Cox, G. J. Rix, E. Rathje, J. Bachhuber, J. French, S. Lasley, and N. Martin (forthcoming). Geotechnical aspects of failures at Port-au-Prince seaport during the 12 January 2010 Haiti earthquake. Earthquake Spectra.

Joh, S. H. (1996). Advances in interpretation and analysis techniques for spectral-analysis-of-surface-waves (SASW) measurements.

PhD diss., Dept. of Civil, Architectural, and Environmental Engineering, University of Texas, Austin, TX, 240 pp.

Johnson, R. A., and D. W. Wichern (2007). Applied Multivariate Statistical Analysis. Upper Saddle River, NJ: Pearson Prentice-Hall.

Nazarian, S., and K. H. Stokoe II. (1984). In situ shear wave velocities from spectral analysis of surface wave tests. Proceedings of the Eighth World Conference on Earthquake Engineering, San Francisco, California, 21–28 July 1984. International Association for Earthquake Engineering (IAEE), 31–38.

Olson, S. M., R. A. Green, S. Lasley, N. Martin, B. R. Cox, E, Rathje, J. Bachhuber, and J. French (forthcoming). Documenting liquefaction and lateral spreading triggered by the 12 January 2010 Haiti earthquake. Earthquake Spectra.

Orense, R. P., T. Kiyota, S. Yamada, Y. Hosono, M. Okamura, and S. Yasuda (2011). Comparison of liquefaction features observed during the 2010 and 2011 Canterbury earthquakes. Seismological Research Letters 82, 905–918.

Pinheiro, J., D. M. Bates, S. DebRoy, D. Sarkar, and the R Core Team (2008). nlme: Linear and Nonlinear Mixed Effects Models. R package version 3.1, 89 pp.

Robinson, K., M. Cubrinovski, and P. Kailey (2011). Field measurements of lateral spreading following the 2010 Darfield earthquake. Proceedings of the Ninth Pacific Conference on Earthquake Engineering, 14–16 April 2011, Auckland, New Zealand, paper no. 52.

Roesset, J. M., D. W. Chang, and K. H. Stokoe II (1991). Comparison of 2-D and 3-D models for analysis of surface wave tests. Proceedings of the Fifth International Conference on Soil Dynamics and Earthquake Engineering, vol. 1, 111–126. International Society for Soil Mechanics and Geotechnical Engineering.

Rosenblad, B. L., K. H. Stokoe II, E. M. Rathje, and M. B. Darendeli (2001). Characterization of Strong Motion Stations Shaken by the Kocaeli and Duzce Earthquake in Turkey. Geotechnical Engineering Report GR01-1, Geotechnical Engineering Center, University of Texas at Austin.

Seed, H. B., K. Tokimatsu, L. F. Harder, and R. Chung (1984). The Influence of SPT Procedures on Soil Liquefaction Resistance Evaluations.

Report no. UCB\EERC-84/15, Earthquake Engineering Research Center, University of California, Berkeley, CA. Skempton, A. W. (1986). Standard penetration test procedures and the effects in sands of overburden pressure, relative density, particle size, aging and overconsolidation. Geotechnique 36 (3), 425–447.

Sowers, G. F., and C. S. Hedges (1966). Dynamic cone for shallow in-situ penetration testing, vane shear and cone penetration resistance testing of in-situ soils. American Society of Testing Materials (ASTM) Select Technical Paper 399, Philadelphia, PA: American Society of Testing Materials.

Stokoe, K. H. II, G. W. Wright, A. B. James, and M. R. Jose (1994).

Characterization of geotechnical sites by SASW method, in Geophysical Characterization of Sites, ed. R. D. Woods, 15–25. New Delhi: Oxford Publishers.

Stokoe, K. H. II, S. H. Joh, and R. D. Woods (2004). Some contributions of in situ geophysical measurements to solving geotechnical engineering problems, in Geotechnical and Geophysical Site Characterization, Proceedings of the International Site Characterization ISC’2 Porto, eds. A. Viana da Fonseca, A. and P. W. Mayne, 97–132. Rotterdam: Millpress.

Stokoe, K. H. II, B. L. Rosenblad, J. A. Bay, B. Redpath, J. G. Diehl., R. A. Steller, I. G. Wong, P. A. Thomas, and M. Luebbers (2003).

Comparison of VS Profiles from Three Seismic Methods at Yucca Mountain. Proceedings of Soil and Rock America 2003 1, 22–25 June 2003, Cambridge MA, 299–306.

Wong, I., and W. Silva (2006). The importance of in-situ shear-wave velocity measurements in developing urban and regional earthquake hazard maps. Proceedings of the 19th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems, 2–6 April 2006, 1,304–1,315. Environmental & Engineering Geophysical Society, CD-ROM. Wong, I., K. H. Stokoe II, B. R. Cox, Y.-C. Lin, and F.-Y. Menq (2011).

Shear-wave velocity profiling of strong motion sites that recorded the 2001 Nisqually, Washington, earthquake. Earthquake Spectra 27 (1), 183–212.

Wotherspoon, L. M., M. J. Pender, and R. P. Orense (2011). Relationship between observed liquefaction at Kaiapoi following the 2010 Darfield earthquake and former channels of the Waimakariri River. Submitted to Engineering Geology.

Youd, T. L., I. M. Idriss, R. D. Andrus, I. Arango, G. Castro, J. T. Christian, R. Dobry, et al. (2001). Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. ACSE Journal of Geotechnical and Geoenvironmental Engineering 127 (10), 817–833.

Department of Civil and Environmental Engineering Virginia Tech 120B Patton Hall Blacksburg, Virginia 24061 U.S.A. (R. A. G.)

938 Seismological Research Letters Volume 82, Number 6 November/December 2011

Performance of Levees (Stopbanks) during the 4 September 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, Earthquakes Russell A. Green, John Allen, Liam Wotherspoon, Misko Cubrinovski, Brendon Bradley, Aaron Bradshaw, Brady Cox, and Thomas Algie

Russell A. Green,1 John Allen, 2 Liam Wotherspoon, 3 Misko Cubrinovski,4 Brendon Bradley,4 Aaron Bradshaw,5 Brady Cox,6 and Thomas Algie7

INTRODUCTION The objective of this paper is to summarize the performance of the levees (or stopbanks) along the Waimakariri and Kaiapoi rivers during the 4 September 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquakes. Shortly after their arrival in the Canterbury area in the mid-nineteenth century European settlers started constructing drainage systems and levees along rivers (Larned et al. 2008).

In particular, flooding of the Waimakariri River and its tributaries posed a constant threat to the Christchurch and Kaiapoi areas. The current levee system is a culmination of several coordinated efforts that started in earnest in the 1930s and is composed of both primary and secondary levee systems. The primary levee system is designed for a 450-year flood. Damage estimates for scenarios where the flood protection system is breached have been assessed at approximately NZ$5 billion (van Kalken et al. 2007). As a result, the performance of the levee system during seismic events is of critical importance to the flood hazard in Christchurch and surrounding areas.

During the 2010 Darfield and 2011 Christchurch earthquakes, stretches of levees were subjected to motions with peak horizontal ground accelerations (PGAs) of approximately 0.32 g and 0.20 g, respectively. Consequently, in areas where the levees were founded on loose, saturated fluvial sandy deposits, liquefaction-related damage occurred (i.e., lateral spreading, slumping, and settlement). The performance summary presented herein is the result of field observations and analysis of 1. Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia U.S.A. 2. TRI Environmental, Duluth, Minnesota U.S.A. 3. University of Auckland, Auckland, New Zealand 4. University of Canterbury, Christchurch, New Zealand 5. University of Rhode Island, Kingston, Rhode Island, U.S.A. 6. University of Arkansas, Fayetteville, Arkansas, U.S.A. 7. Partners in Performance, Sydney, Australia doi: 10.1785/gssrl.82.6.939

aerial images (New Zealand Aerial Mapping 2010, 2011), with particular focus on the performance of the levees along the eastern reach of the Waimakariri River and along the Kaiapoi River.

In the sections that follow, we first present background information about the levee system. This is followed by an overview of the performance of the levees during the Darfield and Christchurch earthquakes. Next, we discuss the relationship between the severity of damage to the levees along the downtown stretch of the Kaiapoi River and the response of the foundation soils. Finally, we present a summary of the findings and draw conclusions.

BACKGROUND OF THE LEVEE SYSTEM The Waimakariri River flows from the Southern Alps across the Canterbury Plains between Christchurch, to the south, and Kaiapoi, to the north, and empties into Pegasus Bay in the east (Figure 1). The river drains a mountainous catchment area of 3,566 km2 and poses the most significant flood hazard in New Zealand (van Kalken et al. 2007). Early efforts

by European settlers to realign and contain the river within its banks were piecemeal and only partially successful (e.g., Wotherspoon et al. 2011). To better coordinate the efforts and to ensure equal flood protection to both Christchurch and Kaiapoi, the Waimakariri River Trust was established in 1923 (Griffiths 1979). In response to the 1926 floods (Figure 2), the Trust implemented a major river improvement scheme in 1930, known as the Hays No. 2 Scheme. Among other things, the scheme entailed an overall improvement of the levee system along the Waimakiriri River. However, these improvements were unable to prevent the major floods in 1940, 1950, and 1957. These floods prompted a further river improvement scheme in 1960, which entailed benching existing levees and constructing new levees.

Seismological Research Letters Volume 82, Number 6 November/December 2011 939

▲▲ Figure 1. Canterbury region of New Zealand.

▲▲ Figure 2. 1926 photograph of the Waimakariri River overflowing its banks in Christchurch. (Te Ara Encyclopedia of New Zealand 2010.)

The mean annual flow of the Waimakariri River is 120 m3/s. However, in 1957 the largest flood on record occurred, with an estimated peak discharge of 4,248 m3/s (Griffiths 1979). This flood initially was estimated to have a 100-year return period (Griffiths 1979) but was later revised to an approximately 450-year return period (Ian Heslop, personal communication 2010) and essentially served as the design basis flood for the levee improvement scheme implemented in the

1960s. Flood protection now includes approximately 100 km of levees.

A typical levee cross-section in the Canterbury region has 3:1 horizontal to vertical slopes on both the river and land sides (Figure 3). They range in height from 3 to 5 m above the subgrade and have a 4-m-wide top, which also serves as an access road. A flood event originating in the headwaters of the Waimakariri River takes approximately 1.5 days to travel downstream before it reaches the levee system that protects Christchurch and surrounding areas. At its crest, the 450-year event would leave 90 cm of freeboard, but may only last for four hours (Heslop, personal communication 2010).

The levees were often constructed by pushing up river gravels and silts. A typical cross-section is made up of a gravel core with 1-m-thick silt cap, which extends from the river side across the top (Figure 3). The levees typically sit on sandy soils at or near the ground water level. A toe filter was also constructed on the land side of the levee to prevent piping of sand during a high-water event. During the 1960 river improvement scheme, some new levees were constructed and benches were added to some of the existing levees, both of which were compacted using vibrating rollers (Tony Boyle, personal communication 2010). However, no compaction control or foundation analysis was conducted (Heslop, personal communication 2010).

940 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 3. Typical geometry and soil composition of levee cross-section.

SEISMIC PERFORMANCE OF THE LEVEES The authors performed damage surveys along stretches of the primary and secondary levees for the Waimakariri River and along the primary levees for the Kaiapoi River following both the 2010 Darfield and 2011 Christchurch earthquakes. The surveys were performed on foot, in an automobile, and from a helicopter. Additionally, the authors used high-resolution aerial images to aid in the damage survey and corresponded with Environment Canterbury (ECan) personnel (Ian Heslop and Tony Boyle). Heslop and Boyle oversaw the post-earthquake damage assessments performed by a local consulting firm and continue to oversee the repairs to the sections of the damaged levees. Below is a summary of the levee performance along the eastern reach of the Waimakariri River and along the Kaiapoi River.

As a result of the damage caused by the Darfield earthquake, ECan estimated that the flood capacity of the Waimakariri levee system had been reduced from a 450-year event (4,730 m3/s) to approximately a 15-year event (1,500 m3/s). Subsequently, concerns were raised when river flows rose to approximately 1,000 m3/s in the days following the Darfield earthquake (personal communications with Boyle and Heslop 2010). ECan proceeded with repairs to the most severely damaged sections of levees within weeks of the Darfield earthquake. Repairs progressed from severely damaged areas to those with only minor damage. By December 2010, the reconstruction had increased the flood protection capacity of the system to 2,500 m3/s or 20-year flood event (Heslop, personal communication 2011). These reconstruction costs, as a result of the Darfield earthquake, were approximately $NZ4 million, which was at the upper bound of the estimate provided

by ECan shortly after the earthquake (Heslop, personal communication 2010).

Damage repair to the levee system on the Waimakariri River was nearly complete at the time of the February 2011 Christchurch earthquake, with the system having been

returned to a 3,000 m3/s or 1-in-30-year event in December 2010. The Christchurch earthquake reduced the flood protection capacity to 2,500 m3/s or a 20-year flood event. As of July 2011, the restoration work has nearly been completed, increasing the capacity to 4,000 m3/s or a 100-year flood event. ECan estimates that an additional $NZ 2 million in damages to the levees were caused by the Christchurch earthquake (Heslop, personal communication 2011). Total restoration to preDarfield earthquake flood capacity is expected by end of 2011.

There was minor damage to the levee system caused by the 13 June 2011 Mw 6.0 aftershock, but it did not result in a reduction in flood capacity.

The majority of the damage to the levees resulting from both the Mw 7.1 Darfield and Mw 6.2 Christchurch earthquakes occurred east of SH1 as depicted in Figure 4 (note, SH1 is shown in Figure 7). In Figure 4, damage severity is categorized using the scale developed by Riley Consultants (2010, 2011). The scale has five grades that range from No Damage to Severe Damage, as summarized in Table 1. As may be observed from Figure 4, the damage patterns to the levees following both earthquakes are very similar, but are in general less severe for the Christchurch earthquake compared to the Darfield earthquake.

Note that some portions of the levees were already under repair by the time the authors were able to inspect them following the Christchurch earthquake. In these cases, the authors supplemented their field observations, to the extent possible, with observations both from high resolution aerial images taken the day after the Christchurch earthquake and field observations made by ECan consultants (Riley Consultants 2011).

The majority of the damage to the levees was a consequence of liquefaction in the foundation soils that resulted in lateral spreading, slumping, and/or settlement. The damage mostly manifested as longitudinal cracks running along the crest of the levees (Figure 5A). Although not desirable, moderate crack widths for this mode of damage are not believed to be critical to the functionality of the levees because they do not provide a direct seepage path from one side of the levee to the other. However, there is always the potential for these longitu-

Seismological Research Letters Volume 82, Number 6 November/December 2011 941

(A)

(B)

▲▲ Figure 4. Observed damage to levees following the A) 4 September 2010 Mw 7.1 Darfield earthquake and B) 22 February 2011 Mw 6.2 Christchurch earthquake. Adapted from Riley Consultants 2010, 2011.

942 Seismological Research Letters Volume 82, Number 6 November/December 2011

TABLE 1 Damage severity categories (Riley Consultants 2010, 2011) Category

Description

No Damage Minor Damage Moderate Damage Major Damage Severe Damage

No observed damage Cracks up to 5 mm wide and/or 300 mm deep. Negligible settlement of crest.

Cracks up to 1 m deep. Some settlement of crest.

Cracks greater than 1 m deep. Evidence of deep seated movement and/or settlement.

Severe damage or collapse. Gross lateral spread and/or settlement, cracks showing deformation of 500 mm or more.

dinal cracks to connect undetected transverse cracks or flaws that only penetrate partway through opposite sides of the levee.

Such a tortuous seepage path could potentially enlarge rapidly due to internal erosion and piping at high river levels.

Transverse cracks in the levees were less commonly observed than longitudinal cracks and were often associated with sharp bends along the length of the levees and/or slumping of the embankment (Figure 5B). Because these cracks provide a direct seepage path from one side of the levee to the other, they can severely impact the functionality of the levees. Even transverse cracks of minor width could potentially enlarge rapidly due to internal erosion and piping at high river levels and lead to the failure of that section of the levee.

Settlement of levee sections resulted from both post-liquefaction consolidation in the foundation soils and bearing capacity failures due to the reduced strength of the liquefied foundation soil. In addition to the degradation in levee functionality due to cracking associated with the settlement (similar to that discussed above), settlement also reduces the amount of freeboard at high river levels. The significance of this loss is dependent on the magnitude of the settlement, but in general it is not thought to be a significant issue with the levee system.

Another liquefaction-related mode of degradation to the levees’ capacity is where liquefaction and/or lateral spreading formed on both sides of the levee. In these cases a potential flow path is formed down through the vertical feeder dike on the river side of the levee, laterally through the source stratum under the levee, and up through the vertical feeder dike on the other side of the levee. Extensive liquefaction was observed on both sides of the levee along an approximately 0.5-km stretch of the Waimakariri River on Coutts Island Road (Figure 6).

From interviews with local land owners and review of maps of the area from 1865, this area was part of an old river channel (Wotherspoon et al. 2011). Additionally, borings performed by the authors using a hand auger showed a deep sand deposit along this 0.5-km stretch of levee and buried sticks and logs on both ends, consistent with an old river channel and river channel banks.

SEVERITY OF DAMAGE AND FOUNDATION SOILS To examine the relationship between the severity of the induced damage to the levees and the liquefaction response of the foundation soil, a stretch of levees along the Kaiapoi River was examined in more depth. As shown in Figure 4, these

levees sustained damage ranging from No Damage to Severe Damage (Table 1). Following the Darfield earthquake, the New Zealand Earthquake Commission (EQC) contracted a local firm to perform a series of cone penetration tests (CPTs), among other in-situ tests, throughout north and south Kaiapoi (Tonkin and Taylor 2010). The locations of the CPT soundings performed on, or adjacent to, the levees along the Kaiapoi River are shown in Figure 7.

Representative CPT soundings from the north and south banks of the Kaiapoi River are presented in Figures 8A and 9A. From interpreting 27 such CPT logs, as well as available borehole data (Tonkin and Taylor 2011), the soil profile along the north bank of the Kaiapoi River east of the Williams Street Bridge is characterized by approximately 4 m of very loose to loose sand overlying approximately 4 m of loose to medium dense gravelly sand. Below approximately 8 m, the sand and gravel layers tend to be significantly denser than the overlying layers. The depth to the ground water table varies, but is approximately 1.5 m deep. Samples of the liquefiable soils taken adjacent to the levees on the north bank had fines contents around 15%, with the fines being non-plastic. On the south bank of the Kaiapoi River east of the Williams Street Bridge, the soil profile is characterized by approximately 6 m of very loose to loose silty sand/sand overlying an approximately 2-m-thick layer of loose to medium dense sand/gravelly sand.

Below approximately 8 m the sand and gravel layers tend to be significantly denser than the overlying layers. The ground water table is approximately 2 m deep.

Using the 27 Kaiapoi levee CPT soundings and two additional soundings performed adjacent to the levees along the southern bank of the Waimakariri River in Kainga and Brooklands, the authors analyzed liquefaction of the foundation soils following the procedures outlined in Youd et al.

(2001). The strong motion seismograph station KPOC, located in north Kaiapoi (Figure 7), recorded motions from both earthquakes. The geometric mean of the horizontal peak ground accelerations (PGAs) of the motions recorded during the Darfield and Christchurch earthquakes were 0.32 g and 0.20 g, respectively. The distance from the strong motion station to the CPT sounding locations ranges from approximately 0.7 to 3.7 km, with the majority of the soundings being located less than 1 km from the station. Because of this close proximity, 0.32 g and 0.20 g PGAs were used to compute the cyclic stress ratios (CSRs) imposed on the soil at all the sounding locations during the Darfield and Christchurch earthquakes, respectively.

Seismological Research Letters Volume 82, Number 6 November/December 2011 943

(A)

(B)

▲▲ Figure 5. Cracks in levee: A) Example of longitudinal cracks running along the crest of the levee; and B) example of transverse (or oblique) crack in levee.

944 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 6. A) Large sand boil on landside of the levee on Coutts Island Rd. B) Large sand boil on river side of the same section of the levee.

Seismological Research Letters Volume 82, Number 6 November/December 2011 945

▲▲ Figure 7. Locations of CPT soundings on or adjacent to levees.

Figures 8B and 9B show the results from the liquefaction evaluation for the two representative CPT soundings mentioned above. In these figures, the cyclic resistance ratio (CRR) for each profile and the CSRs for both events are plotted together, where both the CRR and CSR are adjusted to an Mw 7.5 earthquake. For liquefiable soils (i.e., gravels, sands, and cohesionless silts), liquefaction is predicted to have occurred at depths where the CSR M7.5 > CRR M7.5. Accordingly, for both profiles, liquefaction is predicted to have occurred during the Darfield earthquake for almost the entire depth from the ground water table to the top of the dense gravel/sand layer (i.e., to ~7.5 m and ~11 m for the north and south river banks, respectively). However, during the Christchurch earthquake, marginal liquefaction is predicted to occur at a few isolated depths within both profiles.

In an attempt to relate the severity of the observed levee damage to the liquefaction response of the foundation soil, plots of factor of safety against liquefaction versus damage index (i.e., FSLiq vs. DI) and thickness of the liquefied layer versus damage index (i.e., T vs. DI) were made for the 29 CPT soundings analyzed. Note, damage index corresponds to the damage categories proposed by Riley Consultants (2010, 2011): 1 = No Damage, 2 = Minor Damage, 3 = Moderate Damage, 4 = Major Damage, and 5 = Severe Damage. We performed linear regressions on the data, where first the data from the two earthquakes were kept separate (Figure 10) and then they were combined (Figure 11). In developing these plots,

the sections of the levees that were under repair at the time of the authors’ field inspections were assumed to have DI = 4.

The basis for this is that these sections were given high priority for repair, which implies that the sustained damage was significant. However, because the intensity of shaking during the Christchurch earthquake at these locations was significantly less than that during the Darfield earthquake, it is likely that the levels of damage induced by the Christchurch earthquake were less severe than those from the Darfield earthquake.

Expected trends can be identified in all plots (i.e., the damage index increases as the factor of safety against liquefaction decreases and as the thickness of the liquefied layer increases).

However, the strength of the trends, as indicated by the correlation coefficients (r 2), varies between the two earthquakes when the data is treated separately. For example, for the Darfield earthquake, the lowest correlation coefficient (r 2 = 0.147) is for T vs. DI, but T vs. DI has the highest correlation coefficient (r 2 = 0.625) for the Christchurch earthquake. In contrast, the correlation coefficients for FSLiq vs. DI are relatively consistent for both the Darfield and Christchurch earthquakes (i.e., r 2 = 0.562 and r 2 = 0.595, respectively). When the data from the two earthquakes are combined, r 2 = 0.348 and r 2 = 0.578 for T vs. DI and FSLiq vs. DI, respectively.

From the correlation coefficients, the factor of safety against liquefaction appears to be a better index for damage severity than the thickness of the liquefied layer. This is not altogether surprising given that a lot of the damage to

946 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 8. A) Representative CPT sounding for the north bank of the Kaiapoi River; B) liquefaction evaluation of the site for both the Darfield and Christchurch earthquakes.

(B)

(A)

▲▲ Figure 9. A) Representative CPT sounding for the south bank of the Kaiapoi River; B) liquefaction evaluation of the site for both the Darfield and Christchurch earthquakes.

the levees resulted from lateral spreading, more so than deep seated slumping and settlement/bearing capacity failures. Of these three failure modes, lateral spreading can occur even if a relatively thin layer liquefies, while deep seated slumping and settlement/bearing capacity failures require a thicker layer to liquefy. This is likely the reason for the disparity between the r 2 values for the T vs. DI plots for the Darfield and Christchurch events. In the case of the Darfield earthquake, the levees were subjected to relatively intense shaking and the thickness of the liquefied layer was large. However, because lateral spreading can occur on even a thin liquefied layer, the r 2 value for the T vs. DI plot was very low (i.e., r 2 = 0.147). In contrast, the levees were subjected to less shaking during the Christchurch earthquake and the liquefied layers were relatively thin where liquefaction occurred. However, even these relatively thin liquefied layers were thick enough for lateral spreading to occur, which

resulted in damage to the levees and a relatively high value of r 2 for the T vs. DI plot (i.e., r 2 = 0.625). The implication of this is that liquefaction severity indices that account for both the factor of safety against liquefaction and thickness of the liquefied layer, such as the liquefaction potential index (LPI) (Iwasaki et al. 1982), may not be appropriate for evaluating the risk of damage from liquefaction where lateral spreading is the primary failure mode.

SUMMARY AND CONCLUSIONS The seismic stability of the levees in the Christchurch, New Zealand, area is critically important to the flood protection for the region. Overall, the levee system performed well during both the Mw 7.1 Darfield and Mw 6.2 Christchurch earthquakes. However, portions of the levees along the eastern

Seismological Research Letters Volume 82, Number 6 November/December 2011 947

(A)

(B)

(C)

(D)

▲▲ Figure 10. Correlations relating factor of safety against liquefaction and damage index and thickness of the liquefied layer for the data from the Darfield and Christchurch earthquakes regressed separately. (Note that the low r 2 value in (B) indicates an extremely weak correlation between the thickness of the liquefied layer and damage index for the Darfield earthquake; hence a dotted line is used to show the results of the regression.)

(A)

(B)

▲▲ Figure 11. Correlations relating factor of safety against liquefaction and damage index and thickness of the liquefied layer for combined Darfield and Christchurch earthquake data.

948 Seismological Research Letters Volume 82, Number 6 November/December 2011

reach of the Waimakariri River and along the Kaiapoi River sustained varying levels of damage during both events. In all cases observed by the authors, damage was related to liquefaction in the foundation soils. In an attempt to relate the severity of the observed levee damage to the liquefaction response of the foundation soil, damage severity was correlated to factor of safety against liquefaction and to thickness of the liquefied layer. While damage severity correlated to both of these measures, the factor of safety against liquefaction appears to be the better index of damage severity when lateral spreading is the primary failure mode.

DATA AND RESOURCES Personal communications with Tony Boyle took place 27 September 2010. Personal communications with Ian Heslop took place October and November 2010 and May and July 2011.

ACKNOWLEDGMENTS The primary support for the U.S. GEER team members was provided by grants from the U.S. National Science Foundation (NSF) as part of the Geotechnical Extreme Events Reconnaissance (GEER) Association activity through CMMI00323914 and NSF RAPID grant CMMI-1137977. Also, Dr. Wotherspoon’s position at the University of Auckland is funded by the New Zealand Earthquake Commission (EQC).

However, any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the EQC.

REFERENCES Griffiths, G. A. (1979). Recent sedimentation history of the Waimakariri River, New Zealand. Journal of Hydrology (New Zealand) 18, 6–28.

Iwasaki, T., K. Tokida, F. Tatsuoka, S. Watanabe, S. Yasuda, and H. Sato (1982). Microzonation for soil liquefaction potential using simplified methods. Proceedings of the Third International Conference on Earthquake Microzonation, Seattle WA, 1,319–1,330.

Larned, S. T., D. M. Hicks, J. Schmidt, A. J. H. Davey, K. Dey, M. Scarsbrook, D. B. Arscott, and R. A. Woods (2008). The Selwyn River of New Zealand: A benchmark system for alluvial plain rivers. River Research and Applications 24, 1–21.

New Zealand Aerial Mapping (2010). Kaiapoi (air photo). Wellington, New Zealand.

New Zealand Aerial Mapping (2011). Kaiapoi (air photo). Wellington, New Zealand.

Riley Consultants (2010). Waimakariri and Kaiapoi River Stopbanks Post Earthquake Condition Assessment. Report 10820-B prepared

by Riley Consultants for Environment Canterbury (ECan). Riley Consultants, Christchurch, New Zealand.

Riley Consultants (2011). Waimakariri and Kaiapoi River Stopbanks Findings of Condition Assessment Post 22 February 2011 Earthquake.

Letter Report 10820/2-A from Riley Consultants to Environment Canterbury (ECan). Riley Consultants, Christchurch, New Zealand.

Te Ara Encyclopedia of New Zealand (2010). http://www.teara.govt.nz/ en/floods/6/7.

Tonkin and Taylor (2011). Darfield Earthquake Recovery Geotechnical Factual Report—Kaiapoi North. Report EP-KAN-FAC prepared

by Tonkin and Taylor, Ltd. for the New Zealand Earthquake Commission. Tonkin and Taylor, Christchurch, New Zealand. van Kalken, T., T. Oliver, I. Heslop, and T. Boyle (2007). Impacts of secondary flood embankments on the Waimakariri floodplain, New Zealand. Proceedings of the 32nd Congress of the International Association of Hydraulic Engineering and Researchers, July 1–6, 2007, Venice, Italy.

Wotherspoon, L. M., M. J. Pender, and R. P. Orense (2011). Relationship between observed liquefaction at Kaiapoi following the 2010 Darfield earthquake and former channels of the Waimakariri River. Submitted to Engineering Geology.

Youd, T. L., I. M. Idriss, R. D. Andrus, I. Arango, G. Castro, J. T. Christian, R. Dobry, et al. (2001). Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. ASCE Journal of Geotechnical & Geoenvironmental Engineering 127 (10), 817–833.

Department of Civil and Environmental Engineering Virginia Tech 120B Patton Hall Blacksburg, Virginia 24061 U.S.A. rugreen@vt.edu

(R. A. G.)

Seismological Research Letters Volume 82, Number 6 November/December 2011 949

Performance of Bridges during the 2010 Darfield and 2011 Christchurch Earthquakes Liam Wotherspoon, Aaron Bradshaw, Russell Green, Clinton Wood, Alessandro Palermo, Misko Cubrinovski, and Brendon Bradley

Liam Wotherspoon,1 Aaron Bradshaw, 2 Russell Green, 3 Clinton Wood,4 Alessandro Palermo,5 Misko Cubrinovski,5 and Brendon Bradley5

INTRODUCTION

LOCAL GEOLOGY

The region in and around Christchurch, encompassing Christchurch city and the Selwyn and Waimakariri districts, contains more than 800 road, rail, and pedestrian bridges.

Most of these bridges are reinforced concrete, symmetric, and have small to moderate spans (15–25 m). The 22 February 2011 moment magnitude (Mw) 6.2 Christchurch earthquake induced high levels of localized ground shaking (Bradley and Cubrinovski 2011, page 853 of this issue; Guidotti et al. 2011, page 767 of this issue; Smyrou et al. 2011, page 882 of this issue), with damage to bridges mainly confined to the central and eastern parts of Christchurch. Liquefaction was evident over much of this part of the city, with lateral spreading affecting bridges spanning both the Avon and Heathcote rivers.

The majority of bridge damage was a result of liquefaction-induced lateral spreading, with only four bridges suffering significant damage on non-liquefiable sites. Abutments, approaches, and piers suffered varying levels of damage, with very little damage observed in the bridge superstructure.

However, bridges suffered only a moderate amount of damage compared to other structural systems. Because some bridges critical to the city infrastructure network sustained substantial damage, extensive traffic disruption occurred immediately following the event.

This paper presents a summary of field observations and subsequent analyses on the damage to some of the bridges in the Canterbury region as a result of the Christchurch earthquake. Reference is also made to the performance of bridges following the 4 September 2010 Mw 7.1 Darfield earthquake (Gledhill et al. 2011), and details of damage progression are presented where applicable. The ground motion characteristics for both events and the regional soil conditions are first described.

We provide descriptions of the damage at each selected bridge site and compare observations of liquefaction with predicted response using in situ test data.

The city of Christchurch, shown in Figure 1, is located along the central coast of the Canterbury Plains, an approximately 50-km-wide and 160-km-long region created by the overlapping alluvial fans of rivers flowing east from the Southern Alps. Interbedded marine and terrestrial sediments up to 40 m deep overlie 300 to 400 m of late Pleistocene sands and gravels (Brown and Weeber 1992). Much of the city was originally swampland, beach dune sand, estuaries, and lagoons, which were drained as part of the settlement and expansion of the city (Brown et al. 1995). A high water table, one to two meters below the ground surface in the east of the city, gradually increases in depth moving across the city to the west. To the south of the city are the Port Hills, formed from volcanic activity (Brown and Weeber 1992).

Two spring-fed meandering rivers, the Avon and the Heathcote, cut through Christchurch (Figure 2). The Avon River passes through the city from west of the Christchurch

1. University of Auckland, New Zealand 2 . University of Rhode Island, Kingston, Rhode Island, U.S.A. 3. Virginia Tech, Blacksburg, Virginia, U.S.A. 4. University of Arkansas, Fayetteville, Arkansas, U.S.A. 5. University of Canterbury, Christchurch, New Zealand

▲▲ Figure 1. Overview of Christchurch city and its surroundings, with the epicenters of the Darfield and Christchurch earthquakes shown by stars. Boundaries of moderate bridge damage during the Darfield earthquake at Lincoln and Kaiapoi are represented by circles. The region of interest for Christchurch bridges presented in Figure 2 is bounded by the dashed white rectangle (Google Inc. 2011).

950 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.950

▲▲ Figure 2. Horizontal peak ground accelerations recorded at strong motion sites in Christchurch during the Darfield and Christchurch earthquakes, and the locations of bridges highlighted in this paper (Google Inc. 2011).

central business district (CBD), through the CBD, and to the eastern edge of the city where it enters the Avon-Heathcote estuary. East of the CBD the Avon River widens as it nears the estuary. The Heathcote is a smaller river and runs from west to east in the southern part of the city before entering the estuary.

GROUND MOTION CHARACTERISTICS On 4 September 2010, the Mw 7.1 Darfield earthquake struck 40 km west of the Christchurch CBD at a focal depth of 11 km (Gledhill et al. 2011). The highest recorded ground motions were near the epicenter, having a maximum horizontal PGA of 0.76 g (geometric mean of the horizontal components, applies to all horizontal PGAs stated herein) and a maximum vertical PGA of 1.26 g. These large vertical accelerations are typical

of the near-source strong motion recordings for this event. A maximum horizontal PGA of 0.25 g and maximum vertical PGA of 0.22 g were recorded in the Christchurch CBD, and the PGA generally decreased with distance downstream along the Avon River. The largest vertical PGA in the central and eastern areas of Christchurch was 0.32 g at Pages Road pumping station.

The Mw 6.2 2011 Christchurch earthquake was centered less than 10 km from the Christchurch CBD along the southeastern perimeter of the city in the Port Hills (Figure 1). The close proximity and shallow depth of this event caused higher intensity shaking in Christchurch as compared to the Darfield earthquake. In the city, ground motions were characterized by large vertical accelerations resulting from the close proximity to the fault plane, steeply dipping oblique thrust faulting mecha-

Seismological Research Letters Volume 82, Number 6 November/December 2011 951

(A)

(B) ▲▲ Figure 3. Response spectra of the geometric mean of the horizontal accelerations at strong motion station recordings in central and eastern Christchurch compared to NZS1170.5 design response spectrum for Christchurch, site subsoil class D for 500-year return period. A) Darfield earthquake, B) Christchurch earthquake. Four-letter symbols represent different strong motion stations, positions of which are indicated in Figure 2.

nism, and deep alluvial deposits (Beavan et al. 2011, page 789 of this issue; Bradley and Cubrinovski 2011, page 853 of this issue). The highest recorded ground motions were near the epicenter at the Heathcote Valley primary school, with the horizontal and vertical PGAs 1.41 g and 2.21 g, respectively. In the CBD, horizontal PGAs of between 0.37 g and 0.52 g and vertical PGAs of 0.35 g to 0.79 g were recorded. Horizontal PGAs ranging from 0.22 g to 0.67 g and vertical PGAs from 0.49 g to 1.88 g were recorded in the vicinity of the Avon River (Bradley and Cubrinovski 2011, page 853 of this issue).

The horizontal PGAs for the Darfield and Christchurch earthquakes at the strong motion stations in central and east-

ern Christchurch are summarized in Figure 2. It is clear from the data in Figure 2 that the Christchurch event produced much higher ground motions than the Darfield event in the CBD and along the Avon and Heathcote rivers. While not shown in this figure, the same can be said for the level of vertical accelerations experienced in these areas.

The horizontal acceleration response spectra from five of the strong motions stations in Figure 2 for the Darfield and Christchurch events are compared to the NZS1170.5 design response spectrum for a 500-year return period event in Christchurch (hazard factor Z = 0.22) on a site subsoil class D (Standards New Zealand 2004) in Figure 3.

Because the bridges in the region are typically short to mid span, the natural period can reasonably be assumed as less than 0.8 seconds. Figure 3A shows that during the Darfield event, the spectral acceleration values in this range were generally less than the values that a bridge would have been designed for using current standards (although most bridges were designed according to older standards with lower design levels). Only the spectral accelerations of the ground motion recorded at Heathcote Valley primary school (HVSC) are above the design code values in this range, likely a result of basin wedge effects given its position at the head of the Heathcote Valley in the Port Hills.

In general, the ground motion response spectra from the Christchurch earthquake in Figure 3B were higher than the 500-year-return-period design spectrum over the entire vibration period range. The periods of highest spectral response correspond to the expected natural periods of the bridge structures in the region. Even though bridges likely experienced shaking levels at or above their design levels throughout this region, the majority sustained minimal damage as a result of ground shaking alone. This can be attributed to the sturdy designs typical of bridges constructed in the 1950s and 1960s, which was a period of extensive bridge replacement in Christchurch.

OVERVIEW OF CANTERBURY BRIDGE PERFORMANCE Although liquefaction was widespread in central and eastern Christchurch, only five bridges suffered severe damage and ten developed moderate damage in the 22 February 2011 Christchurch earthquake. Most bridges were reopened within a week of the earthquake, with only one closed for a longer period of time. Because of the location of the earthquake on the southeastern edge of the city, most of the bridge damage was confined to central and eastern regions, where ground shaking was strongest and soil conditions weakest. This paper focuses on the performance of ten of these bridges, the locations of which are indicated in Figure 2. The majority of bridge damage was a result of lateral spreading of river banks, with only four bridges damaged on sites that did not experience liquefaction (locations 1, 8, 9, and 10 in Figure 2). The largest distance from the fault rupture to an affected bridge was 17 km (corresponding to the moderately damaged Chaney’s Overpass). Eleven of the 14 bridges along the Avon River within the CBD suffered

952 Seismological Research Letters Volume 82, Number 6 November/December 2011

only minor damage, mostly to their approaches. Outside the CBD, the two remaining bridges along the Avon that did not suffer moderate or severe damage had only minor approach damage. Compared to the Avon River, bridges crossing the Heathcote River sustained much less damage despite being close to the fault rupture, primarily due to the larger seismic resistance of the foundation soils of these bridges. Apart from three cases, all bridges along the Heathcote River were either undamaged or developed only minor approach damage.

Eight road bridges suffered moderate damage following the 4 September 2010 Darfield earthquake, with five of these closed for five days or longer. Traffic weight limitations and/or restricted lane access was in place for a more extended period, all but one of which instances was due to approach damage as a result of lateral spreading. The Darfield earthquake had a larger magnitude, and thus resulting ground motions affected a much larger region, with bridge damage occurring from Lincoln, 15 km south of central Christchurch, to Kaiapoi, 16 km north.

The most distant bridge damage, at the Williams Street Bridge in Kaiapoi due to lateral spreading, was approximately 30 km from the rupture of the Greendale fault. Within Christchurch city itself, Gayhurst Road Bridge and South Brighton Bridge both experienced moderate damage, principally as a consequence of lateral spreading (Allen et al. 2010; Palermo et al.

2010).

DAMAGE ASSOCIATED WITH LIQUEFACTION

The Christchurch CBD bridges crossing the Avon River generally performed well, with the most common damage being minor lateral spreading, compression or slight slumping of approach material, and minor cracking in abutments.

All bridges were single span and were passable to recovery vehicles in the cordon soon after the event. (The cordon is the restricted-access area of the CBD, put in place due to the widespread earthquake damage in the area.) Compared to the Avon River, bridges crossing the Heathcote suffered much less damage. Apart from the Ferrymead Bridge at the mouth of the Heathcote, all bridges were either undamaged or experienced only minor damage. As previously noted, we infer that this is the result of more resistant foundation soils along the Heathcote River relative to the Avon River. Typical damage was minor approach settlement, with little impact on the bridge abutments and superstructure.

Detailed descriptions of the bridges shown in Figure 4 with the most severe liquefaction-induced damage and the analyses of in situ test data at these sites follow. The PGAs used in the liquefaction evaluations were estimated using the ground motion prediction equations of Bradley (2010) and the spatial correlation model of Goda and Hong (2008). The estimated PGAs at the bridge sites are the geometric mean of the two horizontal components for site class D (Standards New Zealand 2004) and are summarized in Table 1. Further information on the calculation of these PGA values can be found in Green et al. (2011, page 927 of this issue).

Bridges along both the Avon and Heathcote rivers suffered varying levels of damage from lateral spreading due to the Darfield and Christchurch earthquakes, with ground conditions and distance from the epicenter influencing this response as described previously. Even at a given bridge location the level of damage varied significantly from one end of the bridge to the other, with more damage observed on the inner banks of the local river bends, likely a result of the low-energy depositional environment, as compared to the outer banks. In this section of the paper, we present an overview on the heavily damaged Ferrymead Bridge at the mouth of the Heathcote River and on the most affected bridges along the Avon River from the Christchurch earthquake.

The type of bridge damage along the Avon was fairly consistent: settlement and lateral spreading of approaches, backrotation and cracking of the abutments, and some pier damage.

In most cases bridge decks restrained movement of the top of the abutment, resulting in their back-rotation. There was little bridge superstructure damage, with only minor crushing and spalling as a result of pounding and relative movement. Unless otherwise noted, simply supported bridges discussed herein did not have any bearings. All the damaged bridges previously mentioned had pile foundations, with lateral spreading forces placing large demands on the abutment piles and likely resulting in plastic hinging below grade. The approach fill of several bridges subsided by up to a meter, resulting in the bridges being closed up to a week. In most cases, settlement and spreading of the approaches impacted bridge serviceability.

Ferrymead Bridge The Ferrymead Bridge (Figure 4A) was constructed in 1967, runs in the east-west direction, and spans the mouth of Heathcote River (Figure 2). The bridge is a three-span reinforced concrete bridge supported by wall abutments with wingwalls and two four-column bents connected to pile caps.

The west abutment and bents are supported by floating pile foundations, while the eastern bent is supported by end-bearing pile foundations to bedrock, and the east abutment on shallow foundations on bedrock.

Although the Ferrymead Bridge performed well during the 2010 Darfield earthquake, at the time of the Christchurch earthquake it was undergoing a major upgrade to include widening and underpinning of the deck with two reinforced concrete girders supported on two drilled shaft foundations.

These upgrades had been planned before the occurrence of the Darfield earthquake. One of the girders at the east abutment had been completed and the girder at the west abutment was partly completed when the Christchurch earthquake struck.

Also, to allow access for construction cranes and equipment, two temporary steel bridges were erected on both sides of the bridge and were in place at the time of the Christchurch earthquake.

Each abutment consisted of two separate sections, one in front of the other (i.e., one section supporting the superstructure and the other abutment block behind it). Lateral spreading occurred at the east abutment, with the material overlying the bedrock moving both perpendicular and parallel to the bridge

Seismological Research Letters Volume 82, Number 6 November/December 2011 953

(A)

(B)

(C)

(D)

(E)

(F)

▲▲ Figure 4. Bridges damaged primarily as a result of liquefaction: A) Ferrymead, B) South Brighton, C) ANZAC Drive, D) Avondale Road, E) Gayhurst Road, F) Fitzgerald Avenue.

TABLE 1 Estimates of peak ground accelerations during Darfield and Christchurch earthquakes in the absence of liquefaction at bridges presented in Figure 2.

Darfield Earthquake

Bridge Name Moorhouse Ave Bridge Fitzgerald Ave Bridge Gayhurst Rd Bridge Avondale Rd Bridge ANZAC Dr Bridge South Brighton Bridge Ferrymead Bridge Port Hills Overbridge Horotane Overbridge Railway Bridge 3

Christchurch Earthquake

Conditional Median PGA (g)

Conditional Standard Deviation (ln PGA)

Conditional Median PGA (g)

Conditional Standard Deviation (ln PGA)

0.208 0.214 0.206 0.183 0.180 0.188 0.247 0.284 0.292 0.364

0.259 0.293 0.293 0.360 0.379 0.392 0.371 0.350 0.344 0.266

0.412 0.448 0.495 0.344 0.276 0.618 0.673 0.677 0.682 0.814

0.284 0.323 0.319 0.339 0.168 0.404 0.400 0.379 0.373 0.288

954 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C) ▲▲ Figure 5. Ferrymead Bridge field investigation data: A) shear wave velocity (Vs ) profile; B) liquefaction assessment using Vs data, comparing the cyclic resistance ratio CRR 7.5 for the site to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic stress ratios. C) Damage to western abutment and temporary stabilization works.

axis. The lateral spreading caused permanent rotation and cracking of the abutments and a number of the piers. Extensive flexural cracking was evident at the base of the piers at their connection to the pile cap. The rear section of the east abutment back-rotated 2.5° and the section supporting the bridge deck back-rotated 5°. Additionally, surveys showed the east abutment moved vertically upward 10 cm, but there was negligible movement of the eastern pier.

Approximately 8-cm-wide lateral cracks were observed in the vicinity of the drilled shaft supporting the new girder, with the cracks running in both the longitudinal and transverse directions. This caused the top of the new concrete bridge girder to rotate about 2° toward the river and caused approximately 30 cm of ground settlement, measured relative to the bottom surface of the new girder, which was originally cast ongrade.

Severe liquefaction, as evidenced by significant volumes of ejecta, and lateral spreading occurred in the area leading up to

the west abutment. Surveys showed that the west abutment and pier had settled 20 cm and shifted horizontally 20 cm toward the river. The soil in front of the abutment settled approximately 80 cm, but no appreciable rotation of the abutment was observed. The foundations supporting the west bridge pier in Figure 5C had shifted to the east, causing the support columns to be out of plumb. Remedial efforts have been completed to tie back the foundations supporting the western pier that experienced significant tilting to the west abutment using highstrength steel rods.

Following the Christchurch event, Spectral Analysis of Surface Waves (SASW) was performed at a location 60 m to the west of the west abutment. The shear wave velocity (Vs) profile for the west end of the bridge is shown in Figure 5A. The Vs profile shows a soft soil layer between 1.5 and 4 m depth, overlying a much stiffer layer, and the water table at 1.75 m depth.

Using the PGAs listed in Table 1, the cyclic stress ratios (CSRs) for both the Darfield and Christchurch earthquakes were calculated following the methodology outlined in Youd et al.

(2001). The magnitude scaling factors (MSF) recommended by Andrus and Stokoe (2000) were used to scale the CSRs to an Mw 7.5 event (i.e., CSR7.5). Using the shear wave velocity data shown in Figure 5A, the cyclic resistance ratio (CRR7.5) for the profile was calculated following the Andrus and Stokoe (2000) procedure, also outlined in Youd et al. (2001). The overburden correction factor, Kσ, was further used to modify the CRR7.5 values (Hynes and Olsen 1999). This method allows for the direct comparison of the CSR7.5 induced by the two earthquakes with the CRR7.5 for the profile, as shown in Figure 5B. As may be observed from this figure, liquefaction is predicted to have occurred from ~1.5 to 4 m during both the Darfield and Christchurch earthquakes (i.e., CSR7.5 > CRR7.5), with the factor of safety against liquefaction being significantly lower during the Christchurch event. While evidence of severe liquefaction was observed following the Christchurch earthquake, no liquefaction was evident following the Darfield earthquake.

South Brighton Bridge The South Brighton Bridge (Figure 4B) was constructed in 1980, runs in the east-west direction, and spans the Avon River just north of where the river empties into the Avon-Heathcote estuary (Figure 2). The bridge is a three-span skewed reinforced concrete structure with seat-type abutments on rubber bearings and single piers, all of which are supported by raked octagonal precast, prestressed concrete piles. The abutment rubber bearings were removed due to the permanent movements that developed during the Darfield earthquake (Palermo et al. 2010) and were replaced with temporary hardwood packers. The bridge site was a wide wetland prior to the bridge construction. To construct the bridge, two approach embankments approximately 4 m in height were extended out into the wetlands, with the bridge structure spanning the river channel. These embankments were constructed of uncontrolled fill material.

Significant cracking of the approach embankments on both sides of the bridge occurred during the Darfield earth-

Seismological Research Letters Volume 82, Number 6 November/December 2011 955

spalling of the bottom flange of the deck. These displacements represent the cumulative effect of both seismic events. Minor flexural cracking at the base of the central pier from transverse movement due to ground shaking was evident following the Darfield event, with minimal additional damage following the Christchurch event.

▲▲ Figure 6. Settlement of approach material, exposure of raked piles, and cracking of the western abutment of South Brighton Bridge. Note the rotation of the abutment in relation to the girder and cracking at rear of abutment seat.

quake. Slumping of the material adjacent to the abutments developed as a result of movement toward the river, while the approaches developed lateral spreading perpendicular to the river (parallel to the sides of the approach embankment).

Liquefaction ejecta was evident in the area surrounding the approaches, with lateral spreading parallel to the river extending to the north and south of the approaches on both sides.

Similar damage occurred as a result of the Christchurch earthquake, with further severe lateral spreading.

Lateral spreading due to both the Darfield and Christchurch earthquakes caused the east abutment to backrotate by approximately 7°, with spreading of the underlying soils exposing the abutment piles. The piles rotated along with the abutment structure, with evidence of plastic hinge development in both the front and rear rows of piles. The gabion mat used for erosion protection in front of the abutments had moved away from the abutment as the underlying soil spread.

These soil movements were larger than those observed in the Darfield event.

The west abutment back-rotated by approximately 5° and light cracking was observed on the tension face of the abutment piles after that event. The damage to the piles supporting the west abutment caused by the Darfield earthquake was exacerbated during the February earthquake where the abutment had back-rotated by an additional 3° for a total rotation of approximately 8° (Figure 6) and plastic hinging was clearly visible on the abutment piles. Soil beneath the abutment had settled significantly, exposing 80 cm of the supporting piles. Compared to the post-Darfield conditions, there had also been a significant increase in settlement and spreading at this abutment.

Differential movement of the abutments relative to the bridge deck was evident, with the east abutment moving about 22 cm along the line of skew to the north and settled about 3 to 4.5 cm. The west abutment moved 20 cm along the line of skew to the south and settled 8.5 to 9.5 cm, with minor crushing and

ANZAC Drive Bridge ANZAC Drive Bridge was constructed in 2000, runs in the north-south direction on State Highway 74 and spans the Avon River (Figure 2). Shown in Figure 4C, the bridge is a triple-span precast concrete girder structure (hollow core deck) that is supported by two four-column bents and concrete abutment walls with wingwalls. The south approach and abutment were constructed on an embankment fill, while the north end of the bridge was constructed at surrounding grade.

The bridge site experienced marginal liquefaction and minor lateral spreading during the Darfield earthquake, but the bridge and its functionality were not affected by this event. However, the bridge was damaged by the Christchurch earthquake, yet remained functional after regrading the approaches. Severe liquefaction, as evidenced by the large volumes of ejecta, and significant lateral spreading occurred in the areas surrounding the north and south abutments during the latter earthquake, with evidence of liquefaction being more pronounced on the south end of the bridge. There were a significant number of sand boils and ejecta observed in the low-lying areas adjacent to the embankment on the south end.

Additionally, lateral spreading was observed on both the sides of the embankment with the cracks running parallel to the roadway and having widths of about 8 to 18 cm. A short section of the south approach roadway was repaved and showed an abrupt elevation change due to ground settlement in the vicinity of the bridge abutment.

Liquefaction and lateral spreading were less evident on the north end of the bridge. However, a roundabout directly north of the approach possibly obscured some of the evidence.

Cracking parallel to the river developed across the roadway leading up to the north abutment and extended to both sides of the bridge. The higher elevation of the area around the north approach likely resulted in smaller volumes of liquefaction ejecta as compared to the south approach area.

The south abutment back-rotated 6°, as shown in Figure 7, and lateral spreading at the base of the abutment resulted in a 30 to 40 cm gap between the concrete abutment and backfill. Also, a large horizontal gap formed between the abutment and the edge of a walkway running along the riverbank, with the bridge superstructure restraining the horizontal abutment movement. The rotation of the south abutment exposed a row of steel H-piles supporting the abutment, which also appeared to have rotated along with the abutment. Numerous rubber tires were also exposed that had been placed between the abutment and a walkway running along the riverbank. These tires were designed to act as a lateral spreading buffer for the walkway. The north abutment showed similar rotational movements but had less rotation, of 3.5–4°. The lateral spreading along the

956 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C)

(D)

▲▲ Figure 7. Damage to southern abutment of the ANZAC Drive Bridge, with back-rotation of approximately 6° and spreading between abutment and adjacent walkway.

▲▲ Figure 8. ANZAC Drive Bridge pier damage, with cracking and spalling of cover concrete.

base of the abutment was also less, resulting in an 18 to 24 cm gap between the abutment and the backfill. Additionally, the horizontal gap between the abutment and a walkway running along the riverbank was much less relative to the south end.

Both of the bridge piers suffered extensive but superficial cracking to the concrete columns and bent as well as the beamcolumn joint region, with up to 2° of rotation (Figure 8). While the damage first appeared extensive, with apparent shear cracking, further inspection showed that in reality these cracks were limited to the concrete cover. Spalling of the cover concrete appeared to be primarily the result of rotation of the piles, causing stresses to be concentrated at the edges of the members.

These rotations can be attributed to horizontal movement of the pile foundations toward the center of the river due to lateral spreading.

Following the Christchurch event, SASW tests and Dynamic Cone Penetration tests (DCPTs) were carried out

▲▲ Figure 9. ANZAC Drive Bridge field investigation: A) shear wave velocity (Vs ) profile; B) liquefaction assessment using Vs , comparing the cyclic resistance ratio CRR 7.5 for the site to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratios; C) dynamic cone penetration test (DCPT) profile (i.e., NDCPT and equivalent N1,60cs); D) liquefaction assessment using equivalent N1,60cs, comparing the CRR 7.5 for the site to the CSR 7.5 DAR and CSR 7.5 CHC.

50 m southwest of the south abutment. The DCPT N-values (NDCPT) were converted to equivalent standard penetration test (SPT) N-values using a modified relationship to that proposed by Sowers and Hedges (1966). Then, the N-values were further corrected for rod length, hammer energy, effective confining stress, and fines content following the procedures outlined in Youd et al. (2001). The resulting profiles from the SASW tests and DCPTs are shown in Figures 9A and 9C. The Vs data from the SASW test indicates a soft soil layer between depths of 1 and 6 m, and the water table at a depth of 1.5 m.

The CRR7.5 profiles for the site were determined using both the SASW and DCPT data, per Youd et al. (2001) and as outlined previously for the SASW. Using the PGAs listed in Table 1, the cyclic stress ratios (CSRs) for both the Darfield and Christchurch earthquakes were calculated following the

Seismological Research Letters Volume 82, Number 6 November/December 2011 957

methodology in Youd et al. (2001) and as outlined previously.

Comparisons of the CRR7.5 and CSR7.5 for the Darfield and Christchurch earthquakes are presented in Figures 9B and 9D for the SASW and DCP tests, respectively.

As may be observed from Figure 9B (Vs data), liquefaction is predicted to have occurred from ~1.75 to ~6 m during both the Darfield and Christchurch earthquakes, with the factor of safety against liquefaction being slightly lower during the Christchurch event. Similar trends are predicted in Figure 9D (DCPT data), but liquefaction is predicted to have occurred during both earthquakes in a slightly thinner layer, ~2.5 to ~3.25 m. These predictions are consistent with field observations (i.e., liquefaction occurred at the site during both earthquakes, with the liquefaction being more severe during the Christchurch earthquake).

Avondale Road Bridge Avondale Road Bridge (Figure 4D) was constructed in 1962, runs approximately in the north-south direction, and spans the Avon River (Figure 2). The bridge consists of three spans of precast reinforced concrete girders that are supported on two three-column bents and seat-type abutment walls with wingwalls. Since its construction, the bridge has been seismically retrofitted using steel brackets, which are bolted to tie the elements of the bridge together.

The bridge was not damaged during the Darfield earthquake, with the region north of the bridge showing no signs of liquefaction damage. However, just south of the bridge, along the inner bank of the river, there were minor to moderate levels of liquefaction ejecta, with the volume increasing toward the southwest. Liquefaction and lateral spreading were more severe during the Christchurch earthquake, with larger volumes of ejecta and significant lateral spreading adjacent to both sides of the south abutment. To the north, there was also increased volume of ejecta, and moderate spreading 30 m to the west.

There was minimal roadway damage adjacent to the north abutment; however the north abutment back-rotated approximately 3°. At the south, the abutment has back-rotated 7°, with moderate settlement of the approach and damage to roadway and services (Figure 10C). Large lateral spreading cracks extended out from both sides of the abutment, transitioning from perpendicular to the riverbanks to parallel over a distance of approximately 15 m. The superstructure and piers showed no signs of damage after either earthquake.

A cone penetration test (CPT) was performed after the Darfield earthquake, approximately 15 m to the west of the south abutment, with the results shown in Figure 10A (Tonkin and Taylor 2011a). The CRR7.5 profile for the site was determined using the CPT data, per Youd et al. (2001). Using the PGAs listed in Table 1, the cyclic stress ratios (CSRs) for both the Darfield and Christchurch earthquakes were calculated following the methodology outlined in Youd et al. (2001).

Comparisons of the CRR7.5 and CSR7.5 for the Darfield and Christchurch earthquakes are presented in Figure 10B. As may be observed from this figure, the site is predicted to marginally liquefy during the Darfield earthquake, with the severity

(A)

(B)

(C) ▲▲ Figure 10. Avondale Road Bridge field investigation: A) CPT profile; B) liquefaction assessment using CPT data, comparing the cyclic resistance ratio CRR 7.5 to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratio; C) damage to southern abutment and approach.

of the liquefaction increased for the Christchurch earthquake.

These predictions are consistent with field observations (i.e., liquefaction occurred at the site during both earthquakes, with the liquefaction being more severe during the Christchurch earthquake).

Gayhurst Road Bridge Gayhurst Road Bridge was constructed in 1954, runs in approximately the north-south direction, and spans the Avon River (Figure 2). This integral bridge, shown in Figure 4E, consists of three-spans of precast reinforced concrete girders supported by wall piers that were cast in place within the deck and seat-type concrete abutments with wingwalls. Both the piers and the abutments are founded on reinforced concrete piles.

Prior to the earthquakes, both approaches were approximately level with the bridge deck as part of the natural level of the river banks.

Severe liquefaction occurred during the Darfield earthquake, indicated by the significant volume of ejecta to the north

958 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

▲▲ Figure 11. Settlement of the region surrounding the northern abutment of the Gayhurst Road Bridge.

of the bridge on the inner bank of the river, with lateral spreading and large settlements developing throughout the entire area The effects were more severe following the Christchurch earthquake, with an increased volume of ejecta and further lateral spreading and settlement (Figure 11). To the south on the outer bend of the river there was minimal spreading on either side of the bridge and only moderate ejecta volumes after the Christchurch earthquake.

Both earthquakes caused significant damage to the north approach and abutment of this bridge, with their combined effects resulting in approximately one meter of settlement of the approach adjacent to the abutment. The wingwalls on both sides of the north abutment displaced toward the river by about 90 cm at their top and moved laterally between 10 and 15 cm away from the abutment perpendicular to the bridge axis (Figure 12C). Extensive cracking was evident, with total exposure of the reinforcement connecting the wingwalls to the abutment. The north abutment developed 5° of back-rotation, with a fraction of this being initiated during the Darfield earthquake, as were the wingwall movements.

The base of the northern pier rotated toward the center of the river, with one face of the pier cracking horizontally along its length, approximately one meter from the deck soffit. This was initiated in the Darfield event, with crack widening and further rotation during the Christchurch earthquake. Lateral spreading was the cause of this damage, with the lateral force on the pier base developing a large moment at the stiff pier-deck interface and cracking the pier.

At the south abutment there was little indication of settlement of the approach. The wingwalls did not show any appreciable displacement, nor did the abutment show any measureable rotation. The southern pier also did not show any obvious signs of distress.

A CPT was performed after the Darfield earthquake, approximately 5 m east of the north abutment, with the results shown in Figure 12A (Tonkin and Taylor 2011b). Following the procedure outlined in the previous section, CRR7.5 and

(C) ▲▲ Figure 12. Gayhurst Road Bridge field investigation: A) CPT profile; B) liquefaction assessment using CPT data, comparing the cyclic resistance ratio CRR 7.5 to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratio; C) damage to northern approach, with approximately 1 m of slumping of the approach, with damage and movement of wingwalls.

CSR7.5 were developed for the Darfield and Christchurch earthquakes and are compared in Figure 12B. As may be observed from this figure, the site is predicted to marginally liquefy during the Darfield earthquake, with the severity of liquefaction increased for the Christchurch earthquake. While the prediction for the Christchurch earthquake is consistent with field observations (i.e., severe liquefaction), the prediction underestimated the severity of the liquefaction observed during the Darfield earthquake.

Fitzgerald Avenue Bridges Fitzgerald Avenue Bridge, constructed in 1964, runs in the north-south direction and spans the Avon River (Figure 2). The bridge, shown in Figure 4F, consists of two structures supporting southbound traffic and northbound traffic, respectively.

Each bridge consists of double-span precast concrete girders with a single wall pier and pile-supported concrete wall abutments. Retrofit had recently been carried out, involving steel

Seismological Research Letters Volume 82, Number 6 November/December 2011 959

brackets linking the piers and abutments to the deck. At the location of the bridge, the river undergoes a significant change of direction with the north abutment on the inner bank and the south abutment on the outer bank. This bridge was on the edge of the central city cordon setup after the Christchurch earthquake, and consequently, was inaccessible to the general public and was used only by vehicles with cordon access.

As discussed in Bradley et al. (2010), the soil profile underlying the north end of the bridge can be approximated as four distinct layers: (1) sand, ~4.5 m thick, N1 = 10, Vs = 130 m/s; (2) sand with fines, ~6.5 m thick, N1=15, Vs = 160 m/s; (3) sand, ~6.5 m thick, N1 = 10, Vs = 130 m/s; and (4) sand, N1 = 30, Vs = 220 m/s. The soil profile underlying the south end of the bridge is similar to the north end, minus layer (3).

The bridge was undamaged by the Darfield earthquake, with no evidence of liquefaction on either side of the bridge.

However, during the Christchurch earthquake, significant lateral spreading developed on the east side of the north abutment, with cracks running parallel to the riverbank and material moving south toward the river. The north abutment of the western bridge was very near the bend in the river, with a free face both perpendicular and parallel to the bridge.

Lateral spreading was noted with movement occurring both to the south and west. Settlements of approximately 0.5 m were observed on the north approach as well.

Both north abutments showed back-rotation, which— combined with settlement of the river banks at the base of the abutments—exposed the abutment piles. The abutment rotation caused the easternmost pile on the north abutment (Figure 13A) to fail in tension, with the tension face opening up and crack widths measured up to 10 mm. Spalling of the cover concrete on the bottom flange of the deck girder (Figure 13B) developed as a result of relative movement of the superstructure and abutment. Minimal settlement of the approach was observed at the south abutments. Large cracks were noted, however, in the abutment and wingwalls.

This bridge had been previously identified as critical to the bridge network, with an extensive field testing program performed in the late 1990s. The program included multiple CPTs and standard penetration tests (SPTs) performed at the abutments of both the twin bridges. The subsequent analyses showed that the north abutment of the eastern bridge was most vulnerable to liquefaction and structural damage (Bowen and Cubrinovski 2008a, 2008b; Bradley et al. 2010), with liquefaction predicted in the relatively loose sandy soil between 2.5 m and 17.5 m. These predictions are very consistent with the observed response on the bridge during the Christchurch earthquake.

DAMAGE NOT ASSOCIATED WITH LIQUEFACTION Moving away from the Avon and Heathcote rivers, where liquefaction-induced lateral spreading was the main cause of damage, four bridges suffered damage not related to the effects of liquefaction. One bridge, Railway Bridge 3, was damaged due to the seismically induced lateral earth pressures acting on

(A)

(B) ▲▲ Figure 13. Fitzgerald Avenue Bridge damage: A) tension failure of abutment pile and exposure of reinforcement, B) spalling of bottom flange of deck girder.

the abutments. Two bridges, Moorhouse Overbridge and Port Hills Overbridge, were damaged due to shaking effects that activated the transverse response of the structure. The final bridge, Horotane Overbridge, sustained damage as a result of shaking and slope stability issues. The final three bridges did not develop any significant superstructure damage in any of the earthquakes.

Railway Bridge 3 The Railway Bridge 3 was constructed in 1950 and consists of a timber deck with brick masonry wingwall abutments, spanning a roadway between built-up railway embankments approximately 3 m in height (Figure 2).

The bridge was not damaged by the Darfield earthquake, but extreme shaking during the Christchurch earthquake resulted in severe cracking and movement of the abutments.

This caused deformation in the track ballast and tracks, result-

960 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

(B)

(C) ▲▲ Figure 14. Railway Bridge 3 field investigation: A) shear wave velocity (Vs ) profile; B) liquefaction assessment using Vs data, comparing the cyclic resistance ratio CRR 7.5 to the Darfield (CSR 7.5 DAR) and Christchurch (CSR 7.5 CHC) cyclic resistance ratio; C) remediation of bridge using steel frame structure to prop abutments and tracks.

ing in a train derailment soon after the event. The bridge was temporarily remediated in the days following the event using a steel frame between the abutments and stabilizing walls in front of the wingwalls as shown in Figure 14C. An SASW test was performed 20 m to the west of the bridge following the Christchurch event; the shear wave velocity profile at this site is shown in Figure 14A. As indicated by this Vs data, the profile consists of ~4 m of a medium dense layer overlying a denser stratum. Using this shear wave velocity data and the PGAs listed in Table 1, the CRR7.5 for the site and the CSR7.5 induced during the Darfield and Christchurch earthquakes were calculated as outlined above for the other bridges. The results are plotted in Figure 14B. As may be observed from this figure, liquefaction is not predicted to occur during either the Darfield and Christchurch earthquakes (i.e., CSR7.5 < CRR7.5). These predictions are consistent with field observations.

Moorhouse Avenue Overbridge The Moorhouse Avenue Overbridge (Figure 15A) was constructed in 1960 and runs in the east-west direction (Figure 2).

The bridge consists of 11 spans of T-girders that are supported by dual reinforced concrete column bents.

This bridge was not damaged during the Darfield earthquake. Following the Christchurch earthquake, the bridge was out of service to all traffic for about a month due to damage sustained during the event. The damage was primarily to a single column where a deck expansion joint is located. There were no linkages between sections of the deck as this position, while the corresponding pier on the opposite side of the bridge did have these linkages in place. The expansion joint detail extended into the column, increasing the slenderness of the piers (i.e., these columns were of a size comparable to the other columns along the span, except that they were split down the middle by the expansion joint). The columns also had widely spaced transverse reinforcement. The damage was likely caused by a combination of the high accelerations (estimated PGA = 0.41g) and a large velocity pulse, exciting the transverse response of the bridge and resulting in the flexural-buckling failure of the columns shown in Figure 16.

Upon first inspection the bridge had only suffered shear cracking in both columns, but several hours later the bridge was inspected again and it was observed that the damaged columns had started to buckle, putting the central span at risk of collapse. Temporary props were then put in place to provide gravity support for the span until a rehabilitation plan could be implemented. There was also evidence of concrete spalling and bar buckling at the abutment-deck interface.

Port Hills Overbridge The Port Hills Overbridge was constructed in 1970 and runs in approximately the northwest-southeast direction (Figure 2). The bridge, shown in Figure 15B, consists of a dual six-span reinforced concrete voided-slab bridge supported by single pier bents and seat-type abutments. This highway bridge had been recently retrofitted, with spans and abutments linked together with steel brackets and rods to form an integral system. Soil had also been excavated from around the end piers so that the height of the piers would be uniform along the structure.

The bridge was not damaged by the Darfield earthquake, but was approximately 1.5 km from the epicenter of the Christchurch earthquake, with an estimated PGA of 0.68 g at the site. Column damage developed during this event, with the center pier forming a plastic hinge at its base and two of the corner reinforcing bars buckling over a length of 150 mm. This damage was induced by ground shaking, which activated the transverse response of the bridge. The retrofitted links between the spans and the bolts connecting the span to the abutment had elongated, but the bridge was still able to service traffic with the damage it sustained.

Horotane Overbridge The Horotane Overbridge, constructed in 1970, is on State Highway 74 and runs in approximately the northwest-south-

Seismological Research Letters Volume 82, Number 6 November/December 2011 961

(A)

(B)

(C)

▲▲ Figure 15. Bridges with damage not associated with liquefaction: A) Moorhouse Avenue, B) Port Hills, C) Horotane.

rofit method appeared to have worked well in terms of protecting the structure. The ties between spans and at the abutments elongated and pulled out, as they had in the Port Hills Overbridge. Additionally, 60% of the bolts that attached the soffit of the precast concrete beams to the abutment seat extension had sheared off.

The northwest abutment back-rotated 1° and a transverse crack developed at the top of the northwest slope near the concrete abutment; however, this was not continuous across the slope. The southeast abutment back-rotated by 3.4°and a transverse crack developed at the bottom of the slope on the northeast side. A significant transverse crack 10 cm wide and 60 cm deep opened up at the top of the southeast slope and was continuous across the width of the bridge. A transverse scarp also developed near the toe of the 13-m-long slope, extending between the southern piers of the two bridges, suggesting that a slope failure had been initiated in the embankment fill but did not become unstable. Movement was also evident perpendicular to the bridge axis, with cracking in the slope extending through the abutment, resulting in wide cracking and lateral movement of the abutment and superstructure (Figure 17).

CONCLUSIONS ▲▲ Figure 16. Moorhouse Avenue Overbridge pier flexural buckling failure.

east direction (Figure 2). Shown in Figure 15C, the bridge is a dual three-span reinforced concrete bridge supported by single pier bents. The end spans are supported by seat-type abutments, with the structure spanning between two large builtup embankments approximately 9 m high. The embankment slopes beneath the abutments and parallel to the roadway have an angle of about 33° relative to the horizontal (i.e., 1.5H:1V slope). This bridge is ~200 m from the Port Hills Overbridge, and had also been recently retrofitted using a similar approach, with abutment seat extensions and linkages between the bridge elements.

This bridge did not suffer any damage during the Darfield earthquake. During the Christchurch earthquake, the ret-

Overall, the bridges in Christchurch and the Canterbury region performed well during the Darfield and Christchurch earthquakes, given the magnitude of the observed ground motions. Of those bridges that were damaged, the majority were as a result of liquefaction-induced lateral spreading, with only four bridges suffering damage not related to liquefaction effects. Even though the larger-magnitude Darfield event affected a much wider region, the location of the Christchurch event resulted in more significant damage due to the intensity of shaking in a region of the city with many bridges and high liquefaction susceptibility.

As a result of the significant lateral spreading, the most affected components of the bridges were the approaches, abutments, piers, and foundation system. Bridges were able to resist the inertial forces due to shaking, while the compressive lateral spreading forces resulted in abutment rotation and foundation damage. For almost all cases the predicted and observed lique-

962 Seismological Research Letters Volume 82, Number 6 November/December 2011

Events Reconnaissance (GEER) Association activity through a CMMI-00323914 and NSF RAPID grant CMMI-1137977.

However, any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We acknowledge the New Zealand GeoNet project and its sponsors EQC, GNS Science, and Land Information New Zealand for providing ground motion records used in this study.

REFERENCES

▲▲ Figure 17. Abutment damage and superstructure movement of the southeastern abutment of the Horotane Overbridge.

faction occurrences were in close accord, independent of whether the liquefaction was evaluated using the Vs, DCPT, or CPT. Settlement of bridge approaches affected the serviceability of many of the affected bridges, and bridges critical to the network were seriously damaged, causing significant traffic disruption immediately following the event. Nevertheless, the overall network performed well, with only the Moorhouse Avenue Overbridge closed for an extended period of time. This good performance is attributed to the fact that most Christchurch City Council road bridges built in the 1950s and 1960s were robust integral bridges. For the recently constructed bridges, good performance was a result of the significant improvement in bridge seismic safety in New Zealand and retrofitting efforts in the past decade. Additionally, the regular configuration, limited span length, and effective restraining methods were important factors in the reduced vulnerability of the Christchurch bridge network.

ACKNOWLEDGMENTS Dr. Wotherspoon’s position at the University of Auckland is funded by the New Zealand Earthquake Commission (EQC). The primary support for the U.S. GEER team members was provided by grants from the U.S. National Science Foundation (NSF) as part of the Geotechnical Extreme

Allen, J., S. Ashford, E. Bowman, B. Bradley, B. Cox, M. Cubrinovski, R. Green, T. Hutchinson, E. Kavazanjian, R. Orense, M. Pender, M. Quigley, and L. Wotherspoon (2010). Geotechnical reconnaissance of the 2010 Darfield (Canterbury) earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 243–320.

Andrus, R. D., and K. H. Stokoe II (2000). Liquefaction resistance of soils from shear-wave velocity. ASCE Journal of Geotechnical and Geoenvironmental Engineering 126 (11), 1,015–1,025.

Beavan, J., E. Fielding, M. Motagh, S. Samsonov, and N. Donnelly (2011). Fault location and slip distribution of the 22 February 2011 M W 6.2 Christchurch, New Zealand, earthquake from geodetic data. Seismological Research Letters 82, 789–799.

Bowen, H. J., and M. Cubrinovski (2008a). Psuedo-static analysis of piles in liquefiable soils: Parametric evaluation of liquefied layer properties. Bulletin of the New Zealand Society for Earthquake Engineering 41 (4), 234–246.

Bowen, H. J., and M. Cubrinovski (2008b). Effective stress analysis of piles in liquefiable soil: A case study of a bridge foundation. Bulletin of the New Zealand Society for Earthquake Engineering 41 (4), 247– 262.

Bradley, B. A. (2010). NZ-Specific Pseudo-spectral Acceleration Ground Motion Prediction Equations based on Foreign Models. University of Canterbury, Department of Civil Engineering, 319 pp.

Bradley, B. A., and M. Cubrinovski (2011). Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake. Seismological Research Letters 82, 853–865.

Bradley, B. A., M. Cubrinovski, R. P. Dhakal, and G. A. MacRae (2010). Probabilistic seismic performance and loss assessment of a bridge-foundation-soil system. Soil Dynamics and Earthquake Engineering 30 (5), 395–411.

Brown, L. J., R. D. Beetham, B. R. Paterson, and J. H. Weeber (1995).

Geology of Christchurch, New Zealand. Environmental & Engineering Geoscience 1 (4), 427–488.

Brown, L. J., and J. H. Weeber (1992). Geology of the Christchurch Urban Area. Institute of Geological and Nuclear Sciences. Lower Hutt, New Zealand: GNS Science.

Gledhill, K., J. Ristau, M. Reyners, B. Fry, and C. Holden (2011).

The Darfield (Canterbury, New Zealand) Mw 7.1 earthquake of September 2010: A preliminary seismological report. Seismological Research Letters 82 (3), 378–386.

Goda, K., and H. P. Hong (2008). Estimation of seismic loss for spatially distributed buildings. Earthquake Spectra 24, 889–910.

Green, R. A., C. Wood, B. Cox, M. Cubrinovski, L. Wotherspoon, B. Bradley, T. Algie, J. Allen, A. Bradshaw, and G. Rix (2011).

Use of DCP and SASW tests to evaluate liquefaction potential: Predictions vs. observations during the recent New Zealand earthquakes. Seismological Research Letters 82, 927–938.

Guidotti, R., M. Stupazzini, C. Smerzini, R. Paolucci, and P. Rameri (2011). Numerical study on the role of basin geometry and kinematic seismic source in 3D ground motion simulation of the 22 February 2011 M W 6.2 Christchurch earthquake. Seismological Research Letters 82, 767–782.

Seismological Research Letters Volume 82, Number 6 November/December 2011 963

Hynes, M. E., and R. S. Olsen (1999). Influence of confining stress on liquefaction resistance. Proceedings of the International Workshop on Physics and Mechanics of Soil Liquefaction, Baltimore, Maryland, USA, 10–11 September 1998Rotterdam, the Netherlands: Balkema, 145–152.

Palermo, A., M. LeHeux, M. Bruneau, M. Anagnostopoulou, L. Wotherspoon, and L. Hogan (2010). Preliminary findings on performance of bridges in the 2010 Darfield earthquake. Bulletin of the New Zealand Society for Earthquake Engineering 43 (4), 412–420.

Smyrou, E., P. Tasiopoulou, I. Engin Bal, and G. Gazetas (2011). Ground motions versus geotechnical and structural damage in the February 2011 Christchurch earthquake. Seismological Research Letters 82, 882–892.

Sowers, G. F., and C. S. Hedges (1966). Dynamic cone for shallow in-situ penetration testing. In Vane shear and cone penetration resistance testing of in-situ soils. ASTM STP 399. Philadelphia, PA: American Society of Testing Materials, 29–37.

Standards New Zealand (2004). Structural Design Actions, Part 5: Earthquake Actions—New Zealand. Wellington, New Zealand: Standards New Zealand, 82 pp.

Tonkin and Taylor (2011a). Darfield Earthquake Recovery Geotechnical Factual Report—Avondale. Report REP-AVD-FAC prepared

by Tonkin and Taylor, Ltd for the New Zealand Earthquake Commission. Christchurch, New Zealand: Tonkin and Taylor.

Tonkin and Taylor (2011b). Darfield Earthquake Recovery Geotechnical Factual Report—Dallington Lower. Report REP-DAL-FAC prepared by Tonkin and Taylor, Ltd for the New Zealand Earthquake Commission. Christchurch, New Zealand: Tonkin and Taylor.

Youd, T. L., I. M. Idriss, R. D. Andrus, I. Arango, G. Castro, J. T. Christian, R. Dobry, et al. (2001). Liquefaction resistance of soils: Summary report from the 1996 NCEER and 1998 NCEER/NSF workshops on evaluation of liquefaction resistance of soils. ASCE Journal of Geotechnical and Geoenvironmental Engineering 127 (10), 817–833.

964 Seismological Research Letters Volume 82, Number 6 November/December 2011

University of Auckland Private Bag 92019 Auckland 1142 New Zealand

l.wotherspoon@auckland.ac.nz

(L. W.)

New Publications CanGeoRef The American Geosciences Institute (AGI) and the Canadian Federation of Earth Sciences (CFES) have launched CanGeoRef (www.cangeoref.org), a bibliographic database covering the Canadian geoscience literature since the early 1800s.

CanGeoRef is the result of a cooperative arrangement between CFES and AGI to expand access for smaller companies and individuals focused on Canadian geoscience to GeoRef, AGI’s global bibliographic database for the geosciences. Data have already been added for Alberta and Manitoba and Ontario is near completion; Newfoundland/Labrador and British Columbia will follow shortly.

Operational Earthquake Forecasting Report The final report of the International Commission on Earthquake Forecasting for Civil Protection, entitled “Operational Earthquake Forecasting: State of Knowledge and Guidelines for Implementation,” has been published in Annals of Geophysics (vol. 54, no. 4, 315–391, doi: 10.4401/ ag­5350). The complete report can be freely downloaded from http://www.annalsofgeophysics.eu/index.php/annals/article/view/5350. The report has been accepted by the Italian

Department of Civil Protection, which commissioned the study immediately following the L’Aquila earthquake of 6 April 2009. Although written in response to this request, the

doi: 10.1785/gssrl .82.6.965

Commission intends for the report will be useful to other countries developing operational forecasting procedures and protocols.

Rapid Observation of Vulnerability and Estimation of Risk The Federal Emergency Management Agency (FEMA) has released Rapid Observation of Vulnerability and Estimation of Risk (ROVER), free, mobile software for pre- and postearthquake building safety screening. ROVER is available on CD-ROM (FEMA P-154 ROVER CD) from the FEMA Publications Warehouse or via online download. ROVER automates two de facto international standard paper-based seismic safety screening procedures: FEMA P-154, Rapid Visual Screening (RVS) of Buildings for Potential Seismic Hazards, and ATC-20, Postearthquake Safety Evaluation of Buildings. The pre-earthquake module is used by field inspectors to quickly compile an electronic inventory of buildings, record important seismic features of a building, and generate an automatic estimate of the need for detailed seismic evaluation. The postearthquake module is used to quickly perform and manage the safety tagging (red, yellow, and green tags) almost universally applied to buildings after earthquakes. To order FEMA P-154 ROVER on CD from Publications Warehouse, phone 800480-2520 or fax 240-699-0525; to download the free software, visit www.atc-rover.org.

Seismological Research Letters Volume 82, Number 6 November/December 2011 965

SSA 2012 Annual Meeting Announcement

Seismological Society of America Technical Sessions 17–19 April 2012 (Tuesday–Thursday) San Diego, California

IMPORTANT DATES Travel Grant Deadline 30 November 2011 Abstract Submission Deadline 11 January 2012 24 February 2012 Program w/Abstracts Online Meeting Pre-registration Deadline 9 March 2012 Hotel Reservation Cut-Off 24 March 2012 Online Registration Cut-Off 6 April 2012

SSA Annual Meeting Announcement PROGRAM COMMITTEE

CALL FOR PAPERS

Meeting Contacts

Abstract Deadline: Tuesday, 11 January 2012. Electronic submissions required. Instructions will be available on the SSA Web site at www.seismosoc.org/meetings/ on 1 December 2011.

Technical Program Co-Chairs David Oglesby and Raul Castro 2012Program@seismosoc.org

Abstract Submissions Joy Troyer Seismological Society of America 510.559.1784 joy@seismosoc.org

Registration Sissy Stone Seismological Society of America 510.559.1780 sissy@seismosoc.org

Exhibits Katie Kadas Seismological Society of America 510.559.1783

MEETING INFORMATION Registration Registration information will be published in the January and March issues of SRL and will be available online beginning 1 January 2012.

Preliminary Schedule Please note that this year the technical sessions start on Tuesday rather than on Wednesday as in recent years and the sessions end on Thursday. The field trip will be held on Friday. Events will be held at the Town and Country Resort and Convention Center in the Mission Valley area of San Diego, California.

katie@seismosoc.org

Monday, 16 April Board of Directors Meeting (9am–5pm) Registration (3pm–8pm) Icebreaker (6pm–8pm)

Press Relations Nan Broadbent Seismological Society of America 408-431-9885

Tuesday, 17 April Technical Sessions (8am–6pm) Annual Luncheon (12pm–2pm) Town Hall Meeting (7pm–9 pm)

nan0604@msn.com

966 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl .82.6.966

Wednesday, 18 April Technical Sessions (8am–5pm) Lunch (12pm–1pm) Joyner Lecture & Reception (6pm–8pm) Thursday, 19 April Technical Sessions (8am–5pm) Lunch (12pm–1pm) Friday, 20 April Field Trips This schedule is subject to change.

HOTEL INFORMATION Town and Country Resort and Convention Center The Town and Country, a beautiful garden-filled resort, includes five restaurants, two swimming pools and a day spa.

It is next door to a championship golf course and just minutes from downtown, Old Town, and other attractions via trolley. By making your reservation from this URL, you will be insured the SSA conference rate. https://resweb.passkey.com/

Resweb.do?mode=welcome_ei_new&eventID=3640472

Reasons to Stay at the Town and Country When you stay at the conference hotel, you not only stay at the most convenient location for the meeting, you help the Seismological Society. The hotel gives us a significant discount on our meeting rooms and food if we book a certain number of guest rooms. We base the price of meeting registration on making that number. If we don’t meet that projection, SSA loses money on the meeting and our ability to serve our members is reduced. This year we have negotiated an affordable room rate and complimentary in-room Internet. We have blocked rooms in the nicest part of the hotel. Of course, you can stay at other hotels nearby, but you will have to pay for parking at the Town and Country (around $20/day), your room will not be as convenient, and you won’t have the satisfaction of knowing that your room reservation is helping make the SSA Annual Meeting a financial success.

TRAVEL GRANTS Modest travel grants are available to help defray some of the costs for international SSA members and student SSA members who wish to attend the annual meeting in San Diego. The application deadline is 30 November 2011.

Eligibility All grant recipients must be SSA members current through 2012.

• The International Travel Grant is available to SSA members who must travel from outside the US to attend the SSA meeting.

• The Student Travel Grant is available to SSA student members who must travel more than 500 km (310 miles) to attend the meeting.

• The ESC/SSA Travel Grant is available to any student traveling from a member-state of the European Seismological Commission. This grant is provided under a cooperative agreement between SSA and ESC. All grant recipients must present their work in either an oral or poster session at the meeting.

Application To apply for any of these awards, please submit an application electronically via the online form at http://www.seismosoc. org/meetings/2012/travel_grants/. The application should state which award you are applying for, provide reasons why you are a good candidate for the award, and include the text of the abstract for your presentation.

Deadline Applications must be received by SSA no later than 30 November 2011. Applicants will be notified about their award status by 4 January 2012, one week before the SSA deadline for abstract submission. The awards will be announced and the checks presented at the Annual Luncheon during the SSA meeting.

Membership To ensure that your membership is current, please renew your membership by logging into the members area of the SSA website (http://www.seismosoc.org/members). For more information, contact meetings@seismosoc.org.

Seismological Research Letters Volume 82, Number 6 November/December 2011 967

Coming in BSSA Issue 101:6 of the Bulletin of the Seismological Society of America, expected publication December 2011, will present the following articles. indicates that online material will be available at the SSA Web site.

ARTICLES On the Probability of Detecting Picoseismicity Katrin Plenkers, Danijel Schorlemmer, Grzegorz Kwiatek, and the JAQUARS group Source Parameters of Picoseismicity Recorded at Mponeng Deep Gold Mine, South Africa: Implications for Scaling Relations G. Kwiatek, K. Plenkers, G. Dresen, and the JAGUARS Research Group Monitoring the Earthquake Source Process in North America R.B. Herrmann, H. Benz, and C. J. Ammon Investigating the Distributions of Differences between Mainshock and Foreshock Magnitudes Christine Smyth, Jim Mori, and Masumi Yamada Resolution of Seismic-Moment Tensor Inversions from a Single Array of Receivers Ismael Vera Rodriguez, Yu J. Gu, and Mauricio D. Sacchi Magnitude-Scaling Rate in Ground Motion Prediction Equations for Response Spectra from Large, Shallow Crustal Earthquakes John Zhao and Ming Lu Statistical Analysis of the 2002 Mw 7.9 Denali Earthquake Aftershock Sequence Pathikrit Bhattacharya, Mary Phan, and Robert Shcherbakov Moment-Constrained Finite-Fault Analysis Using Teleseismic P waves: Mexico Subduction Zone C. Mendoza, S. Castro Torres, and J. M. Gomez Gonzalez California Integrated Seismic Network (CISN) Local Magnitude Determination in California and Vicinity R. A. Uhrhammer, M. Hellweg, K. Hutton, P. Lombard, A. W. Walters, E. Hauksson, and D. Oppenheimer

Preliminary Probabilistic Seismic Hazard Analysis of the CO2CRC Otway Project Site, Victoria, Australia Mark Stirling, Nicola Litchfield, Matthew Gerstenberger, Dan Clark, Brendon Bradley, John Beavan, Graeme McVerry, Russ Van Dissen, Andy Nicol, Laura Wallace, and Robert Buxton Improvements to Seismic Monitoring of the European Arctic Using Three-Component Array Processing at SPITS S. J. Gibbons, J. Schweitzer, F. Ringdal, T. Kværna, S. Mykkeltveit, and B. Paulsen Recurrent Morphogenic Earthquakes in the Past Millennium along the Strike-Slip Yushu Fault, Central Tibetan Plateau Aiming Lin, Dong Jia, Gang Rao, Bing Yan, Xiaojun Wu, and Zhikun Ren Integration of Paleoseismic Data from Multiple Sites to Develop an Objective Earthquake Chronology: Application to the Weber Segment of the Wasatch Fault Zone, Utah Christopher B. DuRoss, Stephen F. Personius, Anthony J. Crone, Susan S. Olig, and William R. Lund The Crustal and Upper-Mantle Structures beneath the Northeastern Margin of Tibet Xuzhang Shen, Xiuping Mei, and Yuansheng Zhang Crustal Structure in the Southern Appalachians: A Comparison of Results Obtained from Broadband Data and Three-Component, Wide-Angle P and S Reflection Data M. Scott Baker and Robert B. Hawman Stripping Analysis of Ps-Converted Wave Polarization Anisotropy Hitoshi Oda Upper-Crust Shear-Wave Velocity of South Korea Constrained by Explosion and Earthquake Data Heeok Jung, Yong-seok Jang, and Bong Gon Jo

Quantifying a Potential Bias in Probabilistic Seismic Hazard Assessment: Seismotectonic Zonation With Fractal Properties Matteo Spada, Stefan Wiemer, and Eduard Kissling

The Green’s Functions Constructed from 17 Years of Ambient Seismic Noise Recorded at Ten Stations of the German Regional Seismic Network Danuta Garus and Ulrich Wegler

Epistemic Uncertainty in the Location and Magnitude of Earthquakes in Italy from Macroseismic Data W. H. Bakun, A. Gómez Capera, and M. Stucchi

Scattered P′P′ Waves Observed at Short Distances Paul S. Earle, Sebastian Rost, Peter M. Shearer, and Christine Thomas

968 Seismological Research Letters Volume 82, Number 6 November/December 2011

doi: 10.1785/gssrl.82.6.968

Verification of a Spectral-Element Method Code for the Southern California Earthquake Center LOH.3 Viscoelastic Case Florent De Martin Application of the Nearly Perfectly Matched Layer to Seismic-Wave Propagation Modeling in Elastic Anisotropic Media Jingyi Chen and Jianguo Zhao Comparison of the Empirical Green’s Spatial Derivative Method and Empirical Green’s Function Method Michihiro Ohori and Yoshiaki Hisada Application of the Multichannel Wiener Filter to Regional Event Detection Using NORSAR Seismic-Array Data J. Wang, J. Schweitzer, F. Tilmann, R. S. White, and H. Soosalu Scattering and Attenuation of Seismic Waves in Northeastern North America R.D. Cicerone, C.G. Doll Jr., and M. N. Toksöz Real-Time Strong-Motion Broadband Displacements from Collocated GPS and Accelerometers Yehuda Bock, Diego Melgar, and Brendan W. Crowell Analysis of the Origins of κ (Kappa) to Compute Hard Rock to Rock Adjustment Factors for GMPEs Chris Van Houtte, Stéphane Drouet, and Fabrice Cotton Comparison of Site Periods Derived from Different Evaluation Methods D. Motazedian, K. Khaheshi Banab, J. A. Hunter, S. Sivathayalan, H. Crow, and G. Brooks A Stochastic Approach for Evaluating the Nonlinear Dynamics of Vertical Motion Recorded at the IWTH25 Site for the 2008 Mw 6.9 Iwate–Miyagi Inland Earthquake Shigeo Kinoshita Comparison of Nonlinear Structural Responses for Accelerograms Simulated from the Stochastic FiniteFault Approach versus the Hybrid Broadband Approach Gail M. Atkinson, Katsuichiro Goda, and Karen Assatourians Near-Field Response of a 1D-Structure Alluvial Site Denis Sandron, Livio Sirovich, and Franco Pettenati A Predictive Equation for the Vertical-to-Horizontal Ratio of Ground Motion at Rock Sites Based on Shear-Wave Velocity Profiles from Japan and Switzerland Benjamin Edwards, Valerio Poggi, and Donat Fäh

Empirical Distance Attenuation and the Local Magnitude Scale for Northwest Iran Mehdi Rezapour and Reza Rezaei Forearc versus Backarc Attenuation of Earthquake Ground Motion Hadi Ghofrani and Gail Atkinson Regional Correlations of VS30 and Velocities Averaged Over Depths Less Than and Greater Than 30 Meters David M. Boore, Eric M. Thompson, and Héloïse Cadet SHORT NOTES Near-Surface Expression of Early-to-Late Holocene Displacement along the Northeastern Himalayan Frontal Thrust at Marbang Korong Creek, Arunachal Pradesh, India R. Jayangondaperumal, Steven G. Wesnousky, and Barun K. Choudhuri The 16 May 1909 Northern Great Plains Earthquake W. H. Bakun, M. C. Stickney, and G. C. Rogers Location of Aftershocks of the 4 April, 2010 Mw 7.2 El Mayor–Cucapah Earthquake of Baja California, Mexico Raúl R. Castro, José G. Acosta, Víctor M. Wong, Arturo PérezVertti, Antonio Mendoza, and Luis Inzunza Nonvolcanic Tremor in the Aleutian Arc C. L. Peterson, S. R. McNutt, and D. H. Christensen A New Empirical Magnitude Scaling Relation for Switzerland Bettina P. Goertz-Allmann, Benjamin Edwards, Falko Bethmann, Nicholas Deichmann, John Clinton, Donat Fäh, and Domenico Giardini Determination of Love- and Rayleigh-Wave Magnitudes for Earthquakes and Explosions Jessie L. Bonner, Anastasia Stroujkova, and Dale Anderson Inversion of Ground-Motion Data from a Seismometer Array for Rotation Using a Modification of Jeager’s Method Wu-Cheng Chi, W. H. K. Lee, J. A. D. Aston, C. J. Lin, and C. C. Liu Site Effects in Unstable Rock Slopes: Dynamic Behavior of the Randa Instability (Switzerland) Jeffrey R. Moore, Valentin Gischig, Jan Burjanek, Simon Loew, and Donat Fäh

Seismological Research Letters Volume 82, Number 6 November/December 2011 969

COMMENTS AND REPLIES Comment on “Evidence that the 2008 Mw 7.9 Wenchuan Earthquake Could Not Have Been Induced by the Zipingpu Reservoir” by Kai Deng, Shiyong Zhou, Rui Wang, Russell Robinson, Cuiping Zhao, and Wanzheng Cheng Shemin Ge

Reply to “Comment on ‘Evidence that the 2008 Mw 7.9 Wenchuan Earthquake Could Not Have Been Induced by the Zipingpu Reservoir’ by Kai Deng, Shiyong Zhou, Rui Wang, Russell Robinson, Cuiping Zhao, and Wanzheng Cheng” by Shemin Ge Shiyong Zhou and Kai Deng

970 Seismological Research Letters Volume 82, Number 6 November/December 2011

EASTERN

SECTION

RESEARCH LETTERS

Reassessment of Stable Continental Regions of Southeast Asia Russell L. Wheeler

Russell L. Wheeler U. S. Geological Survey

ABSTRACT Probabilistic seismic-hazard assessments of the central and eastern United States (CEUS) require estimates of the size of the largest possible earthquake (Mmax). In most of the CEUS, sparse historical seismicity does not provide a record of moderate and large earthquakes that is sufficient to constrain Mmax.

One remedy for the insufficient catalog is to combine the catalog of moderate to large CEUS earthquakes with catalogs from other regions worldwide that are tectonically analogous to the CEUS (stable continental regions, or SCRs). After the North America SCR, the largest contribution of earthquakes to this global SCR catalog comes from a Southeast Asian SCR that extends from Indochina to southeasternmost Russia.

Integration and interpretation of recently published geological and geophysical results show that most of these Southeast Asian earthquakes occurred in areas exposing abundant alkaline igneous rocks and extensional faults, both of Neogene age (last 23 million years). The implied Neogene extension precludes classification of the areas as SCR crust. The extension also reduces the number of moderate and large Southeast Asian historical earthquakes that are available to constrain CEUS Mmax by 86 percent, from 43 to six.

INTRODUCTION Most probabilistic seismic-hazard assessments of the central and eastern United States (CEUS: east of the Rocky Mountains) and elsewhere worldwide require an estimate of Mmax, the moment magnitude of the largest earthquake that is thought to be possible within a specified area (Wheeler 2009a,b). Wheeler (2009a) cited example assessments, including Risk Engineering Inc. et al. (1986), Johnston et al. (1994), and Petersen et al. (2008). The value of Mmax is important in probabilistic computations for building codes and for design of critical structures such as nuclear power plants (Mueller 2010). doi: 10.1785/gssrl .82.6.971

Accurate estimates of Mmax are more important for nuclear reactors than building codes because reactor designs require consideration of smaller annual probabilities of unexpectedly strong ground motions (Petersen et al. 2008; Office of Nuclear Regulatory Research 2007).

The historical record of the CEUS contains earthquakes of moment magnitude M 7.0 or larger only at the seismic zones of New Madrid, Missouri; Charleston, South Carolina; and perhaps Charlevoix, Quebec (Ebel 1996, 2011; Johnston 1996c; Hough et al. 2000; Bakun and Hopper 2004). Elsewhere in the CEUS, sparse seismicity suggests that large earthquakes may have recurrence intervals longer than the historical record, which is generally two to four centuries long. Wherever sufficient paleoseismic work has been done in the CEUS outside the New Madrid, Charleston, and Charlevoix zones, findings document occurrences of prehistoric earthquakes larger than any in the historical record that occurred at intervals longer than the historical record (Madole 1988; Crone and Luza 1990; Crone, Machette, and Bowman 1997; Crone, Machette, Bradley et al. 1997; Obermeier 1998; McNulty and Obermeier 1999; Tuttle et al. 2006; Cox et al. 2010). If recurrence intervals are that long, then earthquakes larger than any observed historically are possible. If such an earthquake is not in the historical record, then Mmax may not have been observed and it must be estimated by other means. Indirect methods based on physics, statistics, or the geologic properties of small areas have generally given Mmax estimates that lack strong supporting evidence (Chinnery 1979; Coppersmith et al. 1987; Wheeler 2009a). Another approach was needed and the next section summarizes it.

Stable Continental Regions A recent workshop on CEUS Mmax concluded that identification and study of global tectonic analogs of the CEUS and their seismicity is the preferred approach to the problems arising from short historical records (Wheeler 2009b, 141–143).

Seismological Research Letters Volume 82, Number 6 November/December 2011 971

TABLE 1 Criteria for Identifying Stable Continental Regions (SCRs) Time Interval* Neogene Period (0–23 Ma) Paleogene Period (23–65.5 Ma) Late Cretaceous Epoch (65.5–99.6 Ma) Early Cretaceous Epoch (99.6–145.5 Ma)

SCR Identification Criteria† 1. No rifting or major extension 2. No deformation of orogenic 3. No orogenic activity after or transtension after Paleogene foreland after Early Cretaceous Early Cretaceous Allowed Not allowed Not allowed Allowed

Not alowed

Not allowed

Allowed

Allowed

Allowed

* In parentheses, age range of the interval of geologic time, from Gradstein et al. (2004). Ma, millions of years ago. † After Kanter (1994). Allowed: deformation of this age and kind does not disqualify an area from being an SCR. Kanter listed a fourth criterion of no major anorogenic intrusions younger than Early Cretaceous. The criterion is not necessary for evaluation of the Southeast Asian SCRs.

Coppersmith et al. (1987) and Coppersmith (1994) suggested that the CEUS and other areas worldwide that are tectonically analogous to it may have similar values of Mmax. Coppersmith et al. (1987) suggested combining the historical earthquakes of geologically similar regions into a dataset large enough to potentially provide robust lower bounds on Mmax, or perhaps to be candidates for Mmax itself. Accordingly, Kanter (1994) expressed “tectonically analogous” in terms of four criteria that broadly characterize the tectonics of the CEUS and central and eastern Canada (Table 1). Johnston et al. (1994) used the term stable continental region (SCR) for an area that meets all four criteria. Other continental areas are not considered to be tectonic analogs of the CEUS and are classified as active continental crust (ACR).

Participants in the CEUS Mmax workshop were acutely aware that the geologic variables that control the value of CEUS Mmax are poorly known (see discussions throughout Wheeler 2009b). Furthermore, the distinction between SCRs and ACRs is not clear in all continental areas. For example, tectonism young enough to classify an area as active crust according to the criteria of Table 1 may be sparse or unrecognized.

Alternatively, the tectonism might not be clearly rifting, orogenic activity, or deformation of an orogenic foreland. In cases where the distinction between stable and active crust is enigmatic, focusing attention on the brittle upper crust can help to make the distinction. In other cases argument by geologic or tectonic analogy can clarify the distinction. Later sections describe illustrative cases in and around eastern Mongolia and in Indochina, respectively.

With these uncertainties in mind, Kanter (1994) utilized her criteria to define eight SCRs. Each continent contains at least one SCR. The CEUS forms the southern half of the North America SCR. Johnston et al. (1994) compiled geological and seismological information on SCR earthquakes worldwide.

As already mentioned, the recent Mmax workshop produced a recommendation that future estimates of CEUS Mmax for seismic-hazard analyses should utilize the global SCR catalog of Johnston et al. (1994) (Wheeler 2009b, 141–143). The global catalog shows that, after North America, the much

smaller China SCR in Southeast Asia has the most historical earthquakes of M 6.0 or larger (Figure 1). Consequently the 1994 China SCR and its three parts as shown in Figure 1 are important tectonic analogs in estimating CEUS Mmax.

Purpose Since the definition and delineation of SCRs in 1994, many papers on the geophysics and tectonics of Southeast Asia have appeared in English-language Western journals, for example Yin (2010) and papers cited there. My purpose is to reassess the 1994 China SCR and its earthquakes in light of the new information presented in these papers, in order to improve estimates of CEUS Mmax. The 1994 China SCR of Kanter (1994) includes two thin bands of active continental crust that are centered on large, active, strike-slip fault systems. The thin bands divide the 1994 China SCR into three parts that are labeled MO, CH, and IO in Figure 1. The rest of this paper utilizes the new information and the criteria in Table 1 to update the Mongolia, 2011 China, and Indochina SCRs of Figure 1. The update will result in reclassifying most of the 1994 China SCR of Figure 1 as active crust. Nearly all of the Mongolia SCR will retain its classification as SCR crust, as will the southwestern part of the 2011 China SCR.

SOUTHEAST ASIAN SCRs Continental Extension and Alkaline Igneous Rocks Information published since the early 1990s (for example, Yin 2010) shows that much of Southeast Asia is undergoing horizontal extension. Reassessing the SCR with the new information requires determining which parts of the 1994 China SCR have undergone Neogene extension (Table 1). Geodetic and geophysical data and mapped extensional faults and continental rifts provide well-known indicators of continental extension.

It may be less well known that dated alkaline igneous rocks, when combined with geologic field relations showing relative ages of faulting, eruption, and intrusion, can determine both the occurrence of continental rifting and its age. Worldwide,

972 Seismological Research Letters Volume 82, Number 6 November/December 2011

(A)

100°W 60°W

70°N

70°N

(B) 100°E

140°E 50°N

50°N

MO

140°E

CH 30°N

*

30°N

IO

10°N

10°N 0 Kilometers 2000

100°E 0 Kilometers 2000

60°W

100°W

Earthquake epicenter Mainland and coastline of continent Boundary of stable continental region

*

Large earthquakes of New Madrid seismic zone

▲▲ Figure 1. Comparison of sizes and seismicities of North America (A) and China (B) stable continental regions (SCRs). Coastlines and SCR boundaries after Kanter (1994) and Broadbent and Allan Cartography (1994). Epicenters are of earthquakes of magnitude 6.0 or larger on any magnitude scale (Wheeler, in preparation). Both maps use Lambert azimuthal equal area projections and the same scale to aid visual comparison (Broadbent and Allan Cartography 1994). The North America SCR covers 24,007,000 km2 and generated 35 reported earthquakes of magnitude 6.0 or larger over the four-century historical record, whereas the smaller China SCR covers 7,118,000 km2 and has generated 27 such earthquakes over its 15-century historical record (Johnston et al. 1994). Chinese earthquakes appear more numerous in the figure because more of the North American epicenters overlap one another at the scale of the figure. For ease of discussion, I will refer to the single large SCR of part B as the 1994 China SCR, and to its three components as the Mongolia SCR (MO), the 2011 China SCR (CH), and the Indochina SCR (IO).

igneous rocks of alkaline compositions are spatially associated with continental rifts (Bailey 1974; Neumann and Ramberg 1978; Keller and Hoover 1988; McKenzie and Bickle 1988; Wilson 1989). The spatial association is generally accepted as implying that extension produces alkaline melts (Wilson 1989).

Furthermore, common crustal rocks have melting temperaWheeler, Figure 1 tures well below those of basalts. This implies that melting in the mantle generates alkaline basalts. Petrological modeling calculations of McKenzie and Bickle (1988) and of Barry et al.

(2003) imply that the melting of peridotite, a common mantle rock, yields alkaline basaltic melts at depths exceeding 70 km. In laboratory experiments, the initial melting of peridotite under mantle pressures and temperatures produces small amounts of alkaline basaltic melts (Jaques and Green 1980; Olafsson and Eggler 1983; Takahashi and Kushiro 1983). Both the melting experiments and the petrological calculations show that additional melting shifts the composition of basaltic melts away from alkaline toward less-alkaline basaltic compositions. Most riftrelated alkaline igneous rocks are alkaline basalts. In addition, some rifts also contain alkaline volcanic and intrusive rocks of granitic compositions. Alkaline igneous rocks are known in several parts of a rift that contains the New Madrid seismic zone,

the most active seismic zone in the CEUS (Figure 1) (see summary of these alkaline rocks in Wheeler 1997). Importantly for the present study, basaltic rocks dominate in Southeast Asia (Whitford-Stark 1987; Yin 2010), for example in the Baikal rift system of the study area (Wilson 1989; Figure 2 this paper).

Thus, volcanic rocks of alkaline basaltic composition imply a small amount of extension within the upper mantle.

Exposed or shallow normal or transtensional faults demonstrate extension of the upper crust and its seismogenic zone.

The larger the extensional fault slips, the more likely it is that brittle extension penetrates into or spans the seismogenic zone.

Where both rifts and alkaline basaltic volcanic rocks are present and are of similar ages, they indicate that extension affects the upper crust, upper mantle, and therefore perhaps the middle and lower crust as well. Rifts without known alkaline volcanic rocks demonstrate brittle extension of at least the uppermost crust, perhaps including the seismogenic zone. However, alkaline volcanic rocks without recognized, coeval extensional faults are more problematic. Absent known extensional faults, alkaline rocks might indicate that incipient extension in the upper mantle has not extended far enough upward to affect the seismogenic zone.

Seismological Research Letters Volume 82, Number 6 November/December 2011 973

80°

90°

100°

110°

120°

140°

130° SFZ

BRS

Siberia

EXPLANATION

50°

KES 80° 50°

Border of stable continental region (SCR) (Kanter, 1994)

TKFS

o

ng

Borders of China and Mongolia

Mo

Mongolia

Large crustal block (see text)

SCR Ch N ina Ko ort reah

HES

140°

a ssi Ru ina lia Ch

Area of Neogene volcanic rocks 40°

Epicenter in active continental crust

NORTH CHINA BLOCK

Yellow Sea 2011 China SCR East China Sea

Normal fault (hachure on downdropped wall)

SBF

XXF

SOUTH CHINA BLOCK

S

Left lateral strike-slip fault

AS

a

in

Ch

RR

Laos

Ch

ina

Vie tn

am

30° 130°E

Taiwan

M

ECC

20°

Hainan Island

a

rm

Bu

20°

QLFZ

W

Reverse fault (teeth on upthrown wall)

EC

LMSF

30°

Right lateral strike-slip fault

CM

TLF

Epicenter in stable continental crust

40°

LPSFB

IndoChina SCR

SF

South China Sea

Indian Ocean 10°

0

10°

R

ASR

90°

100°

Kilometers

1,000

Outlines of islands other than Hainan and Taiwan are omitted for clarity.

110°

120°

▲▲ Figure 2. Selected tectonic Wheeler, Figure 2 elements of Southeast Asia. Stable continental regions (SCRs) after Broadbent and Allan Cartography (1994) and Kanter (1994) (See Figure 1 for new SCR names introduced here.) North China block and the adjoining part of the Korean peninsula after Zhang et al. (1984), Zhao et al. (1998), and Kwon et al. (2009). South China block after Ren et al. (2002), Liu et al. (2007), J. Adams (see Wheeler 2009b, 83), Yin (2010), and analyses in this paper. Locations of areas of Neogene volcanic rocks are from the continent-scale map in Figure 1 of Yin (2010). For legibility here I generalized the locations. Each solid square represents the center of a group of approximately five of the locations shown by Yin (2010). Epicenters are of reported earthquakes of magnitude 6.0 or larger on any magnitude scale (Wheeler, in preparation). W, epicenter of Wenchuan earthquake (12 May 2008; M 7.9; 31.00°N, 103.32°E; http:// earthquake.usgs.gov/ ). Faults shown have Neogene movement. Their locations and age assignments are after Peizhen et al. (1991), Ren et al. (2002), Jia et al. (2006), Zhu et al. (2010), and Yin (2010). ECCM, eastern China continental margin. Fault names: ASRR, Ailao Shan–Red River shear zone; BRS, Baikal rift system; HES, Hangay extensional system; KES, Khubsugul extensional system; LMSF, Longmen Shan thrust fault; LPSFB, Lanping-Simao fold belt; QLFZ, Qinling fault zone; SBF, Sichuan Basin fault; SF, Sangaing fault; SFZ, Stanovoy fault zone; TKFS, Tunka fault system; TLF, Tanlu fault; XXFS, Xiangshuihe-Xiaojiang fault system. Lambert azimuthal equal area projection centered at 25°N, 100°E.

974 Seismological Research Letters Volume 82, Number 6 November/December 2011

Mongolia SCR The presence or absence of Neogene alkaline igneous rocks and rifting provides a guide to whether the Mongolia SCR should be classified as an SCR or an ACR. The Mongolia SCR includes the eastern half of Mongolia, most of northeastern China, and adjacent areas of Russia (Figure 2). The Mongolia SCR is a comparatively stable region. ACR crust surrounds this SCR; later sections will summarize the active nature of the North China block on the south, a region of active faults and volcanism on the west and north, and the eastern China continental margin on the east. The Tanlu fault separates the Mongolia and 2011 China SCRs. The Mongolia SCR is moving eastward with respect to a fixed Eurasia (Liu et al. 2007; Wang et al. 2011).

The relative motion takes place on a belt of extensional and left-lateral transtensional faulting north of the SCR, between it and the Siberian part of the Eurasian SCR. The belt of faulting comprises the Hangay and Khubsugul extensional systems, the Tunka fault system, the Baikal rift system, and the Stanovoy fault zone (Figure 2). Judging from the motions that Liu et al. used to compute Quaternary rates of fault slips, the Mongolia SCR appears to be moving eastward with respect to Siberia at much less than 1 mm/yr, and possibly as little as 0.1 mm/yr.

Tomography shows that S-wave velocities at 50 km depth are similar across the Mongolia SCR, and the same is true for 100 km depth (Feng and An 2010). From this it appears that crustal and lithospheric thicknesses vary little across the SCR. Seismicity is sparse and geodetically measured velocity and strain are small (Broadbent and Allan Cartography 1994; Liu et al. 2007; Feng and An 2010) (see also the earthquake catalogs at http://earthquake.usgs.gov/earthquakes; last accessed July 21, 2011).

The compilation map of Ren et al. (2002) shows normal faults that bound Paleogene and older basins throughout the SCR, but no Neogene faults or basins. Active systems of northerly striking normal faults and easterly striking left-lateral strike-slip faults in western Mongolia do not appear to extend into the SCR, except in its northwestern corner at the northeast-striking normal faults of the Hangay extensional system (Figure 2) (McCalpin and Khromovskikh 1995; Walker et al.

2007; Yin 2010). Walker (2009) did not find active faults in eastern Mongolia and adjacent China, which include most of the Mongolia SCR. Figure 2 shows that Neogene volcanic rocks are less numerous per unit area within the Mongolia SCR than in more tectonically active regions, such as the Indochina SCR and China east of the South China block and the Tanlu fault (Ren et al.

2002; Liu et al. 2007; Yin 2010). The Neogene volcanic rocks in the Mongolia SCR are largely alkaline although older volcanic rocks range more widely in compositions (Whitford-Stark 1987; Basu et al. 1991). Barry et al. (2003) cited computations

by McKenzie and Bickle (1988), which imply that generation of significant amounts of alkaline melts would require much more Neogene horizontal extension than appears to have occurred in most of the Mongolia SCR. Consequently, since the definition of the Mongolia SCR in 1994, new information does not demonstrate extension younger than Paleogene except

in the northwestern corner of the Mongolia SCR. The rest of the Mongolia SCR meets the criteria of Table 1 and retains its classification as an SCR. 2011 China SCR North China Block The North China craton is the Chinese part of the SinoKorean craton, with the remainder being the northern part of the Korean peninsula (for example, Zhang et al. 1984, Zhao et al. 2009, Yang et al. 2010). I follow Kwon et al. (2009) in calling both cratons “blocks” because, as explained later, they underwent Mesozoic and Cenozoic metamorphism, extension, intrusion, and volcanism so that they are no longer cratonic crust (Figure 2; note that the craton boundary is northwest of the boundary between North and South Korea). The Tanlu fault splits the North China block into two parts. The larger part of the block lies entirely west of the 1994 and 2011 China SCRs, whereas the smaller part is within both versions of the SCR. The smaller part is of more interest here, but most of the information on the North China block comes from the active crust of the larger part. Therefore, I will discuss the North China block as a whole. The block is a triangular region in northern China (Figure 2) that is made of early Precambrian crust (Zhang et al. 1984; Zhao et al. 2001; Kwon et al. 2009).

The North China block is moving eastward with respect to a fixed Siberia and the Mongolia SCR (Yin 2010). Geologic data including slip rates of individual faults indicate eastward movement with respect to Siberia of 1–2 mm/yr in the eastern half of the North China block and 2–4 mm/yr in the western half (Liu et al. 2007). Geodetic data indicate rates consistent with those of Liu et al. (2007) (Wang et al. 2011).

The eastern part of the block is seismically active, whereas the western part is less so (Liu et al. 2007). For example, the 2008 version of the “Centennial” earthquake catalog of Engdahl and Villasenor (2002) lists 17 earthquakes of magnitude 6.0 or larger in the eastern part of the block but only one in the western part. Results of P- and S-wave tomography show a low-velocity zone that extends to 300–400 km depth beneath the eastern part of the block (Zhao et al. 2009). S-wave tomography, deep seismic-reflection profiles, and receiver-function imaging show that the crust thins eastward from approximately 45 km in the western part of the block to about 30 km in the eastern part (Li et al. 2006; Zheng et al. 2006; Chen et al. 2009; Feng and An 2010). Zheng et al. (2006) concluded that most of the thinning took place in the lower crust and in a transitional zone between the crust and mantle. The thinning resulted from extension that began with widespread Early Cretaceous eruption and intrusion of alkaline basaltic and granitic rocks (Ren et al. 2002; Wu et al. 2005; Zhu et al. 2010).

The entire North China block underwent extension by normal and transtensional faulting of early Neogene age, whereas the eastern part of the block and the northern part of the Korean peninsula also underwent late Neogene alkaline igneous activity (Liu et al. 2001; Ren et al. 2002; Zheng et al. 2006; Yu et al. 2008; Yang et al. 2010; Yin 2010). Zhao et al. (2009) inter-

Seismological Research Letters Volume 82, Number 6 November/December 2011 975

preted the low-velocity zone in the mantle beneath the eastern part of the block in terms of warm mantle material that might have caused the rifting and alkaline igneous activity. Extension is demonstrated in the upper crust by the mapped extensional faults, in the lower crust by the geophysical evidence, and deeper than 70 km (McKenzie and Bickle 1988) in the mantle by the compositions of the igneous rocks. The Neogene extension demonstrates that the North China block and the rest of the Sino-Korean block are no longer a craton (Yang et al. 2008, 2010). The extension also requires reclassifying this part of the 1994 China SCR as active crust (Table 1).

Eastern China Continental Margin The eastern China continental margin comprises all of the Yellow Sea, the East China and South China seas within 200–600 km of the mainland Chinese coast, and the land east of the North China and South China blocks (Figure 2; Zhou et al. 1995; Yin 2010). Bathymetric, geologic, and geophysical data show that the continental crust of the margin extends seaward approximately to the edge of the continental shelf (GEBCO World Map Editorial Board 2006; Wang et al.

2006; Yin 2010). Fault-slip data show that the margin is moving south-southwestward past the North and South China blocks (Figure 2; Yin 2010); geodetic data give the rate as 0.7 mm/yr in the north and 1.8 mm/yr in the south (Wang et al.

2011). The continental margin is more seismically active than the South and North China blocks, especially west of Taiwan (Liu et al. 2007; Wang et al. 2011).

The compilations of Yin (2010), Ren et al. (2002), and Sengor and Natal’in (2001) show the continental margin as having undergone Paleogene and older extension. S-wave tomography indicates thick sediments and thin crust and lithosphere under the margin (Feng and An 2010). China east of the North and South China blocks contains numerous basins bounded by normal faults; most of the basins are of Paleogene age (Ren et al. 2002). In the same part of China and in offshore basins on the continental shelf, abundant basaltic volcanic rocks of late Neogene ages are exposed from Hainan Island northeastward to the southern Tanlu fault (Figure 2; Ren et al.

2002; Yin 2010). Many of the basaltic rocks are alkaline (Ho et al. 2003). In the South China Sea, early Neogene thermal subsidence of several kilometers was followed by middle Neogene normal faulting (Zhou et al. 1995). The alkaline basalts and normal faulting indicate Neogene extension of the continental margin from Hainan Island to the southern Tanlu fault.

Yin (2010) restricted the definition of the margin northeast of North Korea to offshore continental crust that has been thinned by back-arc extension. However, three onshore areas between the coast and the Tanlu fault must also be considered because they are in the 1994 China SCR, or adjacent to it and along its tectonic grain (Figure 2): 1. The northern Korean peninsula and the northern Yellow Sea are part of the 1994 China SCR. The northern peninsula underwent Neogene extension as described earlier.

Thus, ACR of the Sino-Korean block and the eastern China continental margin surround the northern Yellow

Sea on three sides (Figure 2). The Neogene extension implies that the northern Korean peninsula, and probably the adjacent part of the Yellow Sea, should be reclassified as active crust (Table 1).

2. Northeast of the North China block and east of the Tanlu fault, northeastern China and adjacent Russia are not in the 1994 China SCR. This area contains numerous Neogene basalts, many of them alkaline (Liu et al. 2001; Ren et al. 2002). The alkaline basalts indicate Neogene extension, and Sengor and Natal’in (2001) show latest Cretaceous–early Neogene grabens in the same area as the basalts. The evidence for Neogene extension implies that the area was correctly classified as active crust in 1994.

3. The southern part of the Korean peninsula is part of the 1994 China SCR. Yin (2010) reported that the peninsula appears to have had little Paleogene or Neogene extension.

The southern part of the peninsula is outside the SinoKorean block. Summaries of Korean geology and tectonics do not show any evidence of foreland deformation or orogenic activity of Cretaceous or younger age (Table 1) (Exxon Production Research Company 1985; Chang 1997; Kwon et al. 2009). However, the southeastern part of the peninsula exposes a few extensional or transtensional faults that bound Neogene basins, as well as sparse Neogene alkaline volcanic rocks (Exxon Production Research Company 1985; Ren et al. 2002; Yang et al.

2010; Yin 2010). Two small islands approximately 170 km east of the mainland and 120 km south of it also expose sparse Neogene alkaline volcanic rocks (Reedman and Kim 1997). Kanter (1994) classified the southern island as being within the same northeast-trending belt of Mesozoic continental crust as the southern part of the Korean peninsula and most of the Yellow Sea (Broadbent and Allan Cartography 1994). The Neogene alkaline rocks exposed on the southern island, together with the Neogene alkaline rocks exposed along trend to the southwest in the eastern China continental margin (Figure 2), suggest that similar Neogene alkaline rocks may be hidden beneath the Yellow Sea. Thus, the evidence for Neogene extension is sparse and whether the southern part of the Korean peninsula and the Yellow Sea are SCR crust or ACR crust is arguable. Overall, I judge that neither area meets the requirements for classification as an SCR (Table 1).

Regional geologic relations are consistent with the classification of the southern part of the Korean peninsula as probable ACR crust. As pointed out earlier in this section, all other areas east of the Tanlu fault, including the northern part of the Korean peninsula, have undergone Neogene extension. As a result, 500–600 km of the active crust of the North China block separates the southern Korean peninsula from the nearest SCR, which is the Mongolia SCR. This isolation allows the possibility that the southern Korean peninsula might be a small fragment of comparatively intact ACR crust that is being carried east-southeastward by Neogene extension of the surrounding more active continental crust. The possibility is consistent with

976 Seismological Research Letters Volume 82, Number 6 November/December 2011

the small horizontal relative motions and dilational strains between the southern part of the peninsula and adjacent ACRs to the west and southwest (Liu et al. 2007; Wang et al. 2011).

The 1994 and 2011 versions of the China SCR extend as far offshore as 700 km southeast of Hainan Island (Figures 1, 2; Broadbent and Allan Cartography 1994). Bathymetric data show that only the near-shore half of this part of the SCR is on the continental shelf, at water depths of 0–200 m (GEBCO World Map Editorial Board 2006). The offshore half is beyond the shelf edge, at depths of 200–5,000 m. As shown on the map of Broadbent and Allan Cartography (1994), the offshore half was interpreted as SCR crust that had been highly extended during the Paleogene Period. However, interpretations of more recent marine seismic-reflection profiles between Hainan Island and southern Taiwan show thinned continental crust in the landward part of the surveyed area and unusually thick oceanic crust in the seaward part (Wang et al. 2006). The profile interpretations imply that the crust that was considered to be highly extended SCR crust should now be reclassified as thick oceanic crust.

In summary, information published since 1994 shows that almost the entire eastern China continental margin underwent Neogene extension. The only potential exception is the southern part of the Korean peninsula.

South China Block The South China block is part of the 1994 China SCR. The block consists of two smaller blocks that joined during late Precambrian time to form crust that is now tectonically stable (Qiu et al. 2000; Li et al. 2002; Zheng et al. 2006; Liu et al.

2007). S-wave tomography shows that the South China block has thinner sedimentary cover and higher upper mantle velocities than the active North China block (Feng and An 2010).

Feng and An (2010) inferred that the different upper-mantle velocities indicate thicker lithosphere in the South China block than in the North China block. Seismicity and geodetic strain rates are approximately as low as those of the Mongolia SCR. These comparisons allowed Liu et al. (2007) to suggest that the South China block is being extruded southeastward, away from the India-Eurasia collision zone, as a single intact entity. Shen et al. (2005) and Liu et al. (2007) used the geodetic data to compute the rate of southeastward motion as 7–8 mm/yr and 4–6 mm/yr, respectively, with respect to a stable Siberia, and Wang et al. (2011) used the data to calculate a left-lateral slip rate of 0.9 mm/yr between the South China and North China blocks.

Active continental crust bounds the South China block on all sides. On the north, the Qinling fold belt of Paleogene or Neogene age separates the South China and North China blocks (Figure 2; Terman 1974; Ren et al. 2002). The fold belt itself is active continental crust because it is the product of orogenic activity younger than Early Cretaceous (Table 1).

The southernmost component of the fold belt is the left-lateral Qinling fault system of Neogene age (Yin 2010). Thus, the Qinling fault system marks the northern boundary of the South China block. On the east, the South China block adjoins the active continental crust of the eastern China continental mar-

gin. On the south, the Neogene Ailao Shan–Red River shear zone separates the South China block from the Indochina SCR. The Ailao Shan–Red River shear zone is tens of kilometers wide, many hundreds of kilometers long, and constitutes active continental crust, as described in a later section on the Indochina SCR. The northern edge of the shear zone marks the south boundary of the South China block. On the west, the left-lateral Xiangshuihe-Xiaojiang fault system forms part of the block’s boundary. The fault system formed during the middle Neogene Period and remains active today. The rest of the western boundary of the South China block is along the east-dipping Sichuan Basin fault and the west-dipping thrust faults that crop out at the eastern front of the Longmen Shan range (Figure 2; Yin 2010; Burchfiel et al. 2008).

The Longmen Shan thrust fault comprises several strands that crop out along different sections of the northeast-trending Longmen Shan range front (Hubbard et al. 2010). Most of the exposed strands of the fault crop out within the range and along its southeastern front. The 400-km-wide Sichuan Basin is southeast of the range front and is part of the 1994 China SCR (Kanter 1994). Jia et al. (2006) and Burchfiel et al.

(2008) explained that the Sichuan Basin is the foreland basin produced by orogenic movements farther west, including those on the Longmen Shan thrust fault. The orogenic movements continue, as shown by the occurrence of the M 7.9 Wenchuan earthquake in 2008 (Figure 2) and by a similar earthquake that paleoseismic and historical evidence dates at approximately 2 ka (Liu et al. 2010).

From the range front southeastward into the Sichuan Basin is a belt of northeast-trending anticlines and northeaststriking, northwest-dipping reverse faults (Burchfiel et al.

1995). Interpretations of well and seismic-reflection data and of geologic mapping imply that the anticlines and reverse faults are underlain by a nearly horizontal strand of the Longmen Shan fault, which propagated southeastward into the basin (Jia et al. 2006; Hubbard et al. 2010). Southeastward movement on the buried fault strand buckled the overlying strata to form the anticlines and reverse faults (Burchfiel et al. 1995, 2008). Elsewhere, similar combinations of anticlines, reverse faults, and one or more underlying thrust faults deformed sedimentary strata of foreland basins and moved the strata outward away from growing mountain ranges, as in the southern, central, and northern Appalachian mountains, the southern Canadian Rocky Mountains, the Idaho-Montana thrust belt, and the Ouachita mountains of Oklahoma and Arkansas (for example, Rich 1934; Boyer and Elliott 1982; Perry et al.

1984; Rodgers 1970; Arbenz 1988; Hatcher et al. 1990). In the Sichuan Basin, deformation of the basin deposits continued as late as Paleogene and Neogene time (Burchfiel et al. 1995; Kirby et al. 2002; Jia et al. 2006). Thus, the northwestern part of the Sichuan Basin is a deformed orogenic foreland that has been active more recently than the Early Cretaceous. This conclusion results in the reclassification of the northwestern part of the basin as active crust (Table 1).

Geologic maps and cross-sections show that several of the reverse faults dip southeastward, particularly at or near the

Seismological Research Letters Volume 82, Number 6 November/December 2011 977

southeast edge of the foreland deformation (Burchfiel et al.

1995; Kirby et al. 2002; Jia et al. 2006; Burchfiel et al. 2008; Hubbard et al. 2010). Such backward-dipping reverse faults at or near the fronts of fold-and-thrust belts are recognized elsewhere (for example, Rodgers 1950; Malik et al. 2010).

Accordingly, a line drawn along the southeasternmost outcrops of southeast-dipping thrust faults may approximate the eastern limit of deformation in the Sichuan Basin. Figure 1 of Yin (2010) shows such a line as a southeast-dipping reverse fault labeled the Sichuan Basin fault. I use the Sichuan Basin fault to approximate the western boundary of the South China block (Figure 2).

Figure 2 shows an isolated right-lateral strike-slip fault within the northeastern South China block. Yin (2010) shows the fault as having Neogene movement but provides no other information on the fault. The North America SCR contains four similarly isolated active faults: the normal Cheraw fault in eastern Colorado, the strike-slip Meers fault in southern Oklahoma, the reverse Reelfoot fault in the New Madrid seismic zone of southeastern Missouri, and the reverse Ungava fault in northern Quebec (Russ 1979; Madole 1988; Crone and Luza 1990; Adams et al. 1991; Crone, Machette, Bradley et al. 1997). The active faults inside the North America SCR imply that the fault inside the South China block need not disqualify that part of the block from being SCR crust. Neither Yin (2010) nor Ren et al. (2002) show any Neogene igneous rocks or additional faults in the South China block. To summarize, the South China block retains its classification as SCR crust except for the northwestern Sichuan Basin.

Indochina SCR Since middle Paleogene time the Indochina SCR has been extruded southeastward in response to subduction of the Indian plate beneath the Eurasian plate (Figure 2; Leloup et al. 1995; Yin 2010). The SCR is moving rapidly southeastward relative to a fixed Siberia (Liu et al. 2007; Simons et al. 2007). Wang et al. (2011) calculated rates of left-lateral strike slip of 18 and 10 mm/yr along two sections of the Xiangshuihe-Xiaojiang fault system (Figure 2). The northeastern boundary of the extruding mass is the northwest-striking Ailao Shan–Red River shear zone. The western boundary of the extruding mass is a complex fault network that includes the Sangaing fault and other northand northwest-striking, mostly right-lateral faults (Leloup et al. 1995; Ren et al. 2002; Yin 2010) and diversely oriented, offshore Paleogene and Neogene grabens (Sengor and Natal’in 2001). Since early Neogene time most of the interior of the Indochina SCR was fragmented by right-lateral transtensional faulting and underwent eruption of alkaline basalts (Rangin et al. 1995; Ren et al. 2002; Ho et al. 2003). S-wave tomography and teleseismic receiver function analyses indicate thinned crust and lithosphere under the Indochina SCR (Feng and An 2010; Bai et al. 2010). Seismicity is negligible in the SCR (Broadbent and Allan Cartography 1994; Tarr et al. 2010).

The Ailao Shan–Red River shear zone is 20–200 km wide and 800–1,200 km long onshore, with another 500–1,400 km of length suggested under the South China Sea (Terman

1974; Exxon Production Research Company 1985; Leloup et al. 1995). The shear zone underwent roughly 500–700 km of left-lateral strike slip during 40–15 Ma (Leloup et al. 1995).

The zone reversed its slip sense at approximately 5 Ma and has accumulated 20–50 km of right-lateral slip since that time. The shear zone contains a core of high-temperature metamorphic and igneous rocks that were uplifted from mid-crustal depths.

Flanking the core on its northeast and southwest sides are belts of lower-temperature metamorphic rocks and, farther out from the core, sedimentary basins whose strata were folded and cut by reverse faults that dip inward toward the core. Presumably most of the characteristics of the shear zone and related structures formed during the left-lateral majority of the zone’s evolution. For this reason Figure 2 shows the dominant left-lateral slip sense.

One of the groups of structures related to the shear zone is the Lanping-Simao fold belt (Leloup et al. 1995), which overlaps the north tip of the Indochina SCR (Figure 2). The fold belt comprises elongated folds and related reverse faults, which together deformed Late Cretaceous–early Paleogene shales of one of the sedimentary basins that flank the core of the Ailao Shan–Red River shear zone. The folds and reverse faults accommodated approximately 40 km of horizontal transport to the west-northwest. This structural style is the same as those of the northwestern Sichuan Basin and the North American analogs listed in the earlier description of the Sichuan Basin. The direction of horizontal transport is kinematically consistent with the orientation and movement sense of late Paleogene and early Neogene left-lateral movement on the Ailao Shan–Red River shear zone. Leloup et al. (1995) described field relations that led them to conclude that the fold belt formed at the same time as the shear zone and formed in the same stress field. These characteristics of the Lanping-Simao fold belt and its similarities to the Sichuan Basin and the North American analogs of the Sichuan Basin motivate me to classify the Lanping-Simao fold belt as a deformed foreland that is younger than the Early Cretaceous Epoch. Thus, new information that demonstrates Neogene rifting and post-Cretaceous foreland deformation requires reclassification of the Indochina SCR as active crust (Table 1).

DISCUSSION In this section I draw on the preceding descriptions to summarize whether the Mongolia SCR, North China block, northern and southern parts of the Korean peninsula, Yellow Sea, eastern China continental margin, South China block, and Indochina SCR satisfy the criteria of Kanter (1994) as summarized in Table 1. Figure 3 summarizes the following discussion.

The paucity of alkaline basaltic rocks, the absence of reported extensional faulting, and the laboratory experiments in which heating of mantle rock under mantle pressures produced alkalic melts first, when considered together indicate that the Mongolia SCR may have undergone incipient Neogene extension at upper-mantle melting depths exceeding 70 km. The extension does not appear to have propagated upward to rift

978 Seismological Research Letters Volume 82, Number 6 November/December 2011

80°

120°

140°

Mongolia SCR

EXPLANATION Border of stable continental region (SCR) (Kanter, 1994)

100°

140°

80°

40° Border of stable continental region (SCR) (this paper)

40°

South China SCR

Epicenter in active continental crust Epicenter in stable continental crust

20° 20°

0

1,000 Kilometers

100°

120°

▲▲ Figure 3. Summary of the changes from the 1994 stable continental regions (SCRs) to the SCRs of this paper (see Discussion text).

the upper-crustal seismogenic zone. Therefore, nearly all of the Mongolia SCR retains its SCR classification, at least in the upper crust and for the purpose of reassessing the 1994 China SCR to improve estimates of CEUS Mmax.

Similarly, the South China block should retain its classification as SCR crust (Figure 3). Its lithosphere is thick. Low seismicity and strain rates led Liu et al. (2007) to the interpretation that the block is being extruded southeastward as an intact block. I did not find any published evidence of Neogene extensional faulting, Neogene alkaline igneous rocks, or foreland deformation or orogeny younger than Early Cretaceous within the South China block. I suggest that the block be called the South China SCR. In contrast, the North China block, eastern China continental margin, and Indochina SCR fail to meet the requirements for classification Wheeler, Figureas3 SCR crust (Figure 3). All three areas contain widespread Neogene extensional faulting and abundant Neogene alkaline basaltic rocks. Thus, all three areas underwent Neogene rifting. The Korean peninsula and the Yellow Sea contain lesser amounts of Neogene extensional faulting and alkaline rocks. In addition, the northern tip of the Indochina SCR overlaps the Lanping-Simao fold belt, in which folding and reverse faulting deformed foreland-basin strata after Early Cretaceous time. A better classification for all five areas is active continental crust.

In the introduction I explained that moderate to large historical earthquakes in the 1994 China SCR provide valuable constraints on the value of CEUS Mmax. The realization that most of the 1994 China SCR is active crust reduces the number of Southeast Asian earthquakes that are available to constrain CEUS Mmax. Figure 2 shows epicenters of 43 earthquakes

of magnitude 6.0 or larger on any magnitude scale (Wheeler, in preparation). Some epicenters coincide so that the figure appears to show only 39 of them. Thirty-seven of the earthquakes, or 86 percent of the 43, are now recognized as having occurred in ACRs, leaving only six SCR earthquakes. The 37 active-crust earthquakes had estimated moment magnitudes ranging up to approximately 8. Two of the six stable-crust earthquakes occurred in the eastern part of the Mongolia SCR and four were in or on the border of the South China SCR (Figure 3). Table 2 lists the six SCR earthquakes, and double circles identify their epicenters in Figure 2.

Moderate earthquakes can provide lower bounds on estimates of Mmax, but larger earthquakes can provide tighter constraints on Mmax. Specifically, the tightest constraints on CEUS Mmax are sizes of earthquakes with about M 6.7 and larger (for example, see Figure 3 of Petersen et al.

2008). As explained earlier, many of Earth’s larger historical earthquakes occurred in China. As explained in this paper, recently published information indicates that nearly all of those larger earthquakes occurred in ACR crust instead of SCR crust. These reclassified earthquakes are lost to estimation of CEUS Mmax. Fortunately, the development of paleoseismology in recent decades could counterbalance this loss.

Paleoseismic data allow estimation of the sizes, locations, and ages of moderate to large prehistoric earthquakes (Tuttle 2001; McCalpin 2009). Wheeler (2008) estimated that earthquakes of approximately M 6.5 and larger are the most likely to yield paleoseismic estimates of their magnitudes, locations, and ages.

Therefore, paleoseismic magnitude estimates of large prehistoric SCR earthquakes may partly counterbalance the loss of large Chinese earthquakes. Determination of the impact of

Seismological Research Letters Volume 82, Number 6 November/December 2011 979

TABLE 2 Moderate to Large Earthquakes in the Mongolia and South China SCRs* SCR †

Date (YearMoDay)

Origin Time (HrMnSecs)

Source 1‡

Latitude

Longitude

Source 2 §

M

Source 3||

SCH

16310814

unknown

J94

29.300

111.700

J94

5.8

J94, J96

SCH

19170124

004812.00

ISS

31.000

114.000

ISS

6.5

C08, J96

MO

19220814

114104.70

C08

52.069

130.539

C08

6.6

C08, J96

SCH

19360401

unknown

J94

22.500

109.400

J94

6.8

J94, J96

MO

19410505

151827.00

J94

46.500

126.900

ISS

6.0

J94, J96

SCH

20080525

082149.99

PDE

32.560

105.420

PDE

6.1

GCMT

* Stable Continental Regions (see text). † SCR in which earthquake occurred. SCH, South China SCR; MO, Mongolia SCR. ‡ Source catalog for date and origin time. C08, 2008 version of “Centennial” catalog of Engdahl and Villasenor (2002); ISS, International Seismological Service (available at http://www.isc.ac.uk /); J94, Johnston et al. (1994); PDE, Preliminary Determination of Epicenters, U. S. Geological Survey (available at http://earthquake.usgs.gov/earthquakes/eqarchives/epic /). § Source catalog for latitude and longitude of epicenter. || Source catalog for moment magnitude M. GCMT, Global Centroid Moment Tensor (available at http://www.globalcmt.org/ CMTsearch.html ). First entry is source catalog of original size estimate (MS or maximum intensity). Second entry indicates that size estimate was converted to M with the look-up tables of Johnston (1996a,b).

this counterbalance on CEUS Mmax is beyond the scope of this paper and will be examined in a future report.

CONCLUSIONS 1. New information that was not available in the early 1990s shows that most of the 1994 China SCR is active continental crust. Only the South China and Mongolia parts of the 1994 SCR meet the criteria for retaining their classifications as SCRs. 2. This finding reduces the number of Southeast Asian moderate to large earthquakes that are available to constrain CEUS Mmax from 43 to six.

ACKNOWLEDGMENTS A. C. Johnston and his colleagues produced the global SCR catalog. The catalog and its extensive documentation of the geologic and tectonic contexts of each earthquake form the most valuable source of information with which to constrain CEUS Mmax. Many of the ideas in this paper stemmed from discussions with J. Adams, J. Ake, A. C. Johnston, C. S. Mueller, R. L. Wesson, and the other participants in the 2008 Mmax workshop. Suggestions by R. Gold, C. S. Mueller, M. D. Petersen, and two anonymous reviewers improved the manuscript.

REFERENCES Adams, J., R. J. Wetmiller, H. S. Hasegawa, and J. Drysdale (1991). The first surface faulting from a historical intraplate earthquake in North America. Nature 352, 617–619.

Arbenz, J. K., comp. (1988). Ouachita system: Cross sections. In The Appalachian-Ouachita Orogen in the United States, ed. R. D. Hatcher Jr., W. A. Thomas, and G. W. Viele, plate 11. Volume F-2

of The Geology of North America. Boulder, CO: Geological Society of America.

Bai, L., X. Tian, and J. Ritsema (2010). Crustal structure beneath the Indochina peninsula from teleseismic receiver functions. Geophysical Research Letters 37 (5), L24308, doi:10.1029/2010GL044874.

Bailey, D. K. (1974). Continental rifting and alkaline magmatism. In The Alkaline Rocks, ed. H. Sorensen, 148–159. New York: John Wiley and Sons.

Bakun, W. H., and M. G. Hopper (2004). Magnitudes and locations of the 1811–1812 New Madrid, Missouri, and the 1886 Charleston, South Carolina, earthquakes. Bulletin of the Seismological Society of America 94, 64–75.

Barry, T. L., A. D. Saunders, P. D. Kempton, B. F. Windley, M. S. Pringle, D. Dorjnamjaa, and S. Saandar (2003). Petrogenesis of Cenozoic basalts from Mongolia: Evidence for the role of asthenospheric versus metasomatized lithospheric mantle sources. Journal of Petrology 44, 55–91.

Basu, A. R., W. Junwen, H. Wankang, X. Guanghong, and M. Tatsumoto (1991). Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic volcanic rocks of eastern China: Implications for their origin from suboceanic-type mantle reservoirs. Earth and Planetary Science Letters 105, 149–169.

Boyer, S. E., and D. Elliott (1982). Thrust systems. American Association of Petroleum Geologists Bulletin 66, 1,196–1,230.

Broadbent, T., and Allan Cartography (1994). Maps of the stable continental regions. In The Earthquakes of Continental Regions, vol. 5, ed. A. C. Johnston, K. J. Coppersmith, L. R. Kanter, and C. A. Cornell, appendix G. Report for Electric Power Research Institute (EPRI), Palo Alto, CA. EPRI TR-102261, 15 plates, scales 1:12,500,000 and 1:20,000,000.

Burchfiel, B. C., L. H. Royden, R. D. van der Hilst, B. H. Hager, Z. Chen, R. W. King, C. Li, J. Lu, H. Yao, and E. Kirby (2008). A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People’s Republic of China. GSA Today 18; doi:10.1130/GSATG18A.1.

Burchfiel, B. C., C. Zhiliang, L. Yuping, and L. H. Royden (1995).

Tectonics of the Longmen Shan and adjacent regions, Central China. International Geology Review 37, 661–735.

980 Seismological Research Letters Volume 82, Number 6 November/December 2011

Chang, K. H. (1997). Korean peninsula. In Encyclopedia of European and Asian Regional Geology, ed. E. M. Moores and R. W. Fairbridge, 465–473. London: Chapman and Hall.

Chen, L., C. Cheng, and Z. Wei (2009). Seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western North China craton. Earth and Planetary Science Letters 286, 171–183.

Chinnery, M. A. (1979). Investigations of the Seismological Input to the Safety Design of Nuclear Power Reactors in New England. Nuclear Regulatory Commission Report NUREG/CR–0563, 72 pp.

Coppersmith, K. J. (1994). Introduction to The Earthquakes of Continental Regions, ed. A. C. Johnston, K. J. Coppersmith, L. R. Kanter, and C. A. Cornell, vol. 1, 1:1–1:10. Report for Electric Power Research Institute (EPRI), Palo Alto, CA. Coppersmith, K. J., A. C. Johnston, A. G. Metzger, and W. J. Arabasz (1987). Methods for Assessing Maximum Earthquakes in the Central and Eastern United States. EPRI RP-2556–12, report for Electric Power Research Institute (EPRI), Palo Alto, CA, 312 pp.

Cox, R. T., J. Gordon, S. Forman, T. Brezina, M. Negrau, A. Hill, C. Gardner, and S. Machin (2010). Paleoseismic sand blows in north Louisiana and south Arkansas. Seismological Research Letters 81, 1,032–1,047.

Crone, A. J., and K. V. Luza (1990). Style and timing of Holocene surface faulting on the Meers fault, southwestern Oklahoma. Bulletin of the Geological Society of America 102, 1–17.

Crone, A. J., M. N. Machette, and J. R. Bowman (1997). Episodic nature of earthquake activity in stable continental regions revealed by palaeoseismicity studies of Australian and North American Quaternary faults. Australian Journal of Earth Science 44, 203– 214.

Crone, A. J., M. N. Machette, L. A. Bradley, and S. A. Mahan (1997).

Late Quaternary Surface Faulting on the Cheraw Fault, Southeastern Colorado. USGS Miscellaneous Investigations Map I–2521 (1 sheet), various scales, 7 pp.

Ebel, J. E. (1996). The seventeenth-century seismicity of northeastern North America. Seismological Research Letters 67, 51–68.

Ebel, J. E. (2011). A new analysis of the magnitude of the February 1663 earthquake at Charlevoix, Quebec. Bulletin of the Seismological Society of America 101, 1,024–1,038.

Engdahl, E. R., and A. Villasenor (2002). Global seismicity, 1900– 1999. In International Handbook of Earthquake and Engineering Seismology, Part A, ed. W. H. K. Lee, H. Kanamori, P. C. Jennings, and C. Kisslinger, 665–690. Amsterdam: Academic Press.

Exxon Production Research Company (1985). Tectonic Map of the World.

Twenty-one sheets, scale 1:10,000,000 at the equator. Tulsa, OK: American Association of Petroleum Geologists Foundation.

Feng, M., and M. An (2010). Lithospheric structure of the Chinese mainland determined from joint inversion of regional and teleseismic Rayleigh-wave group velocities. Journal of Geophysical Research 115, B06317; doi:10.1029/2008JB005787.

GEBCO World Map Editorial Board (2006). World Ocean Bathymetry, General Bathymetric Chart of the Oceans (GEBCO). One sheet, scale 1:35,000,000 at the equator; available at http://www.gebco. net/. Last accessed July 21, 2011.

Gradstein, F., J. Ogg, and A. Smith, eds. (2004). A Geologic Time Scale 2004. Cambridge: Cambridge University Press, 589 pp., 1 folded plate.

Hatcher, R. D. Jr., P. H. Osberg, A. A. Drake Jr., P. Robinson, and W. A. Thomas (1990). Tectonic map of the U.S. Appalachians. In The Appalachian-Ouachita Orogen in the United States, ed. R. D. Hatcher Jr., W. A. Thomas, and G. W. Viele, plate 1. Volume F-2 of The Geology of North America. Boulder, CO: Geological Society of America.

Ho, K.-S., J.-C. Chen, C.-H. Lo, and H.-L. Zhao (2003). 40Ar–39Ar dating and geochemical characteristics of late Cenozoic basaltic rocks from the Zhejiang-Fujian region, SE China: Eruption ages, magma evolution and petrogenesis. Chemical Geology 197, 287–318.

Hough, S. E., J. G. Armbruster, L. Seeber, and J. F. Hough (2000). On the modified Mercalli intensities and magnitudes of the 1811–1812 New Madrid earthquakes. Journal of Geophysical Research 105, 23,839–23,864.

Hubbard, J., J. H. Shaw, and Y. Klinger (2010). Structural setting of the 2008 Mw 7.9 Wenchuan, China, earthquake. Bulletin of the Seismological Society of America 100, 2,713–2,735.

Jaques, A. L., and D. H. Green (1980). Anhydrous melting of peridotite at 0–15 kb pressure and the genesis of tholeiitic basalts. Contributions to Mineralogy and Petrology 73, 287–310.

Jia, D., G. Wei, Z. Chen, B. Li, Q. Zeng, and G. Yang (2006). Longmen Shan fold-thrust belt and its relation to the western Sichuan Basin in central China: New insights from hydrocarbon exploration.

American Association of Petroleum Geologists Bulletin 90, 1,425– 1,447.

Johnston, A. C. (1996a). Seismic moment assessment of earthquakes in stable continental regions: I. Instrumental seismicity. Geophysical Journal International 124, 381–414.

Johnston, A. C. (1996b). Seismic moment assessment of earthquakes in stable continental regions: II. Historical seismicity. Geophysical Journal International 125, 639–678.

Johnston, A. C. (1996c). Seismic moment assessment of earthquakes in stable continental regions: III. New Madrid 1811–1812, Charleston 1886 and Lisbon 1755. Geophysical Journal International 126, 314–344.

Johnston, A. C., K. J. Coppersmith, L. R. Kanter, and C. A. Cornell (1994). The Earthquakes of Stable Continental Regions. Five vols.

Report for Electric Power Research Institute (EPRI), Palo Alto, CA, EPRI TR-102261.

Kanter, L. R. (1994). Tectonic interpretation of stable continental crust.

Chapter 2 iin The Earthquakes of Stable Continental Regions, ed. A. C. Johnston, K. J. Coppersmith, L. R. Kanter, and C. A. Cornell, vol. 1, 1–98. Report for Electric Power Research Institute (EPRI), Palo Alto, CA, EPRI TR 102261.

Keller, G. R., and J. D. Hoover (1988). A comparison of the Newark and Rio Grande rift systems and associated magmatism. In TriassicJurassic Rifting, ed. W. Manspeizer, 911–932. New York: Elsevier.

Kirby, E., P. W. Reiners, M. A. Krol, K. X. Whipple, K. V. Hodges, K. A. Farley, W. Tang, and Z. Chen (2002). Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences from 40Ar/39Ar and (U-TH)/He thermochronology. Tectonics 21, 1,001; doi:10.1029/2000TC0012146.

Kwon, S., K. Sajeev, G. Mitra, Y. Park, S. W. Kim, and I.-C. Ryu (2009).

Evidence for Permo-Triassic collision in Far East Asia: The Korean collisional orogen. Earth and Planetary Science Letters 279, 340– 349.

Leloup, P. H., R. Lacassin, P. Tapponnier, U. Scharer, Z. Dalai, L. Xiaohan, Z. Liangshang, J. Shaocheng, and P. T. Trinh (1995). The Ailao Shan–Red River shear zone (Yunnan, China), Tertiary transform boundary of Indochina. Tectonophysics 251, 3–84.

Li, S., W. D. Mooney, and J. Fan (2006). Crustal structure of mainland China from deep seismic sounding data. Tectonophysics 420, 239– 252.

Li, Z.-X., X.-H. Li, H. Zhou, and P. D. Kinny (2002). Grenvillian continental collision in south China: New SHRIMP U-Pb zircon results and implications for the configuration of Rodinia. Geology 30, 163–166.

Liu, J., J. Han, and W. S. Fyfe (2001). Cenozoic episodic volcanism and continental rifting in northeast China and possible link to Japan Sea development as revealed from K-Ar geochronology. Tectonophysics 339, 385–401.

Liu, J. F., J. Chen, J. H. Yin, Y. C. Lu, A. Murray, L. C. Chen, J. Thompson, and H. L. Yang (2010). OSL and AMS14C dating of the penultimate earthquake at the Leigu trench along the Beichuan fault, Longmen Shan, in the northeast margin of the Tibetan Plateau. Bulletin of the Seismological Society of America 100, 2,681–2,688.

Seismological Research Letters Volume 82, Number 6 November/December 2011 981

Liu, M., Y. Yang, S. Zhengkang, S. Wang, W. Min, and W. Yongge (2007). Active tectonics and intracontinental earthquakes in China; the kinematics and geodynamics. In Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues, ed. S. Stein and S. Mazzotti, 299–318. Geological Society of America Special Paper 425. Boulder, CO: Geological Society of America.

Madole, R. F. (1988). Stratigraphic evidence of Holocene faulting in the mid-continent: The Meers fault, southwestern Oklahoma. Bulletin of the Geological Society of America 100, 392–401.

Malik, J. N., A. A. Shah, A. K. Sahoo, B. Puhan, C. Banerjee, D. P. Shinde, N. Juyal, A. K. Singhvi, and S. K. Rath (2010). Active fault, fault growth and segment linkage along the Janauri anticline (frontal foreland fault), NW Himalaya, India. Tectonophysics 483, 327–343.

McCalpin, J. P., ed. (2009). Paleoseismology. Amsterdam: Academic Press, 649 pp.

McCalpin, J. P., and V. S. Khromovskikh (1995). Holocene paleoseismicity of the Tunka fault, Baikal rift, Russia. Tectonics 14, 594–605.

McKenzie, D., and M. J. Bickle (1988). The volume and composition of melt generated by extension of the lithosphere. Journal of Petrology 29, 625–679.

McNulty, W. E., and S. F. Obermeier (1999). Liquefaction evidence for at least two strong Holocene paleo-earthquakes in central and southwestern Illinois, USA. Environmental and Engineering Geoscience 5, 133–146.

Mueller, C. S. (2010). The influence of maximum magnitude on seismichazard estimates in the central and eastern United States. Bulletin of the Seismological Society of America 100, 699–711.

Neumann, E. R., and I. B. Ramberg (1978). Paleorifts: Concluding remarks. In Tectonics and Geophysics of Continental Rifts, ed.

I. B. Ramberg and E. R. Neumann, 409–424. Dordrecht, the Netherlands: D. Reidel Publishing Co. Obermeier, S. F. (1998). Liquefaction evidence for strong earthquakes of Holocene and latest Pleistocene ages in the states of Indiana and Illinois, USA. Engineering Geology 50, 227–254.

Office of Nuclear Regulatory Research (2007). A Performance-based Approach to Define the Site-specific Earthquake Ground Motion.

Nuclear Regulatory Commission Guide 1.208, Nuclear Regulatory Commission, 53 pp.; http://www.nrc.gov/reading-rm/doc-colLast lections/reg-guides/power-reactors/rg/01-208/01-208.pdf. accessed July 21, 2011.

Olafsson, M., and D. H. Eggler (1983). Phase relations of amphibole, amphibole-carbonate, and phlogopite-carbonate peridotite: Petrologic constraints on the asthenosphere. Earth and Planetary Science Letters 64, 305–315.

Peizhen, Z., B. C. Burchfiel, P. Molnar, Z. Weiqi, J. Dechen, D. Qidong, W. Yipeng, L. Royden, and F. Song (1991). Amount and style of late Cenozoic deformation in the Liupan Shan area, Ningxia Autonomous Region, China. Tectonics 10, 1,111–1,129.

Perry, W. J., D. H. Roeder, and D. R. Lageson, comps. (1984). North American Thrust-faulted Terranes: Selected Papers Reprinted from the AAPG Bulletin and other Geological Journals. Tulsa, OK: American Association of Petroleum Geologists, 466 pp.

Petersen, M. D., A. D. Frankel, S. C. Harmsen, C. S. Mueller, K. M. Haller, R. L. Wheeler, R. L. Wesson, Y. Zeng, O. S. Boyd, D. M. Perkins, N. Luco, E. H. Field, C. J. Wills, and K. S. Rukstales (2008). Documentation for the 2008 Update of the United States National Seismic Hazard Maps. USGS Open-File Report 2008– 1128, 119 pp.; http://earthquake.usgs.gov/hazards/products/conterminous/2008/. Last accessed July 21, 2011.

Qiu, Y. M., S. Gao, N. J. McNaughton, D. I. Groves, and W. Ling (2000).

First evidence of >3.2 Ga continental crust in the Yangtze craton of south China and its implications for Archean crustal evolution and Phanerozoic tectonics. Geology 28, 11–14.

Rangin, C., P. Huchon, X. Le Pichon, H. Bellon, C. Lepvrier, D. Roques, N. D. Hoe, and P. V. Quynh (1995). Cenozoic deformation of central and south Vietnam. Tectonophysics 251, 179–196.

Reedman, A. J., and D. H. Kim (1997). Korea—South Korea. In Encyclopedia of European and Asian Regional Geology, ed. E. M. Moores and R. W. Fairbridge, 473–483. London: Chapman and Hall.

Ren, J., K. Tamaki, S. Li, and Z. Junxia (2002). Late Mesozoic and Cenozoic rifting and its dynamic setting in eastern China and adjacent areas. Tectonophysics 344, 175–205.

Rich, J. L. (1934). Mechanics of low-angle overthrust faulting as illustrated by Cumberland Thrust Block, Virginia, Kentucky, and Tennessee. Bulletin of the American Association of Petroleum Geologists 18, 1,584–1,596.

Risk Engineering Inc., Geomatrix Consultants Inc., Woodward-Clyde Consultants, and Cygna Corporation (1986). Seismic Hazard Methodology for the Central and Eastern United States. NP-4726, 11 vols. Report for Seismicity Owners Group and Electric Power Research Institute, Palo Alto, CA. Rodgers, J. (1950). Mechanics of Appalachian folding as illustrated

by Sequatchie anticline, Tennessee and Alabama. American Association of Petroleum Geologists Bulletin 34, 672–681.

Rodgers, J. (1970). The Tectonics of the Appalachians. New York: Wiley, 271 pp.

Russ, D. P. (1979). Late Holocene faulting and earthquake recurrence in the Reelfoot Lake area, northwestern Tennessee. Bulletin of the Geological Society of America 90, 1,013–1,018.

Sengor, A. M. C., and B. A. Natal’in (2001). Rifts of the world. In Mantle Plumes: Their Identification through Time, ed. R. E. Ernst and K. L. Buchan, 389–482. Geological Society of America Special Paper 352. Boulder, CO: Geological Society of America.

Shen, Z.-K., J. Lu, M. Wang, and R. Burgmann (2005). Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau. Journal of Geophysical Research 110, B11409, doi:10.1029/2004JB003421.

Simons, W. J. F., A. Soquet, C. Vigny, B. A. C. Ambrosius, S. H. Abu, C. Promthong, C. Subarya, D. A. Sarsito, S. Matheussen, P. Morgan, and W. Spakman (2007). A decade of GPS in southeast Asia— resolving Sundaland motion and boundaries. Journal of Geophysical Research 112, B06420, doi:10.1029/2005JB003868.

Takahashi, E., and I. Kushiro (1983). Melting of a dry peridotite at high pressures and basalt magma genesis. American Mineralogist 68, 859–879.

Tarr, A. C., A. Villasenor, K. P. Furlong, S. Rhea, and H. M. Benz (2010).

Seismicity of the Earth 1900–2007. USGS Scientific Investigations Map I-3064 (1 sheet), scale 1:25,000,000.

Terman, M. J. (1974). Tectonic map of China and Mongolia. Two sheets, scale 1:5,000,000. Boulder, CO: Geological Society of America.

Tuttle, M. P. (2001). The use of liquefaction featurees in paleoseismology—lessons learned in the New Madrid seismic zone, central United States. Journal of Seismology 5, 361–380.

Tuttle, M. P., H. Al-Shukri, and H. Mahdi (2006). Very large earthquakes centered southwest of the New Madrid seismic zone 5,000– 7,000 years ago. Seismological Research Letters 77, 755–770.

Walker, R. T. (2009). The active tectonics of NE Asia. Abstracts and Programs of the Geological Society of America 41, 658 (abstract).

Walker, R. T., E. Nissen, E. Molor, and A. Bayasgalan (2007).

Reinterpretation of the active faulting in central Mongolia. Geology 35, 759–762.

Wang, H., M. Liu, J. Cao, X. Shen, and G. Zhang (2011). Slip rates and seismic moment deficits on major active faults in mainland China. Journal of Geophysical Research 116, B02405; doi:10.1029/2010JB007821.

Wang, T. K., M.-K. Chen, C.-S. Lee, and K. Xia (2006). Seismic imaging of the transitional crust across the northeastern margin of the South China Sea. Tectonophysics 412, 237–254.

Wheeler, R. L. (1997). Boundary separating the seismically active Reelfoot rift from the sparsely seismic Rough Creek graben, Kentucky and Illinois. Seismological Research Letters 68, 586–598.

982 Seismological Research Letters Volume 82, Number 6 November/December 2011

Wheeler, R. L. (2008). Paleoseismic targets, seismic hazard, and urban areas in the central and eastern United States. Bulletin of the Seismological Society of America 98, 1,572–1,580.

Wheeler, R. L. (2009a). Methods of Mmax Estimation East of the Rocky Mountains. USGS Open–File Report 2009–1018, 43 pp.; http:// pubs.usgs.gov/of/2009/1018. Last accessed July 21, 2011.

Wheeler, R. L. (2009b). Sizes of the largest possible earthquakes in the Central and Eastern United States: Summary of a workshop, September 8—9, 2008, Golden, Colorado, U.S. Geological Survey Open–File Report 2009–1263, 304 pp. (available at http://pubs. usgs.gov/of/2009/1263/, accessed July 21, 2011).

Whitford-Stark, J. L. (1987). A Survey of Cenozoic Volcanism on Mainland Asia. Geological Society of America Special Paper 213.

Boulder, CO: Geological Society of America, 74 pp.

Wilson, M. (1989). Igneous Petrogenesis. London: Chapman and Hall, 466 pp.

Wu, F.-Y., J.-Q. Lin, S. A. Wilde, X. O. Zhang, and J.-H. Yang (2005).

Nature and significance of the Early Cretaceous giant igneous event in eastern China. Earth and Planetary Science Letters 233, 103–119.

Yang, J.-H., S. O’Reilly, R. J. Walker, W. Griffin, F.-Y. Wu, M. Zhang, and N. Pearson (2010). Diachronous decratonization of the SinoKorean craton: Geochemistry of mantle xenoliths from North Korea. Geology 38, 799–802.

Yang, J.-H., F.-Y. Wu, and S. A. Wilde (2008). Mesozoic decratonization of the North China block. Geology 36, 467–470.

Yin, A. (2010). Cenozoic tectonic evolution of Asia: A preliminary synthesis. Tectonophysics 488, 293–325.

Yu, Z., S. Wu, D. Zou, D. Feng, and H. Zhao (2008). Seismic profiles across the middle Tan-Lu fault zone in Laizhou Bay, Bohai Sea, eastern China. Journal of Asian Earth Science 33, 383–394.

Zhang, Z. M., J. G. Liou, and R. G. Coleman (1984). An outline of the plate tectonics of China. Bulletin of the Geological Society of America 95, 295–312.

Zhao, G., S. A. Wilde, and P. A. Cawood (1998). Thermal evolution of Archean basement rocks from the eastern part of the North China craton and its bearing on geologic setting. International Geology Review 40, 706–721.

Zhao, G., S. A. Wilde, P. A. Cawood, and M. Sun (2001). Archean blocks and their boundaries in the North China craton: Lithological, geochemical, structural and P-T path constraints and tectonic evolution. Precambrian Research 107, 45–73.

Zhao, L., R. M. Allen, T. Zheng, and S.-H. Hung (2009). Reactivation of an Archean craton: Constraints from P- and S-wave tomography in North China. Geophysical Research Letters 36, L17306; doi:10.1029/2009GL039781.

Zheng, J., W. L. Griffin, S. Y. O’Reilly, M. Zhang, N. Pearson, and Y. Pan (2006). Widespread Archean basement beneath the Yangtze craton. Geology 34, 417–420.

Zheng, T., L. Chen, L. Zhao, W. Xu, and R. Zhu (2006). Crust-mantle structure difference across the gravity gradient zone in North China craton: Seismic image of the thinned continental crust.

Physics of Earth and Planetary Interiors 159, 43–58.

Zhou, D., K. Ru, and H.-Z. Chen (1995). Kinematics of Cenozoic extension on the South China Sea continental margin and its implications for the tectonic evolution of the region. Tectonophysics 251, 161–177.

Zhu, G., M. Niu, C. Xie, and Y. Wang (2010). Sinistral to normal faulting along the Tan-Lu fault zone: Evidence for geodynamic switching of the east China continental margin. Journal of Geology 118, 277–293.

U.S. Geological Survey MS 966, Box 25046 Lakewood, Colorado 80225 U.S.A.

wheeler@usgs.gov

Seismological Research Letters Volume 82, Number 6 November/December 2011 983

Moment Magnitude (MW) Conversion Relations for Use in Hazard Assessment in Eastern Canada Allison L. Bent

Allison L. Bent

Canadian Hazards Information Service, Geological Survey of Canada

ABSTRACT To be unbiased and uniform across a wide geographical area, seismic hazard assessments based primarily on earthquake recurrence rates require that the same magnitude scale be used for all earthquakes evaluated. Increasingly, moment magnitude, M W , is seen as the magnitude of preference. Moment magnitude, however, was not routinely calculated in the past for earthquakes in Canada, necessitating the conversion from other magnitude types in common use. This step is complicated by the fact that several magnitude scales are routinely reported for Canadian earthquakes with the choice being influenced primarily by geography and to a lesser extent by the size of the earthquake. This paper focuses on eastern Canada, where mN is the most commonly used magnitude scale. Conversions to M W are established and evaluated. The simple conversion of applying a constant is sufficient. However, the conversion is time dependent with the constant changing from 0.41 to 0.53 in the mid-1990s.

INTRODUCTION Magnitude recurrence rates are an important factor in seismic hazard assessment in Canada and elsewhere. The (Canadian) National Earthquake Database (NEDB 2010), hereafter referred to as the NEDB, routinely reports several earthquake magnitude scales for Canadian earthquakes, with mN and ML being the most commonly used for eastern Canada.1 When evaluating magnitude recurrence curves for use in seismic hazard assessment there exists the possibility that a mixed data set will lead to non-uniform or even erroneous results. Thus, it becomes imperative to use the same magnitude scale for all 1. For hazard purposes a conversion equation only for offshore events with M L is required. For these, mN is not appropriate because their S wavetrain lacks Lg energy or the Lg energy is clearly attenuated.

There are at least two other types of M L used in eastern Canada: pre1980 onshore earthquakes for which magnitudes were computed before mN was defined (mNs for most of these back to about 1940 have subsequently been determined from amplitude data but some events remain as M Ls) and small earthquakes up to the present for which there is no amplitude data at a station beyond 50 km (these are not important for seismic hazard).

earthquakes in the data set. Moment magnitude, or M W, has become the preferred magnitude scale as it can be related to the physical properties of the earthquake rupture and does not saturate at high magnitudes. However, this magnitude has not been routinely calculated in the past in Canada and using it for hazard assessment requires that reliable MWs be determined for all earthquakes used in the hazard calculations. This paper focuses on eastern Canada but similar studies have been undertaken to derive M W conversions for western Canada (Ristau et al. 2003, 2005).

Moment magnitude has been determined for many of the largest earthquakes in eastern Canada and for some of the moderate ones. In a recent study Bent (2009) evaluated data for the 150 largest earthquakes that met the completeness criteria for use in hazard assessment in eastern and northern Canada and determined M Ws for each of them. While instrumentally determined M Ws were given preference, conversions from other magnitude types or felt information were sometimes necessary given the long time period covered. Furthermore, it is now almost always possible to determine MW for eastern and northern Canadian earthquakes of magnitude 5.0 or greater and also possible for many of the magnitude 4 to 5 earthquakes.

Therefore, in developing a conversion scale, the emphasis is on the smaller (less than magnitude 5.0) earthquakes.

The approach used in this study is to assemble a database of eastern and northern Canadian earthquakes for which an instrumentally determined mN and M W are available, establish a conversion relation, and evaluate it with respect to several previously published relations based on smaller data sets. The M W data set for offshore regions where M L is the primary magnitude was insufficient to establish a reliable conversion relation.

It may be necessary to employ a two-step conversion from ML to mb and then mb to M W . This conversion relation is under investigation by the author.

A variety of conversions of varying complexity were tested but the author verified that the more complex conversions did not result in a statistically significant improvement over the simple application of a constant, and in some cases did not provide a better absolute fit in terms of mean residual. These new simple conversions were, however, found to be statistically significant improvements over previously published relations.

984â&#x20AC;&#x192; Seismological Research Lettersâ&#x20AC;&#x192; Volume 82, Number 6â&#x20AC;&#x192; November/December 2011

doi: 10.1785/gssrl.82.6.984

A significant outcome of this study is that the mN – M W conversion was found to be time dependent with the constant changing by 0.1 around 1995, which corresponds to a time when the Canadian National Seismograph Network (CNSN) underwent several changes and upgrades and the procedure used for routine earthquake locations and magnitude calculations in eastern Canada was also updated. Further studies are underway to better understand the exact nature of the reason(s) for this change.

mN to MW A data set of earthquakes for which both mN and MW were independently determined from instrumental data was established by searching the NEDB, published literature, and relevant Web sites. Except in a few cases where it was unavailable, the mN value is that found in the NEDB. Note that based on the recommendations of Wetmiller and Drysdale (1982), 1) the Nuttli (1973) formula for distances greater than 4° (444 km) is used for all distances, and 2) the magnitudes are calculated over a wider range of frequencies than that for which the scale was originally defined. The Nuttli magnitude scale was derived for use with World-wide Standard Seismograph Network (WWSSN) instruments. Amplitudes used for magnitude determination in the NEDB are read from the raw waveforms and corrected for instrumental magnification at the period at which they are read.

That is, there is no transfer to a WWSSN instrument response.

However, the instrument responses for the earlier versions of the CNSN were very similar to those of the WWSSN instruments. With the most recent upgrades to the network, that is no longer the case, as the “flat” frequency range has been extended for both the short-period and broadband instruments.

The M Ws used in this study come from a wide variety of sources (noted in Table 1) but the author verified that they were instrumentally determined using well established methods, primarily moment tensor inversion, forward waveform modeling, and spectral analysis. The final data set consists of 154 earthquakes (Table 1, Figure 1). Note that mN 2.5 is the minimum magnitude used in this study as it is generally the lowest magnitude used in hazard calculations for reasons of completeness.

A precursory evaluation of the data showed that for the most part there was a narrow range of MWs for any given mN (Figure 2). There were, however, a few events for which the relation appeared anomalous. On closer inspection it was found that all of these events occurred in the Byam Martin Channel at the northwest extreme of the area of interest. All Byam Martin Channel events (six total) were excluded from further analysis. Therefore, the resulting mN – M W conversion relation may not be applicable to that region.

The simplest conversion relation would be the straightforward application of a constant, which was determined by subtracting M W from mN and calculating the mean value. The best fit for the complete data set is MW = mN – 0.43 with a standard deviation of 0.18. Subdividing the data set into magnitude bins did not result in a significantly different constant for any bin.

In the course of updating the hazard calculations for eastern Canada (J. Adams, personal communication 2006) it was noticed that there was a slight change in the magnitude recurrence curves for the lower magnitudes when the data set was updated by adding events that occurred since 1990 (the added earthquakes were mostly lower magnitudes). Evaluating M W – mN as a function of time suggested that there was a change in the relation around 1995. This date corresponds well to a previously identified date (Bent, unpublished data) when a number of factors that could influence magnitude changed.

These factors include • a decrease in the average period at which magnitude was measured; • an increase in the number of stations used to calculate magnitude; • an increase in the average station-epicenter distance (probably related to station increase); and • an improvement in the precision to which amplitudes and periods were calculated.

All of these are likely linked to improvements and changes made to the CNSN as well as a change in the software used to calculate locations and magnitudes that occurred in the mid-1990s.

In light of the above, the calculations were redone subdividing the data into pre-1995 and post-1995 bins with the 1995 events included in the post-1995 bin. The resulting constants were 0.41 ± 0.18 for the pre-1995 events (116 events) and 0.53 ± 0.19 for the post-1995 group (32 events). The calculations were redone using dividing dates ranging from 1993 to 1998 but the slight differences in the constants were not statistically significant and the exact date at which the change occurred could not be further refined. To help equalize the size of the data sets the calculations were redone using only those events of magnitude 4.0 or greater. The same time dependence was seen. Given that the network changes occurred over a period of several years, there may not be an exact date at which the relation changed. All further calculations use 1995 as the dividing date. Subdividing each group further by magnitude did not affect the results, and all further calculations use data at all magnitudes unless stated otherwise. The data set was sent to another researcher (R. Youngs, written communication 2010), who confirmed that there was a time dependence in the difference; 1997 was his preferred date as it gave the lowest standard deviation but again the date is not precisely constrained.

Least squares straight line fits were also determined for the data, with the results as follows: M W = 0.99mN – 0.36 ± 0.16 (pre-1995) M W = 0.93mN – 0.22 ± 0.19 (post-1995) The F-test was used to compare the fit of the straight line and constant for each group of data. For the pre-1995 group the constant with a mean residual (converted M W vs. true M W) of –0.004 results in a slightly better fit than the straight line with a mean residual of 0.007. The p value from the F-test is 0.601, implying that the difference between the two is not statistically

Seismological Research Letters Volume 82, Number 6 November/December 2011 985

TABLE 1 Earthquakes for mN – MW Conversion

TABLE 1 (continued) Earthquakes for mN – MW Conversion

yyyymmdd Location

mN* MW Source†

yyyymmdd Location

mN* MW Source†

19391019 19660101 19670613 19711002 19720121 19721227 19721121 19721119 19721228 19790627 19790819 19800311 19800403 19810616 19810704 19810713 19810918 19810930 19811028 19820109a 19820109b 19820109c 19820109d 19820111 19820113 19820115 19820117 19820119 19820316 19820402 19820411 19820418 19820506 19820623 19820713 19820728 19820806 19820813 19820903 19821026 19821204 19830117 19830513a 19830513b 19830516 19830529 19830812

5.6 5.3 B09, J94 4.7 4.3 H78 4.5 4.1 H78 5.1 4.7 B09, J94 5.1 4.6 B09, J94 5.4 6.3 B09, J94 5.7 6.0 B09, J94 5.6 5.9 B09, J94 5.1 5.9 B09, J94 5.0 5.0 B09, J94 5.0 4.8 B09, J94 3.7 3.4 Bo94 4.0 3.6 Bo94 3.6 3.2 Bo94 3.8 3.2 Bo94 3.8 3.3 Bo94 3.6 3.0 Bo94 3.4 3.0 Bo94 3.9 3.5 Bo94 5.8§ 5.6 B09, J94 5.0 § 4.9 B09, J94 3.8 3.4 Bo94 3.7 3.3 Bo94 5.5 5.0 B09, J94 4.0 3.2 Bo94 3.8 3.4 Bo94 3.6 3.3 Bo94 4.5 4.5 Bu87 3.5 3.3 Bo94 4.3 3.7 Bo94 4.0 3.5 Bo94 4.1 3.5 Bo94 4.0 3.5 Bo94 3.5 2.9 Bo94 3.8 3.3 Bo94 3.7 3.3 Bo94 3.7 3.1 Bo94 4.3 3.6 Bo94 3.7 3.2 Bo94 3.5 3.1 Bo94 3.9 3.4 Bo94 4.1 3.6 Bo94 3.5 3.1 Bo94 3.9 3.7 Bo94 3.8 3.4 Bo94 4.1 3.7 Bo94 3.5 3.0 Bo94

19831007a 19831007b 19831011 19831117 19831228 19840224 19840411 19840923 19841130 19850303 19850412 19851005 19851019 19860111 19860131 19860806 19860818 19860919 19861109 19870318 19870713 19870806 19870926 19871023 19871111a 19871111b 19880102 19880124 19880128 19880310 19880313 19880424 19880509 19880515 19880809 19880826 19881020 19881123 19881125 19881126 19881126 19881126 19881211 19890119 19890131 19890210 19890309

5.3 4.8 NS89 3.6 3.2 Bo94 4.1 3.6 Bo94 3.7 3.3 Bo94 3.5 2.9 Bo94 3.7 3.3 Bo94 3.8 3.4 Bo94 3.6 3.3 Bo94 3.8 3.4 Bo94 3.1 2.8 Bo94 3.5 3.0 Bo94 4.0 3.5 Bo94 4.1 3.6 Bo94 4.0 3.4 Bo94 5.0 4.6 N88 3.5 3.2 Bo94 3.0 2.7 Bo94 4.2 3.6 Bo94 4.2 3.7 Bo94 3.3 2.8 Bo94 4.1 3.6 Bo94 3.4 2.9 Bo94 3.8 3.3 Bo94 3.7 3.2 Bo94 3.5 3.0 Bo94 3.2 3.0 Bo94 3.6 3.1 Bo94 3.1 2.7 Bo94 3.8 3.6 Bo94 3.7 3.3 Bo94 3.1 2.8 Bo94 3.6 3.3 Bo94 3.5 3.1 Bo94 3.3 3.0 Bo94 3.4 2.8 Bo94 3.8 3.5 Bo94 3.9 3.5 Bo94 4.8 4.3 H96 6.5 5.9 B09, N89 4.1 3.4 H96 2.9 2.5 H96 2.6 2.5 H96 2.8 2.5 H96 3.6 3.3 Bo94 3.1 2.9 Bo94 4.3 3.8 Bo94 4.3 3.8 Bo94

Charlevoix QC Attica NY Attica NY W of Coral Harbor NU Baffin Bay Byam Martin Channel‡ Byam Martin Channel‡ Byam Martin Channel‡ Byam Martin Channel‡ NW of Spence Bay Charlevoix QC St-Basile QC Lwr St Lawrence Charlevoix QC ON-QC-NY border Lwr St. Lawrence Ste-Adele QC Lac-du-Cerf QC Lwr St. Lawrence Miramichi NB Miramichi NB Miramichi NB Miramichi NB Miramichi NB Miramichi NB Miramichi NB Miramichi NB Gaza NH Miramichi NB Miramichi NB Miramichi NB Miramichi NB Miramichi NB SE of Val d’Or QC St-Jovite QC Miramichi NB Western QC N of North Bay ON Western QC Miramichi NB Charlevoix QC Lwr St. Lawrence Miramichi NB Miramichi NB Charlevoix NB Lac-Megantic QC St-Stephen NB

Goodnow NY Goodnow NY Kanata, ON Miramichi NB Western QC Miramichi NB Lwr St. Lawrence Southern NB Miramichi NB Charlevoix QC Maine Miramichi NB Pennsylvania Charlevoix QC Painesville OH Western QC Charlevoix QC Charlevoix QC Lwr St. Lawrence Charlevoix QC Ashtabula OH Charlevoix QC New York Kilmar QC Western QC Western QC Charlevoix QC Charlevoix QC Lwr St. Lawrence Western QC Charlevoix QC Southern NB Miramichi NB S of Ottawa ON Cornwall ON Miramichi NB Northern NH Saguenay QC Saguenay QC Sageunay QC Saguenay QC Saguenay QC Saguenay QC Saguenay QC Charlevoix QC Lwr St. Lawrence Charlevoix QC

986 Seismological Research Letters Volume 82, Number 6 November/December 2011

TABLE 1 (continued) Earthquakes for mN – MW Conversion

TABLE 1 (continued) Earthquakes for mN – MW Conversion

yyyymmdd Location

mN* MW Source†

yyyymmdd Location

19890311 19890316 19890810 19891013 19891104 19891112 19891116 19891122 19891225 19900303 19900313 19900421 19900423 19901007 19901019 19901021 19901218 19901220 19910101 19910131 19910801 19911127 19911208 19930807 19931116 19940116 19940116 19950616 19960314 19960607 19960821 19960924 19970524 19971028 19971106 19971206 19980730 19980925 19990316 20000101 20000420 20010126

4.4 3.6 SA01 5.7 5.0 B09, BH92 3.5 3.3 Bo94 3.2 3.0 Bo94 3.4 2.9 Bo94 3.4 2.9 SA01 4.0 3.6 Bo94 3.4 3.0 Bo94 6.1 6.2 B09, B94 3.6 3.5 Bo94 3.2 2.9 Bo94 3.1 2.9 Bo94 3.0 2.7 Bo94 3.9 3.6 Bo94 5.0 4.5 B09, L94 3.3 3.0 Bo94 3.3 3.1 Bo94 2.7 2.4 H96 2.9 2.5 H96 3.2 2.7 H96 2.6 2.0 H96 2.5 2.2 H96 4.3 3.7 SA01 3.1 2.5 SA01 4.1 3.9 D03 4.0 3.9 D03 4.6 4.6 D03 3.8 3.7 D03 4.4 4.2 BCMT 3.1 2.6 SA01 3.6 3.4 D03 3.1 2.4 SA01 4.2 3.6 D03 4.7 4.3 D03 5.1 4.9 B09, BCMT 5.7 5.0 B09, BCMT 4.4 3.7 D03 5.4 4.5 B09, USGS 5.1 4.5 B09, L04 5.2 4.7 B09, B02 4.0 3.6 D03 4.4 3.9 D03

20010814 20010814 20020420 20020605 20030613 20040628 20040804 20040826 20050306 20051020 20060109 20060225 20061129 20061207 20060407 20061003 20081115 20090321 20090721

Charlevoix QC E coast of Ungava QC Blackville NB Charlevoix NB Western QC Charlevoix QC Western QC Charlevoix QC Ungava QC Charlevoix QC Charlevoix QC Charlevoix QC Charlevoix QC Mont-Laurier QC Mont-Laurier QC Charlevoix QC Charlevoix QC Mont Laurier QC Mont Laurier QC Mont-Laurier QC Mont-Laurier QC Mont-Laurier QC Charlevoix QC Charlevoix QC Napierville QC Reading PA Reading PA Lisbon NH Ste-Agathe QC Charlevoix QC Berlin NH Charlevoix QC Christieville QC Charlevoix QC Quebec City Wager Bay NU La Conception QC OH-PA border Lwr St. Lawrence Kipawa QC Saranac L. NY Ashtabula OH

* †

‡ §

Byam Martin Channel‡ Byam Martin Channel‡ Au Sable Forks NY Radisson QC Charlevoix QC Illinois Lake Ontario Wager Bay NU Charlevoix QC Thornbury QC Huntingdon QC Thurso QC Sudbury ON Cochrane ON Charlevoix QC Maine Charlevoix QC Ivujivik QC Lwr St. Lawrence

mN* MW Source† 5.6 5.2 B09, GCMT 4.9 5.2 GCMT 5.5 5.1 B09, GCMT 4.5 3.7 BCMT 4.2 3.8 BCMT 4.7 4.2 H10 3.8 3.1 K06 5.0 4.3 B09, BCMT 5.4 4.7 B09, BCMT 4.3 3.6 H10 4.2 3.6 BCMT 4.5 3.9 BCMT 4.1 3.7 A08 4.2 3.7 BCMT 4.1 3.8 H10 4.3 3.9 H10 4.2 3.6 H10 4.4 3.8 H10 4.3 3.6 BCMT

mN from NEDB (2010) unless otherwise stated.

Sources for MW A08: Atkinson et al. (2008) B02: Bent et al. (2002) B09: Bent (2009) B94: Bent (1994) BCMT: Bent unpublished CMT BH92: Bent and Hasegawa (1992) Bo94: Boatwright (1994) Bu87: Burger et al. (1987) D03: Du et al. (2003) GCMT: Global CMT Project (2010) H10: Herrmann (2010) H78: Herrmann (1978) H96: Haddon (1996) J94: Johnston et al. (1994) and references therein K06: Kim et al. (2006) L04: Lamontagne et al. (2004) L94: Lamontagne et al. (1994) N88: Nicholson et al. (1988) N89: North et al. (1989) NS89: Nábélek and Suaréz (1989) SA01: Sonley and Atkinson (2001) USGS: United States Geological Survey (2010) Event not used in analysis (see comments in text). mN from Chael (1987).

Seismological Research Letters Volume 82, Number 6 November/December 2011 987

▲▲ Figure 1. Map showing locations of events used to determine the mN – MW conversion relation.

significant. Given that the constant is simpler to apply and provides a marginally better fit, there is clearly no advantage to using a more complicated relation. For the post-1995 group the mean residual is −0.005 for both the straight line and the constant and the p value is 0.922, meaning that there is no advantage to using one over the other. Thus, the constant is retained as the preferred value.

There are several existing M W – mN conversion relations in the published literature. Table 2 shows a comparison of the results from this study with existing conversion relations. It is not surprising that the conversion relations from this study give the lowest residuals, since they were derived from the data set used in the comparison while the others were based on various subsets of it. Additionally, some relations were stated as being valid only for a specific range of magnitudes and therefore might not be expected to provide a good fit for every event in this data set. The principal question, however, is whether the lower residuals of this study represent a statistically significant improvement over the earlier conversion relations. Applying the F-test to all of the relations included in Table 2 gives a p value of 0.000 for both the pre- and post-1995 groups, implying that the differences are statistically significant at a 99.9% or higher level. The worst fitting relations in terms of residuals were removed and the comparison was redone for those models where the absolute value of the mean residual was less than 0.1—the two models from this study as well as Atkinson (1993) and Sonley and Atkinson (2005). For the pre-1995 data

the differences are again significant at the 99.9% level. For the post-1995 data the results are equivocal. The p value of 0.508 indicates that it is about equally likely that the differences are real or due to random chance.

The conversion relations presented here are sufficiently well determined and tested that they may be used for converting large data sets for use in hazard assessments. However, more work needs to be done to better understand the time dependence. Some preliminary tests have been performed to evaluate the likelihood that specific changes made during the 1990s led to the apparent change in magnitude. By rounding the amplitude data of the recent events to the precision of the earlier events and recalculating the magnitudes, the added precision could be ruled out. The other factors (number and distribution of stations, frequency, processing software) could not be ruled out easily and more detailed examination of them is required to determine which is/are the cause(s) and why.

CONCLUSIONS Relations for converting from mN to M W for eastern Canada have been established. The conversion from mN requires simply the subtraction of a constant although there is a time-dependence to that value. Prior to 1995 the value is 0.41 and for earthquakes occurring since 1995 it is 0.53. The cut-off date is not precise, but differences caused by changing the cut-off date by up to two years in either direction were not found to be sta-

988 Seismological Research Letters Volume 82, Number 6 November/December 2011

▲▲ Figure 2. Earthquakes used in the mN – MW conversion. Gray symbols represent the pre-1995 events, black symbols the post-1995 ones, and white symbols the Byam Martin Channel events not used in the analysis. The corresponding diagonal lines show the best fit straight line in a least squares sense to each group. Most dots represent individual earthquakes but some represent two to six earthquakes that lie on the same point.

TABLE 2 Comparison of MW – mN Conversion Relations Pre-1995 Relation Constant (this study) Line (this study) Atkinson (1993) Boore and Atkinson (1987) Johnston et al. (1994) Hasegawa-a(1983) * Hasegawa-b (1983) * Nuttli (1983) Sonley and Atkinson (2005)

Mean Residual –0.004 0.007 –0.061 0.149 0.405 –0.192 –0.391 0.216 –0.088

Post-1995 S. D. 0.16 0.16 0.16 0.22 0.30 0.31 0.24 0.29 0.17

Mean Residual –0.005 –0.005 0.046 0.124 0.301 –0.284 –0.140 0.147 0.048

S.D. 0.19 0.19 0.19 0.21 0.26 0.27 0.27 0.25 0.20

* a refers to relation for mN < 4.2 and b for mN 4.2–6.6.

tistically significant. This date corresponds to known changes in the CNSN and operating system. Further investigation is underway to better understand the underlying reason for the change in the magnitude relation.

It should be cautioned that these relations were established using earthquakes of mN 2.5 or greater and may not be appropriate for smaller magnitudes. While it is also possible that they may not be appropriate for the largest earthquakes, this

factor should not be a major problem as it should always be possible to determine M W directly for these events. Comparisons with instrumentally determined moment magnitudes show that these conversions are very reliable on average when a large data set is considered but will not always give the correct M W for any individual earthquake. While it would be expected that the conversions should hold true for eastern North America in general, it must be emphasized that they are based on magni-

Seismological Research Letters Volume 82, Number 6 November/December 2011 989

tudes calculated by the GSC and should be used with caution for magnitudes calculated by other agencies unless it can be verified that the same formulae and procedures were used. In particular, until the underlying causes of the mN – M W timedependence are better understood, it should not be assumed that this time dependence applies to all data bases for eastern North America.

ACKNOWLEDGMENTS I thank Bob Youngs for providing a second opinion on the time dependence of mN. John Adams, John Cassidy, and Martin Chapman provided constructive reviews. Janet Drysdale verified the pre-1995 magnitude calculation procedure. Natural Resources Canada contribution number 20110070.

REFERENCES Atkinson, G. M. (1993). Earthquake source spectra in eastern North America. Bulletin of the Seismological Society of America 83, 1,778– 1,798.

Atkinson, G. M., S.-L. I. Kaka, D. Eaton, A. Bent, V. Peci, and S. Halchuk (2008). A very close look at a moderate earthquake near Sudbury, Ontario. Seismological Research Letters 79, 119–131.

Bent, A. L. (1994). The 1989 (MS 6.3) Ungava, Quebec earthquake: A complex intraplate event. Bulletin of the Seismological Society of America 84, 1,075–1,088.

Bent, A. L. (2009). A Moment Magnitude Catalog for the 150 Largest Eastern Canadian Earthquakes. Geological Survey of Canada Open-File Report 6080, 23 pp.

Bent, A. L., and H. S. Hasegawa (1992). Earthquakes along the northwestern boundary of the Labrador Sea. Seismological Research Letters 63, 587–602.

Bent, A. L., M. Lamontagne, J. Adams, C. R. D. Woodgold, S. Halchuk, J. Drysdale, R. J. Wetmiller, S. Ma, and J.-B. Dastous (2002). The Kipawa, Quebec “Millennium” earthquake. Seismological Research Letters 73, 285–297.

Boatwright, J. (1994). Regional propagation characteristics and source parameters of earthquakes in eastern North America. Bulletin of the Seismological Society of America 84, 1–15.

Boore, D. M., and G. M. Atkinson (1987). Stochastic prediction of ground motion and spectral response parameters at hard-rock sites in eastern North America. Bulletin of the Seismological Society of America 77, 440–467.

Burger, R. W., P. G. Somerville, J. S. Barker, R. B. Herrmann, and D. V. Helmberger (1987). The effect of crustal structure on strong ground motion attenuation relations in eastern North America. Bulletin of the Seismological Society of America 77, 420–439.

Chael, E. P. (1987). Spectral scaling of earthquakes in the Miramichi region of New Brunswick. Bulletin of the Seismological Society of America 77, 347–365.

Du, W.-X., W.-Y. Kim, and L. R. Sykes (2003). Earthquake source parameters and state of stress for the northeastern United States and southeastern Canada from analysis of regional seismograms.

Bulletin of the Seismological Society of America 93, 1,633–1,648.

Global CMT Project (2010). Online database, http://www.globalcmt. org.

Haddon, R. A. W. (1996). Use of empirical Green’s functions, spectral ratios, and kinematic source models for simulating strong ground motion. Bulletin of the Seismological Society of America 86, 597–615.

Hasegawa, H. S. (1983). Lg spectra of local earthquakes recorded by the Eastern Canada Telemetered Network and spectral scaling.

Bulletin of the Seismological Society of America 73, 1,041–1,061.

Herrmann, R. B. (1978). A seismological study of two Attica, New York earthquakes. Bulletin of the Seismological Society of America 68, 641–651.

Herrmann, R. (2010). Online database, http://www.eas.slu.edu/eqc/ eqc_mt/MECH.NA/.

Johnston, A. C., K. J. Coppersmith, L. R. Kanter, and C. A. Cornell (1994). The Earthquakes of Stable Continental Regions. Assessment of Large Earthquake Potential, TR-102261-V1. Five-volume proprietary report prepared for Electric Power Research Institute, Palo Alto, CA. Kim, W.-Y., S. Dineva, S. Ma, and D. Eaton (2006). The 4 August 2004, Lake Ontario, earthquake. Seismological Research Letters 77, 65–73.

Lamontagne, M., A. L. Bent, C. R. D. Woodgold, S. Ma, and V. Peci (2004). The 16 March 1999 mN 5.1 Côte-Nord earthquake: The largest earthquake ever recorded in the Lower St. Lawrence seismic zone, Canada. Seismological Research Letters 75, 299–316 Lamontagne, M., H. S. Hasegawa, D. A. Forsyth, G. G. R. Buchbinder, and M. Cajka (1994). The Mont-Laurier, Quebec, earthquake of 19 October 1990 and its seismotectonic environment. Bulletin of the Seismological Society of America 84, 1,506–1,522.

Nábélek, J., and G. Suaréz (1989). The 1983 Goodnow earthquake in the central Adirondacks, New York: Rupture of a simple, circular crack.

Bulletin of the Seismological Society of America 79, 1,762–1,778.

National Earthquake Database (2010). Digital database, http://www. seismo.nrcan.gc.ca, Geological Survey of Canada, Ottawa, Ontario.

Nicholson, C., E. Roeloffs, and R. L. Wesson (1988). The northeastern Ohio earthquake of 31 January 1986: Was it induced? Bulletin of the Seismological Society of America 78, 188–217.

North, R. G., R. J. Wetmiller, J. Adams, F. M. Anglin, H. S. Hasegawa, M. Lamontagne, R. DuBerger, L. Seeber, and J. Armbruster (1989). Preliminary results from the November 25, 1988 Saguenay (Quebec) earthquake. Seismological Research Letters 60, 89–93.

Nuttli, O. W. (1973). Seismic wave attenuation and magnitude relations for eastern North America. Journal of Geophysical Research 78, 876–885.

Nuttli, O. W. (1983). Average seismic source-parameter relations for mid-plate earthquakes. Bulletin of the Seismological Society of America 73, 519–535.

Ristau, J., G. Rogers, and J. Cassidy (2003). Moment magnitude calibration for earthquakes off Canada’s west coast. Bulletin of the Seismological Society of America 93, 2,296–2,300.

Ristau, J., G. C. Rogers, and J. F. Cassidy (2005). Moment magnitudelocal magnitude calibration for earthquakes in western Canada.

Bulletin of the Seismological Society of America 95, 1,994–2,000; doi:10.1785/0120050028.

Sonley, E., and G. M. Atkinson (2001). Apparent source spectra for earthquakes in the Charlevoix seismic zone: A comparison of direct and empirical Green’s function methods. Bulletin of the Seismological Society of America 91, 1,729–1,740.

Sonley, E., and G. M. Atkinson (2005). Empirical relationship between moment magnitude and Nuttli magnitude for small-magnitude earthquakes in southeastern Canada. Seismological Research Letters 76, 752–755.

United States Geological Survey (2010). Online database, http://neic. usgs.gov.

Wetmiller, R. J., and J. A. Drysdale (1982). Local magnitude of eastern Canadian earthquakes by an extended mb(Lg) scale. Earthquake Notes 53 (3), 40.

Canadian Hazards Information Service Geological Survey of Canada 7 Observatory Crescent Ottawa, Ontario K1A 0Y3 Canada

990 Seismological Research Letters Volume 82, Number 6 November/December 2011

bent@seismo.nrcan.gc.ca

M

Meeting Calendar C 2011 4 November. Consortium of Organizations for Strong Motion Observation Systems (COSMOS) Annual Meeting and Technical Session, “Recent Major Earthquakes and their Influence on Strong Ground Motion Determinations and Design,” Emeryville, California. www.cosmos-eq.org.

6–7 December. Geotechnical Short Course, Virginia Tech, Blacksburg, Virginia. www.cpe.vt.edu/gee/ 2012 11–13 January. Magmatic Rifting and Active Volcanism Conference, Addis Ababa, Ethiopia. http://www.see.leeds.ac.uk/afar/new-afar/conference/conference.html

22–25 January. 7th Gulf Seismic Forum (GSF 2012), Jeddah, Saudi Arabia. http://7gsf.info/.

28 February−2 March. 10th International Workshop on Seismic Microzoning and Risk Reduction, Tsukuba, Japan. http://www.jaee.gr.jp/event/10IWSMRR/index.html

E

A

E

L

T

E

I

N

N

D

G

A

R

10−14 April. Earthquake Engineering Research Institute Annual Meeting/Federal Emergency Management Agency National Earthquake Conference, Peabody Hotel, Memphis, Tennessee. www.eeri.org

17−19 April. 2012 SSA Annual Meeting, San Diego, California www.seismosoc.org and page 966 for details.

23−27 April. National Earthquake Conference, Memphis, Tennessee www.earthquakeconference.org

13–15 June. Incorporated Research Institutions for Seismology (IRIS) Workshop, Boise, Idaho. www.iris.edu

26−29 June. 45th Rock Mechanics / Geomechanics Symposium, San Francisco, California www.armasymposium.org

5−10 August. 4th International Geological Congress, Brisbane, Australia. www.34igc.org

24 −28 September. 15th World Conference on Earthquake Engineering (15WCEE), Lisbon, Portugal www.15wcee.org

3–4 March. International Symposium One Year after the 2011 Eastern Japan Earthquake, Kenchiku-kaikan Hall, Tokyo.

4–7 November. Geological Society of America Annual Meeting, Charlotte, North Carolina.

kawashima.k.ae@m.titech.ac.jp

www.geosociety.org/meetings/2012/

Please send notices of meetings you would like to appear in the “Meeting Calendar” three months before the expected date of publication. Send announcements to SRL Editor Jonathan M. Lees in care of the SRL managing editor at srl@seismosoc.org.

doi: 10.1785/gssrl .82.6.991

Seismological Research Letters Volume 82, Number 6 November/December 2011 991

992 Seismological Research Letters Volume 82, Number 6 November/December 2011

201 Plaza Professional Bldg. • El Cerrito, CA 94530 Phone: +1-510-525-5474 • Fax: +1-510-525-7204 • http://www.seismosoc.org

MEMBERSHIP APPLICATION Title: ☐ Dr. ☐ Mr. ☐ Ms. ☐ Mrs. ☐ Miss ☐ Fr. Name:__________________________________________________________________ Company/Institution:___________________________________________________ Address:_ ______________________________________________________________ ________________________________________________________________________ Phone: ____________ Fax: ____________ E‑mail: __________________________ SSA publishes two journals: The Bulletin of the Seismological Society of America (BSSA) and Seismological Research Letters (SRL.) Both of these journals are available in electronic form to SSA members. In addition, some membership options include the print version of one or both journals. All members are entitled to meeting and print journal discounts.

SSA Membership for 2012 All memberships include online access to both BSSA and SRL. ____ ☐ Membership with print BSSA and print SRL: $140 in North America, $150 other countries. ____ ☐ Membership with just print SRL: $110 in all countries ☐ Membership with only electronic journals: $100 all countries ____ ____ ☐ Student membership (all electronic, must have current student id): $25 ☐ Student membership (print SRL, must have current student id): $35 ____ ____ ☐ Developing country (all electronic, must reside in country not listed as high-income by the World Bank): $45

Additional for expedited shipping of BSSA outside the United States: $75 ____

Additional for priority shipping of BSSA within the United States: $35 ____

Additional for expedited shipping of SRL outside the United States: $35 ____

Additional for priority shipping of SRL within the United States: $30 ____

Eastern Section Membership $5 ____

Subtotal ____ For applications dated after 1 July, applicants may opt for an 18-month membership (e.g., 1 July 2012 through 31 December 2013). Calculate the additional cost by multiplying the subtotal by 0.5.

____

Total enclosed

____

Payment Options: ☐ Check drawn on U.S. bank in U.S. dollars payable to SSA ☐ International Postal Money Order ☐ Credit Card: ☐ Visa ☐ Mastercard ☐ American Express Account Number _________________________________ Expiration Date ______________________ Signature_ _________________________________________________________________________ Check to receive further information on: ☐ EERI Associate membership ☐ Institutional subscriptions

☐ Back issues of BSSA ☐ Corporate membership ☐ Other: ____________________________

Seismological Research Letters Volume 82, Number 6 November/December 2011 993

Permission to Reproduce Material from SRL The Seismological Society of America (SSA) no longer requires that permission be obtained from SSA or the author(s) for the use of tables, figures, or short extracts of papers published in SSA journals, provided that the source be appropriately and accurately cited. The author’s permission must be obtained for modifying the material in any way beyond simple redrawing, however.

SSA recommends that the credit line read “authors, article title, journal title, volume number, page number(s), year, © Seismological Society of America.”

SSA does require that the article be published by SSA before any material may be reproduced therefrom.

BACK ISSUES AVAILABLE Back issues of volumes 66 to date of SRL are still available. Limited quantities of issues of volumes before 66 are also available. Issues from volumes 66 to 72 are $10 each, except for 68:1 ($25 each) and 68:4 ($20 each). Issues from volumes 73 and 74 are $15 each, except for 73:5 and 74:5, which are $25 each. Issues from volume 75 forward are $20 each. Available issues from volumes before 66 are $15 each, except for issue 58:4, which is $25. Orders should be sent to the Seismological Society of America, 201 Plaza Professional Building, El Cerrito, California 94530. For more information, visit http://www.seismosoc.org/publications/orders/srl-back-issue.php.

RECOMMENDATION FOR MEMBERSHIP SEISMOLOGICAL SOCIETY OF AMERICA I suggest the following for membership in the Society.

Name (please print or type)_ _________________________________________________ Address_ ________________________________________________________________ Name___________________________________________________________________ Address_ ________________________________________________________________ You (may) (may not) use my name in your invitation.

Signed_ _________________________________________________________________ Please print name__________________________________________________________ Address_ ________________________________________________________________ Return this form to: The Seismological Society of America 201 Plaza Professional Bldg. • El Cerrito, CA 94530 Phone +1-510-525-5474; Fax +1-510-525-7204

http://www.seismosoc.org

994 Seismological Research Letters Volume 82, Number 6 November/December 2011

SUBSCRIBE* TO Seismological Research Letters Seismological Research Letters (SRL) is published bimonthly by the Seismological Society of America. SRL is a unique journal— the first to serve as a general forum for informal communication among seismologists, as well as between seismologists and those nonspecialists interested in seismology and related disciplines.

The contents of SRL include contributed articles on topics of broad seismological and earthquake engineering interest, opinion pieces on current seismological topics, news and notes about seismology and seismologists in the U.S. and internationally, special earthquake reports, the Electronic Seismologist column, strong-motion network reports, equipment news, and letters to the editor. A special section in SRL for the Eastern Section of the Seismological Society of America focuses on

eastern North American seismology. Abstracts of papers to be presented at the Annual Meeting of the Seismological Society of America and the annual meeting of the SSA Eastern Section also appear in SRL. The subscription rate for North American institutions is $150*; for countries outside North America the rate is $160*.

Subscription rates include normal shipping; for priority mail delivery within the United States please include $30, outside the United States please include an additional payment of $35.

Note: Subscribers to both SRL and our sister publication Bulletin of the Seismological Society of America (BSSA) are eligible for a combined discounted rate if both are ordered together.

See below for combined rate information.

☐ We wish to receive SRL for 2012 (vol. 83) Subscription within North America $150* per year Subscription outside North America $160* per year Additional for priority mail outside the United States $35 per year $30 per year Additional for priority mail inside the United States ☐ We wish to receive a combined print subscriptions to both BSSA and SRL for 2012 Subscription within North America $690* per year Subscription outside North America $710* per year Additional for priority mail outside the United States $110 per year Additional for priority mail inside the United States $65 per year ☐ We wish to receive an electronic subscription to BSSA and print subscription SRL for 2012 Subscription within North America $665* per year Subscription outside North America $675* per year Additional for priority mail outside the United States $35 per year $30 per year Additional for priority mail inside the United States ☐ We wish to receive only the electronic subscription to BSSA for 2012 Electronic BSSA only subscription $540* per year Total

_____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____

Payment Options: ☐ Check drawn on U.S. bank in U.S. dollars payable to SSA ☐ International Postal Money Order ☐ Credit Card: ☐ Visa ☐ Mastercard ☐ American Express Account Number _________________________________ Expiration Date ______________________ Signature_ _________________________________________________________________________ Name_____________________________________________________________________________ Address_ __________________________________________________________________________ Phone/Fax_________________________________________________________________________

The Seismological Society of America 201 Plaza Professional Bldg. • El Cerrito, CA 94530 Phone +1-510-525-5474; Fax +1-510-525-7204 http://www.seismosoc.org

For institutions and other nonmembers.

Seismological Research Letters Volume 82, Number 6 November/December 2011 995

Last Page

996 Seismological Research Letters Volume 82, Number 6 November/December 2011

SSA OFFICERS Christa von Hillebrandt-Andrade, President William U. (Woody) Savage, Vice-President

Keith L. Knudsen, Secretary Mitchell M. Withers, Treasurer

SSA BOARD OF DIRECTORS Rick Aster (2014) Laurie Baise (2014) Eric Calais (2014) Michael Campillo (2014) Steven M. Day (2012) Robert Graves (2012)

Christa von Hillebrandt-Andrade (2013) Klaus-G. Hinzen (2013) Thomas H. Jordan (2013) Lisa Grant Ludwig (2013) William U. (Woody) Savage (2012) David Wald (2012)

EASTERN SECTION OF SSA BOARD OF DIRECTORS Janet Drysdale, President Martin Chapman, Vice-President Chris Cramer, Secretary

Charles Scharnberger, Treasurer David Eaton, Fifth Member

SSA PUBLICATIONS COMMITTEE Andy Michael (Chair) Roland Burgmann Steven Day David Wald

Ex Officio: Christa von Hillebrandt-Andrade Diane I. Doser Jonathan M. Lees Kim Olsen

CORPORATE MEMBERS, SEISMOLOGICAL SOCIETY OF AMERICA Bechtel Corporation P.O. Box 193965 San Francisco, California 94119 Degenkolb Engineers Natalia Schoeck 350 Sansome Street Suite 900 San Francisco, California 94104 ExxonMobil Upstream Research Company Technical Information Center URC-URC-SW104 PO Box 2189 Houston, Texas 77252-2189 Fugro William Lettis & Associates, Inc. Robin K. McGuire 4155 Darley Avenue, Suite A Boulder, Colorado 30305

Geotech Instruments Attn.: Lani Oncescu 10755 Sanden Dr. Dallas, Texas 75238-1366 Guralp Systems Limited Horst Rademacher 12 Southwood Dr. Orinda, California 94563 Kinemetrics Inc. Ogie Kuraica 222 Vista Avenue Pasadena, California 91107

Nanometrics, Inc. Neil Spriggs 250 Herzberg Road Kanata, Ontario Canada K2K 2A1 Pacific Gas & Electric Co. Lloyd S. Cluff, Geosciences Dept.

245 Market Street, Room 403 San Francisco, California 94105 Puerto Rico Strong Motion Program P.O. Box 9041 Mayagüez, Puerto Rico 00681

Lighthouse R & D Enterprises, Inc. 16945 Northchase Drive Refraction Technology Suite 100 1600 Tenth Street, Suite A Houston, Texas 77060 Plano, Texas 75074

URS Corporation Ivan G. Wong Seismic Hazards Group 1333 Broadway, Suite 800 Oakland, California 94612

Seismological Society of America 201 Plaza Professional Building El Cerrito, CA 94530