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How are focal mechanisms determined by seismic data?

How are focal mechanisms determined by seismic data?. Focal mechanisms are visualization models that show: direction of slip in an earthquake attitude of movement along the fault orientation of stress. Fault Plane. Auxiliary Plane. Strike Slip Right Lateral.

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How are focal mechanisms determined by seismic data?

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  1. How are focal mechanisms determined by seismic data? Focal mechanisms arevisualization models that show: • directionof slip in an earthquake • attitude of movementalong the fault • orientationof stress Fault Plane Auxiliary Plane Strike Slip Right Lateral The Great California ShakeOut

  2. Seismologists analyze the first P-waves from the Earth that either compress or dilatethe ground at multiple stations. The Great California ShakeOut

  3. At each seismograph station:First motion P-waves are analyzed to show orientation of stress and slip along the fault. Reverse Fault A B A B A A Lower Hemisphere Projection Areas of compression and dilation are plotted on stereonet projection and the best fit line separating the zones of stress is drawn, creating the “beach ball” diagram. Illustrations by AniPytlewski The Great California ShakeOut

  4. How do geologists determine the Fault Plane ? • Aftershocks • Seismic history • Field evidence for strike, dip, and rake of the fault (correlate to focal mechanism) • Ray Path geometry Additional field data is needed to differentiate between the auxiliary and fault plane. The Great California ShakeOut

  5. References Cronin, V., 2004, A draft primer on focal mechanism solutions for geologists, Baylor University, p. 1-14 Johnson, Jenda. “Focal mechanism.” Incorporated Research Institutions for Seismology. 2013. Web. 18 June 2013. <http://www.iris.edu/hq/programs/education_and_outreach/ animations/25 >. Lillie, R. J., 1998, Earthquake seismology, Whole earth geophysics: An introductory textbook for geologists & geophysicists, p. 185-220 Rowan, Chris. “5 focal mechanisms.” 29 December 2009. Web. 18 June 2013. <http://all-geo.org/highlyallochthonous/2009/12/5-focal-mechanisms/>. The Great California ShakeOut

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