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Department of Earth SciencesKFUPM
Introduction to Seismology
Fault Mechanism
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Ali [email protected].
sa
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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