Seismic Waves Seismic Waves and and

Earthquake LocationEarthquake Location

Walter D. MooneyUS Geological Survey

CaliforniaMooney @ usgs.gov

Wave propagation animation

Seismogram showing different phases

SEISMIC WAVE VELOCITYSEISMIC WAVE VELOCITY

Body Waves

P-wave

S-wave

Surface Waves

Love Wave V S1 < VL< V S2

Rayleigh Wave VR < 0.92 VS

2

1

2

pV

2

1

Vs

Vp ~ √3 Vs.

Conversion of Seismic Phases at Rock BoundariesConversion of Seismic Phases at Rock Boundaries

Sini

Sinr

V

V2

1

2

P*

PnPg

Seismogram showing converted phases

Magnetic Tape Record Analog Record

Guralp CMG-3T

Teledyne GS-13

Streckeisen STS-2

High frequency

seismometers

Vault of an observatory

IDA MK8 DAS

Another popular model

Major Discontinuities of the Earth

Teleseismic Analog record

Digital Record of the 2004 Sumatra Earthquake Mw 9.3

T-phase of the Largest Aftershock of the 2004 Sumatra Earthquake Recorded at the VISK Station, India

Travel Time Curves

EARTHQUAKE LOCATION EARTHQUAKE LOCATION

Manual Method for Location

Single-station Location

Multi-station Location

Computational Methods for Location

Computer Programs for Location

HYPO 71

SEISAN

Manual Method

C

SEISMIC STATION

A

* EPICENTRE

D

B

EPICENTRE

HYPOCENTRE

H

*

CHAPTER 3 FIG. 28 a

X SEISMOGRAPHSTATION

D Ts Tp 8

Ts Tp DVs Vp

1 1

Seismogram

Computational Method

Input data: P & S Arrival Data

Station Coordinates

Station Correction

P-wave Velocity Model

Vp/Vs

Maximum Amplitude

Signal Duration

Geiger's (1912) method of least squares Technique

Computation :

Ri = i - t i RMS

N

ii

N

R

2

1

1

STATION CORRECTIONSTATION CORRECTION

3-Component Seismograms of an aftershock of the 2004 Sumatra Earthquake

Wadati Plot

Vp/Vs = 1.73

CHAPTER 3 FIG. 34

A

CHAPTER 3 FIG. 35

B

(a) Earthquake occurs in a Circular Array

(b) Earthqauke occurs outside the Array

Note: At least one S-phase is required to control the Hypocentre location

Limitations in Precise Location:

EARTHQUAKE RELOCATION

Master Event Method

Average Residual Method

Homogeneous Station Method

Double-Difference (DD) Method

Joint Hypocentre Method

rij = tij – (ij + Si)

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ik

obsjk

ik

ijk ttttdr )()(

THANK YOU

Fremantle, Perth