HR: 15:25h
AN: T53C-08    [Abstracts]
TI: Variations of the Velocity Contrast and Rupture Properties of M6 Earthquakes Along the Parkfield Section of the San Andreas Fault
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
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: Zhao, P
EM: pzhao@gatech.edu
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, United States
AU: Shi, Z
EM: zheqians@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089, United States
AU: Lewis, M
EM: malewis@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089, United States
AB: We perform a comprehensive high-resolution imaging of bimaterial interfaces along the Parkfield section of the San Andreas Fault (SAF), based on analysis of fault zone head waves (FZHW) that refract along bimaterial fault interfaces. The employed seismic data are generated by 8993 earthquakes since 1984, and recorded by the NCSN and HRSN permanent seismic networks along with two temporary PASSCAL deployments: the 2001-2002 PASO and the 2004 Parkfield Guided Waves experiment. We stack waveforms of events in repeating earthquake clusters to increase the signal-to-noise ratio and confidence level of FZHW identification. Next we align the peak or 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. Clear FZHW are observed for many stations on the NE (slow) side of the fault in the creeping section of the SAF north of Middle Mountain (MM), indicating a presence of a sharp bimaterial interface in that region with variable values of velocity contrasts. The obtained velocity contrasts are 5-10% north of MM, and systematically decrease towards Gold Hill (GH). No clear head waves are observed at stations on the NE side of the SAF for ray paths sampling the fault around GH, indicating an absence or reversal of the velocity contrast in this region. The obtained along-strike variations of velocity contrasts are consistent with geological observations of a sliver of high-velocity rock immediately to the NE of the SAF associated with the GH fault and 3D seismic tomography results. The existence of a local reversal of velocity contrast near GH offers a simple explanation for the opposite propagation directions of the M6 1966 and 2004 Parkfield earthquakes, and could also partially explain the apparent segmentation of the M6 events. The 1966 earthquake nucleated near MM and propagated to the SE, as expected for rupture on the bimaterial fault interface in that region. The local reversal of the velocity contrast near GH may have prevented the rupture from propagating further to the SE. On the other hand, the 2004 earthquake nucleated near GH and propagated to the NE, again as expected for rupture on a bimaterial interface. The rupture stopped at MM where the preferred rupture direction is to the SE.
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 7250 Transform faults
SC: Tectonophysics [T]
MN: 2007 Fall Meeting