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

2. Seismologists analyze the first P-waves from the Earth that either compress or dilate the ground at multiple stations. First MotionGround Movement CompressionUP DilationDOWN NullNo apparent motion 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 ABA AA B Illustrations by Ani Pytlewski 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. 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.. 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.. The Great California ShakeOut