HR: 0800h
AN: S21A-0237    [Abstracts]
TI: Velocity Contrast along the Hayward Fault From Analysis of Fault Zone Head Waves
AU: * Ohlendorf, S
EM: summer_joi@berkeley.edu
AF: Berkeley Seismological Lab, University of California, Berkeley, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760, United States
AU: Peng, Z
EM: zpeng@gatech.edu
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, United States
AU: Ben-Zion, Y
EM: benzion@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089, United States
AB: The Hayward fault is a major branch of the San Andreas system in northern California. It juxtaposes the Franciscan Complex (fast) to the SW side against the Great Valley Sequence (slow) to the NE side. Previous studies based on 3D seismic tomography suggested ~5-10% seismic velocity contrast in the upper 10 km, consistent with geological observations. Here we systematically investigate the velocity contrast along the entire Hayward fault using fault zone head waves (FZHW) that refract along the fault interface. The FZHW provide the most diagnostic seismic signal for the existence of sharp bimaterial interfaces, and the highest-resolution tool for imaging their seismic properties. A total of 10,952 earthquakes recorded by the Northern California Seismic Network (NCSN) between January 1984 and June 2007 are used in the study. We perform waveform cross correlation for all possible event pairs, and group them into similar event clusters if they are located within 3 km and have a median cross-correlation coefficient of at least 0.85. A total of 250 clusters have been identified between 20 km north and 70 km south of Point Pinole along the Hayward fault strike. The waveforms generated by each event clusters are stacked for each station within 10 km on the slower (NE) side of the fault. Next we align the peak and trough of the direct P waves assuming right-lateral strike-slip focal mechanisms, pick the FZHW arrivals, and plot the waveforms against the along-fault-interface distances. The results at many stations on the slow side of the fault show clear propagation of FZHW from most event clusters, implying a velocity contrast that is geometrically coherent along the entire 90 km of the Hayward fault. The strength of the velocity contrast varies somewhat along strike and with depth. For stations in the central portion, the travel time moveout between the FZHW and direct P waves increases continuously with distance over ~75 km, whereas for stations closer to the ends the moveout is continuous only over distances of ~30 km. The moveout analysis indicates average velocity contrasts of ~5-11%, with higher values for the central region and the upper 6 km. The FZHW do not show clear breaks between the Hayward and Mission faults, suggesting a continuous transition between these two fault zones. The existence of a coherent bimaterial interface in the structure of the Hayward fault can have significant implications for properties of earthquake ruptures on this fault.
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 7250 Transform faults
SC: Seismology [S]
MN: 2007 Fall Meeting