Tectonics_Lecture-5

Seismology Measuring the interior

Seismology Group IIT Kharagpur

Introduction Basic Concepts: Earthquakes (Passive Source)

Generates Seismic Waves

Propagate away from source and samples the Earth structure Free Surface ground motions caused by these propagating waves Æ recorded at surface detectors (SEISMOMETERS)

Recorded ground motion is SEISMOGRAM

SEISMOGRAM = Source * medium of propagation * Receiver (Earth Filter) response (Observation) elastic waves Origin Time Location & Nature of Source

&

Arrival Time

Travel Time Phase & Amplitude

Seismology Group IIT Kharagpur

Speed of waves in the medium

SEISMOGRAM = Source * medium of propagation * Receiver (Earth Filter) response (Observation) elastic waves Our goal is to understand the way in which the features of the observed seismogram are related to the properties of the source and structure of the Earth • Little of the Earth is accessible by direct observations Deepest Well ~ 13Kms deep (Russia) • Seismology is a primary (powerful) tool for exploring the Earth’s interior.

• Provides information on Æ Variations of Velocity Æ Density Æ Attenuation with depth in the Earth.

• Existence of the Crust, Mantle, Liquid Outer Core and Solid Inner Core are inferred from the variations in velocity with depth, • The idea of variations in chemical composition within the Earth is also based on Seismological Data.

DEEP STRUCTURE

Compositional Structure of the Earth Average 35 km (continent) 7-8 km (ocean)

John Milne Lord Rayleigh Lord Rutherford

Andrya Mohorovicic (Moho) 1909 Mantel or ‘coat’ by Emil Wiechert (in German) -Discovered by R.D. Oldham 1906.

- Correctly delineated by Beno Gutenberg in 1912 from earthquake data.

-Fluid Outer Core (1926) From works on tides by Sir Harold Jeffreys.

Crust (rich in silica)

Mantle

Derived from the mantle over the aeons by series of melting & reworking

Outer Core

Inner Core 1929 Buller earthquake south Island of New Zealand.

Inge Lehmann 1936 – solid Inner Core within the liquid Outer Core.

DEEP STRUCTURE

Mechanical Layering of the Earth Continental crust Oceanic crust

D’’ Layer

Combined View (A comparison)

Distribution of seismic velocities and density within the Earth

No S-wave

SHALLOW STRUCTURE Seismic Reflection profiling

• Detail crustal images that reveal information about location of economic resources (oil and minerals)

Seismology is the primary method for studies of Earthquakes e.g. nature of Faulting determined from resulting seismograms

Radiation Pattern Fault Plane solutions giving fault plane geometry using P wave polarity

Earthquake Distribution and Tectonic Plate Most Earthquakes result from motion of plates and are distributed along the plate margins.

Jigsaw of Plates

Seismometers Earthquakes (Passive Source)

Generates Seismic Waves

Propagate away from source and samples the Earth structure Free Surface ground motions caused by these propagating waves Æ recorded at surface detectors (SEISMOMETERS)

Recorded ground motion is SEISMOGRAM

Seismometers Pendulum Seismograph

Range of ground motion (in dB) and the period of ground motion spanned by the broad-band seismic system of IRIS-GDSN compared to the WWSSN instruments.

Inertial-Pendulum Vertical and Horizontal Seismographs * Damped Harmonic Oscillators * Natural Frequency of vibration is given by : fo=1/2Ď€(k/m)1/2

Damping dash pot

Inertial Mass Seismology Group IIT Kharagpur

Seismograph Response

z(t) = Z(ω)eiωt u(t) = U(ω)eiωt Frequency Response Function Æ

H(ω)= Z(ω)/U(ω)

Response Function Curves The Frequency response function can be written as: H(ω) = ω2/(ω02 – 2εiω - ω2) Where, ω = 2πf ε is the Damping parameter.

The strength of the damping relative to the stiffness of the spring is described by: h = ε /ω0 Æ Damping Constant

Frequency of signal Natural Frequency

Theoretically the absence of damping (h=0) results in an infinite response at resonance (ω0) Critical damping: h=1 (No Oscillatory character of the Response) Optimum damping is applied at h=0.707 (Flat Frequency Response)

Modern Seismographs: Force Balance Feedback Principle Ground Vibration

Motion in the Coil/Magnet

From EM

Strength of current Proportional to the Ground Velocity

Induces a voltage proportional to the motion

Generates a Feedback Current to restore the mass

Three-Component Broadband Seismometers Z V

Back Azimuth Æ OH

dZ

tan φ = dE / dN

d i O dE

dN

φ

i

Incidence Angle Æ i H

E

N

tan i = OH / OV = [(dE2 + dN2)1/2 / dZ]

From Incorporated Research Institutions in Seismology (IRIS) Seismology Group IIT Kharagpur

Indian Network

Types of Installations and Seismometers

Seismology Group IIT Kharagpur

Seismographs and Accelerographs

Weak motion

Medium motion

Strong motion

CMG-3T CMG-3TB CMG-3ESP CMG-3ESP Compact

CMG-6T CMG-6TD CMG-40T

CMG-5T CMG-5TB CMG-5TD CMG-5U

Seismology Group IIT Kharagpur

Digitizers and data modules CMG-DM16R8 CMG-DM24S6 CMG-DM24S3 CMG-DCM CMG-AM

Basic Station Setup Timing & Location

GPS

Transmit via Telemetric Link

Data Storage Digitizer

Sensor

Seismometer Setup Data Download and Servicing Seismology Group IIT Kharagpur

Seismic Waves

Earthquakes (Passive Source)

Generates Seismic Waves

Propagate away from source and samples the Earth structure Free Surface ground motions caused by these propagating waves Æ recorded at surface detectors (SEISMOMETERS)

Recorded ground motion is SEISMOGRAM

Seismic Body Waves P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel. V = Ö [(K + 4/3m )/r ]

Vp=[(Κ+4/3μ)/ρ]1/2

S-Waves - Secondary waves, also called shear waves. They travel with a velocity that depends only on the rigidity and density of the material through which they travel: V = Ö [( m )/r ]

Vs=[μ/ρ]1/2

Seismic Surface Waves Rayleigh Waves Æ Named after Lord Rayleigh. Rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.

Love Waves Æ Named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It's the fastest surface wave and moves the ground from side-to-side.

Local and Regional Observations Broad-band seismogram of the North Ridge earthquake recorded in Southern California Sg Pg SVD – 133 km

Sg Pg Pn NEE – 368 km

Teleseismic Observation

Broad-band seismogram of the North Ridge earthquake recorded at HRV in Harvard, Massachusetts, about 5000 km epicentral distance.

Global Record Section: 1994 North Ridge, California Earthquake and corresponding propagation paths

Seismic Phases Whole Earth Phase Direct Arrivals and Shadow Zones

Seismic Phases

Seismic Phases

(a) Seismograms and (b) Ray path for a deep focus earthquake beneath Tonga recorded on an array of seismographs in central Australia

Earthquake location determined from the arrival times of various seismic phases

Measurements made from a seismic body-wave arrival