Chapter 11
Earthquakes Dr. Irina Goll Natural Science Department, Blinn College, Bryan Campus, TX Friday, April 9, 2010
1
What is an earthquake?
An earthquake (EQ) is the vibration of Earth produced by the rapid release of energy released radiates in all directions from its source, the focus Energy is in the form of waves Energy
Friday, April 9, 2010
2
Energy from an earthquake radiates in all directions much like waves created by a pebble
Friday, April 9, 2010
3
Earthquake’s glossary
Terms Focus - the
place within Earth where earthquake waves originate
Epicenter – location
on the surface
directly above the focus
Friday, April 9, 2010
4
There are several reasons which causes an earthquake: • volcanic eruption • nuclear test • collapses of empty oil pool
Friday, April 9, 2010
However the majority of EQs are connected to existing faults in
5
What is an earthquake?
Earthquakes (EQ) and faults Faults
are large fractures in Earth’s crust Earthquake is a result of movements between crustal blocks along fault surface Most of the motion along faults can be explained by the plate tectonics theory
Friday, April 9, 2010
http://videos.howstuffworks.com/ howstuffworks/236-how-earthquakes6
Mechanism of an earthquake
Elastic rebound (Reid) Mechanism
for earthquakes was first explained by H.F. Reid Rocks
on both sides of an existing fault are deformed by tectonic forces The forces are pushing the blocks of rocks into opposite directions Rocks bend and store elastic energy When frictional resistance holding the rocks together is overcome Friday, April 9, 2010
7
Mechanism of an earthquake
Elastic rebound
Slippage happens at the weakest point (the
focus)
Vibrations (earthquakes) occur as the
deformed rock “springs back” to its original shape (elastic rebound)
Friday, April 9, 2010
8
Friday, April 9, 2010
9
Offset produced by the 1906 San Francisco earthquake
Figure 11.4 Friday, April 9, 2010
10
Transform Fault San Andreas Fault: an Active Earthquake Zone
Friday, April 9, 2010
11
San Andreas: An active earthquake zone
San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long Process described as strike-slip motion Some portions exhibit slow, gradual displacement known as fault creep Other segments regularly slip producing small earthquakes
Friday, April 9, 2010
12
San Andreas: An active earthquake zone
Displacements along the San Andreas fault There are segments which store elastic energy for hundreds of years before rupturing in great earthquakes Great earthquakes should occur about every 50 to 200 years along these sections
http://videos.howstuffworks.com/hsw/9216-investigatingearthquakes-earths-moving-crust-video.htm
Friday, April 9, 2010
13
What is an earthquake?
Foreshocks and aftershocks earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Small
Friday, April 9, 2010
14
Seismology
The study of earthquake waves, seismology, dates back almost 2000 years to the Chinese Seismographs, instruments that record seismic waves Principle idea of seismograph is based on inertia Inertia – is defined as a phenomenon when objects at rest tend to remain at rest, and objects in motion tend to stay in motion
Friday, April 9, 2010
15
Oldest Seismograph (Made in China)
Friday, April 9, 2010
16
Modern Seismograph (made in ď Ž
Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
Friday, April 9, 2010
17
Seismology ď Ž
Seismographs ď Ž More
than one type of seismograph is needed to record both vertical and horizontal ground motion
Friday, April 9, 2010
18
A horizontal ground motion seismograph
Friday, April 9, 2010
19
A vertical ground motion seismograph
Figure 11.8 Friday, April 9, 2010
20
ANIMATION How a seismograph works? ď Ž
http://www.wwnorton.com/college/ geo/egeo/flash/8_3.swf
Friday, April 9, 2010
21
REVIEW questions
What is an earthquake? How are faults, foci and epicenters related? Who was the first to explain the actual mechanism by which earthquakes are generated? Explain what is meant by elastic rebound? What is the principle idea of seismograph?
Friday, April 9, 2010
22
Seismology ď Ž
Records obtained are called seismograms
Friday, April 9, 2010
Seismogram from Sumatra Earthquake
23
Seismogram showing P, S, and L or surface waves
Friday, April 9, 2010
24
Types of seismic waves
Body waves Travel
through Earth’s interior Two types based on mode of travel Primary (P) waves Push-pull (compress and expand) motion, changing the volume of the intervening material Travel through solids, liquids, and gases
Friday, April 9, 2010
25
Body waves: Primary (P) waves
Push-pull (compress and expand) motion Friday, April 9, 2010
26
Types of seismic waves
Body waves Secondary
(S) waves “Shake” motion at right angles to their direction of travel Travel only through solids Slower velocity than P waves Slightly greater amplitude than P waves
Friday, April 9, 2010
27
Body waves: Secondary (S) waves
“Shake� motion at right angles to their direction of travel Friday, April 9, 2010
28
Types of seismic waves ď Ž
Body waves in any solid material, P waves travel about 1.7 times faster than S waves
ď Ž Generally,
Seismic wave motion http://www.wwnorton.com/college/geo/egeo/ flash/8_2.swf Friday, April 9, 2010
29
Types of seismic waves
Surface waves (L waves) Travel
along outer part of Earth Complex motion Cause greatest destruction Exhibit greatest amplitude and slowest velocity Waves have the greatest periods (time interval between crests) Often referred to as long waves, or L waves Friday, April 9, 2010
30
Friday, April 9, 2010
31
Friday, April 9, 2010
32
Locating the source of earthquakes ď Ž Three station recordings are needed to locate an epicenter
station determines the time interval between the arrival of the first P wave and the first S wave at their location ď Ž The larger S - P interval the larger the distance from seismic station to the epicenter of an earthquake ď Ž Each
Friday, April 9, 2010
33
Three seismograms from 3 seismic stations P
S 3 S - P interval ~ 4 min
P
S 2
S - P interval ~ 5 min
P
S 1
S - P interval ~ 6 min Friday, April 9, 2010
Seismogra m 1 has the largest S–P interval & therefore the distance from Station #1 to the epicenter of the earthquake 34
How to connect the S minus P interval with a distance to EQ focus?
A travel-time graph is used to determine each station’s distance to the epicenter The graph links An Interval between the time when the first Swave arrived minus first P-wave arrived and A Distance from the seismic station to the epicenter through the calculated curve.
Friday, April 9, 2010
35
A travel-time graph
Friday, April 9, 2010
36
A travel-time graph
Friday, April 9, 2010
36
Locating the source of earthquakes
Locating the epicenter of an earthquake The
center of a circle is placed at the location of the seismic station The radius of a circle drawn around each station is equal to the distance from this particular station to the epicenter This should be done for all 3 stations The point where all three circles intersect is the earthquake epicenter Friday, April 9, 2010
37
Finding an earthquake epicenter
EPICENTER
Friday, April 9, 2010
38
REVIEW questions
What types of waves do we call a body waves? Describe the P-wave? (speed, mode of travel, materials it can travel through)
Describe the S-wave (speed, mode of travel, materials it can travel through)
Which type of seismic wave causes the greatest destruction to buildings? How many stations do you need in order to locate a focus of an
Friday, April 9, 2010
39
Earthquake belts
Earthquake belts About
95 percent of the energy released by earthquakes originates in a few relatively narrow zones that wind around the globe Major earthquake zones include Circum-Pacific belt, Mediterranean Sea region Himalayan complex Oceanic ridge system
Friday, April 9, 2010
40
Location of Shallow, Intermediate and Deep Earthquakes
Friday, April 9, 2010
41
Locating the source of earthquakes
Earthquake depths Earthquakes
originate at depths ranging from 5 to nearly 700 kilometers Earthquake foci arbitrarily classified as Shallow (surface to 70 kilometers) Intermediate (between 70 and 300 kilometers) Deep (over 300 kilometers)
Friday, April 9, 2010
42
Locating the source of earthquakes
Earthquake depths Definite
patterns exist
Shallow
focus occur along the oceanic ridge system Almost all deep-focus earthquakes occur in the circum-Pacific belt, particularly in regions situated landward of deep-ocean trenches
Friday, April 9, 2010
43
Friday, April 9, 2010
44
Measuring the size of earthquakes
Two measurements that describe the size of an earthquake are Intensity
– a measure of the degree of earthquake shaking at a given locale based on the amount of damage Magnitude – estimates the amount of energy released at the source of the earthquake
Friday, April 9, 2010
45
Measuring the size of earthquakes
Intensity scales Mercalli Intensity Scale was developed using California buildings as its standard The drawback of intensity scales is that destruction may not be a true measure of the earthquake’s actual severity Modified
Friday, April 9, 2010
46
Measuring the size of earthquakes
Magnitude scales Richter
magnitude - concept introduced by Charles Richter in 1935 Richter scale Based on the amplitude of the largest seismic wave recorded Accounts for the decrease in wave amplitude with increased distance
Increasing distance Friday, April 9, 2010
47
Measuring the size of earthquakes
Magnitude scales Richter scale Largest
magnitude recorded on a WoodAnderson seismograph was 9.5 Magnitudes less than 2.0 are not felt by humans Each unit of Richter magnitude increase corresponds to a 10-fold increase in wave amplitude and a 32-fold energy increase
Friday, April 9, 2010
48
Friday, April 9, 2010
49
Friday, April 9, 2010
50
Earthquake destruction
Destruction from seismic vibrations Liquefaction of the ground Unconsolidated
materials saturated with water turn into a mobile fluid
Friday, April 9, 2010
51
Sandboils created by Liquefaction
Friday, April 9, 2010
52
Effects of Liquefaction building rests on unconsolidated sediment that
Friday, April 9, 2010
53
Earthquake destruction
Seiches The
rhythmic sloshing of water in lakes, reservoirs, and enclosed basins Waves can weaken reservoir walls and cause destruction www.lakesuperiorstore.com/resources/ glinfo.htm
Friday, April 9, 2010
54
Earthquake destruction
Tsunamis, or seismic sea waves Destructive waves that are often inappropriately called “tidal waves” Result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake
Friday, April 9, 2010
55
Earthquake destruction
Tsunamis, or seismic sea waves In
the open ocean height is usually < 1 meter In shallower coastal waters the water piles up to heights that occasionally exceed 30 meters Can be very destructive
Friday, April 9, 2010
56
Formation of a tsunami
Figure 11.20
Friday, April 9, 2010
57
Tsunami
http://www.pep.bc.ca/tsunamis/ causes_2.htm
http://www.pbs.org/wnet/ savageearth/animations/tsunami/ index.html
Friday, April 9, 2010
58
Earthquake destruction
Landslides Ground subsidence Fire
Ways to prevent a destruction caused by earthquake:
http://www.physorg.com/news154888577.html
Friday, April 9, 2010
59
Can Earthquake be predicted?
Short range Predictions Goal
– warning of a location and magnitude of a large earthquake within a narrow time frame Research has concentrated on monitoring possible precursors – phenomena that precede and thus provide warning of a forthcoming earthquake
Friday, April 9, 2010
60
Can Earthquake be predicted?
Short range Predictions Monitoring
changes in groundwater
levels Electrical conductivity of rocks Animal behavior
Currently no reliable method exists
Friday, April 9, 2010
61
Can Earthquake be predicted?
Long range Predictions Paleoseismology Looking
for the cycles Calculating probabilities
Friday, April 9, 2010
62
REVIEW questions
Shallow-focus EQ are associated with _______ and _________ plate boundaries? Deep-focus EQ occur several hundred km bellow what prominent features on the deep-ocean floor? Distinguish between :
Mercalli Scale Richter Scale
Friday, April 9, 2010
63