Earthquake Fault Mechanism

Department of Earth SciencesKFUPM

Introduction to Seismology

Fault Mechanism

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Ali Onceloncel@kfupm.edu.

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Previous Lecture

•Faults and Their Types•Initial Ground Motions for P-waves •Initial P-wave Radiation Pattern •Case Work: Fault interpretation

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Review: Earthquake Fault Mechanism

The radiation patterns of P-waves are used to construct a graphical representation of earthquake faulting geometry.

The symbols are called “Focal Mechanisms” or “Beach Balls”, and they contain information on the fault orientation and the direction of slip.

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Review: Earthquake Fault Mechanism The type of faulting may

be identified (reverse, normal, strike-slip) but the particular fault plane cannot be determined. So they both are shown as the possible solutions.

The three stresses: two horizontal plus the vertical.

Why is the significance to understand the style of faulting?

Because, they are also telling us about the stresses acting within Earth.

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Faulting Stresses

Intermediate Vertical Stress- Strike-Slip Faulting.Compressional stresses

Largest vertical stress - Normal FaultingExtension from Tensional stresses.

Smallest vertical stress - Reverse Faulting/ Compressional stresses

σmax

σmin

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Radiation Patterns

In seismology we call the direction a receiver is from a source the azimuth:

source

receiver

North

The azimuth is always measured clockwise from North and varies between 0 and 360 degrees.

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Azimuth

First Motions

source

receiver

North

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Example: 2004 Sumatra EQ

Source: http://iisee.kenken.go.jp/staff/yagi/eq/Sumatra2004/Sumatra2004.htmlIn

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What types of Earthquake Software?

1. Based on Waveform Modeling: Teleseismic Body-Wave Inversion Program from:

http://www.eri.u- tokyo.ac.jp/ETAL/KIKUCHI/index.html

2. Based on First Motion Polarities (FOCMEC): http://www.geol.vt.edu/outreach/vtso/focmec/

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What are the sources of Seismic Data?

Over 16,000 seismometers are permanently deployed around the world

Seismometers are “on” 24 hrs, 7 days a weekMost seismic data is free and available over the

internet in near real time

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Netw

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1. visit to IRIS PAGE from http://www.iris.edu/seismon/In

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2. Then, click http://www.iris.edu/seismon/Intr

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3.Later, click http://www.iris.edu/seismon/last30days.phtml/

And click the event you want to get dataIntr

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4. Now, the station list is given where data is available.In

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5. Select names of those station you want to have dataIn

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6. Then, just click the proceed under the same page.Intr

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7. Now, prefer the data for any component to plot

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Let’s come back to discuss earthquake fault mechanism

b d

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Cross section of reverse fault, earthquake focus and quadrants of compression and dilatation.

First motions observed at the surface reveal patterns of compression and dilatation.

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Focal mechanism solution for a fault is commonly a lower focal sphere projection.

For a dip-slip fault, this projection is equivalent to the compression/dilatation pattern viewed by a bird flying over the earthquake focus.

Black = Compression

White = Dilatation

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Cross sections of Dip-Slip Faults

C= CompressionD= Dilatation

The focal mechanism solution for a reverse fault (a) has a compression (black) in the inside portion of the circle, surrounded by regions of dilatation (white)

The opposite pattern is observed for a normal fault (b)

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A=?

B1=? B2=?

C=?

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