# Earthquake Fault Mechanism

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• 1. Department of Earth Sciences KFUPM Introduction to Seismology Fault Mechanism Introduction to Seismology-KFUPM Ali Oncel [email_address]
• 2. Previous Lecture
• Faults and Their Types
• Initial Ground Motions for P-waves
• Case Work: Fault interpretation
Introduction to Seismology-KFUPM
• 3. 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.
Introduction to Seismology-KFUPM
• 4. 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. Introduction to Seismology-KFUPM
• 5. Faulting Stresses
• Intermediate Vertical Stress- Strike-Slip Faulting.
• Compressional stresses
• Largest vertical stress - Normal Faulting
• Extension from Tensional stresses.
Smallest vertical stress - Reverse Faulting/ Compressional stresses σ max σ min Introduction to Seismology-KFUPM
• 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. Introduction to Seismology-KFUPM Azimuth
• 7. First Motions Introduction to Seismology-KFUPM
• 8. Example: 2004 Sumatra EQ Source: http://iisee.kenken.go.jp/staff/yagi/eq/Sumatra2004/Sumatra2004.html Introduction to Seismology-KFUPM
• 9. What types of Earthquake Software?
• Based on Waveform Modeling : Teleseismic Body-Wave Inversion Program from:
• http://www.eri.u- tokyo.ac.jp/ETAL/KIKUCHI/index.html
• Based on First Motion Polarities (FOCMEC):
• http://www.geol.vt.edu/outreach/vtso/focmec/
Introduction to Seismology-KFUPM
• 10. 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 week
• Most seismic data is free and available over the internet in near real time
Global Seismic Network Introduction to Seismology-KFUPM
• 11. 1. visit to IRIS PAGE from http://www.iris.edu/seismon/ Introduction to Seismology-KFUPM
• 12. 2. Then, click http://www.iris.edu/seismon/ Introduction to Seismology-KFUPM
• 13.
• Later, click http://www.iris.edu/seismon/last30days.phtml/
• And click the event you want to get data
Introduction to Seismology-KFUPM
• 14. 4. Now, the station list is given where data is available. Introduction to Seismology-KFUPM
• 15.
• Select names of those station you want to have data
Introduction to Seismology-KFUPM
• 16.
• Then, just click the proceed under the same page.
Introduction to Seismology-KFUPM
• 17.
• Now, prefer the data for any component to plot
Introduction to Seismology-KFUPM
• 18. Let’s come back to discuss earthquake fault mechanism b d Introduction to Seismology-KFUPM
• 19. 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. Introduction to Seismology-KFUPM
• 20. 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 Introduction to Seismology-KFUPM
• 21. Cross sections of Dip-Slip Faults C= Compression D= 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) Introduction to Seismology-KFUPM
• 22. Introduction to Seismology-KFUPM A=? B1=? B2=? C=?
• 23. Introduction to Seismology-KFUPM