1. Imaging Bimaterial Interface of the Parkfield section of the San Andreas Fault Using Fault Zone Head Waves Peng Zhao, Georgia Tech Dr. Zhigang Peng, Georgia Tech

2. Outline • Bimaterial interface along a fault structure • Introduction of fault zone head waves • Results of the Parkfield section of the San Andreas fault (SAF) • Conclusions

3. Franciscan Complex (Sedimentary rock, slow) Salinian Block (Granite, Fast) Parkfield section of the SAF Major faults such as the San Andreas fault (SAF) juxtapose different types of rocks, resulting in well defined fault interface.

4. Earthquake rupture on bimaterial interfaces has preferred rupture direction, which is the slip direction of the slower side (Andrews and Ben-Zion, 1997). • Earthquake rupture directivity leads to significant ground shakings in the forward direction. • Better predictions of the rupture propagation directions is extremely useful for seismic hazard mitigation Simulations for earthquakes on southern SAF with different rupture propagation directions http://visservices.sdsc.edu/projects/scec/terashake/compare/

5. Fault Zone Head Waves (Ben-Zion, 1989) fault zone head waves are generated along a bimaterial interface fault zone head waves have opposite polarities with the direct P waves

6. Slow Fast

8. The average velocity contrasts can be estimated from

9. Summary of velocity contrast along the Parkfield section of the San Andreas fault Or reversal of velocity contrast?

10. Comparison with previous studies and implications for earthquake rupture directions (Eberhart-Phillips and Michael, 1993)

11. Conclusion • Fault zone head waves provide a useful tool for detecting the existence of bimaterial interface and measuring the velocity contrasts at seismogenic depth • We found clear variations of velocity contrast along the Parkfield section of the San Andreas fault • Our results may have significant implications for predicting the rupture directions of large earthquakes.

12. Thank You !

13. Stacking Processes • Enhance the signal and suppress incoherent noise • Increase the confidence level of phase picking • Reduce the number of traces

17. Identification of Repeating Clusters • Compute the inter-event distances along and perpendicular to the fault strike. • Estimate the source radius for each event from its magnitude, with a circular crack model. • Include an event into a cluster if it is within the circular rupture patch of all other events in that cluster.

18. (Eberhart-Phillips and Michael, 1993)