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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
    • Initial P-wave Radiation Pattern
    • 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
  • 6. 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. 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