HR: 0800h
AN: T51C-0680    [Abstracts]
TI: The Velocity Contrast Across the Parkfield Section of the San Andreas Fault Near the SAFOD Drill Site
AU: * Lewis, M A
EM: malewis@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 University Parkway, ZHS 117, Los Angeles, CA 90089-0740, United States
AU: Ben-Zion, Y
EM: benzion@usc.edu
AF: Department of Earth Sciences, University of Southern California, 3651 University Parkway, ZHS 117, Los Angeles, CA 90089-0740, United States
AU: Peng, Z
EM: zpeng.seismo@gmail.com
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, ES&T Building, Rm. 2256 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 University Parkway, ZHS 117, Los Angeles, CA 90089-0740, United States
AU: Zhao, P
EM: peng.zhao.gatech@gmail.com
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, ES&T Building, Rm. 2256 311 Ferst Drive, Atlanta, GA 30332, United States
AB: We investigate the existence and strength of velocity contrasts in the Parkfield region of the San Andreas fault (SAF), focusing on the 15 km to the NW and SE of the SAFOD drill site. The research is based on observations of fault zone head waves (FZHW) that refract along an interface separating materials with different seismic velocities. When the fault separates contrasting materials the FZHW provide the best diagnostic and imaging tool of the properties of the fault interface. The study is part of a larger project on imaging bimaterial interfaces in the Parkfield region using data of multiple seismic networks. Here we examine waveforms from the temporary PASO deployment and nearby stations from the permanent High Resolution Seismic Network (HRSN) and Northern California Seismic Network (NCSN) over the time intervals July-December 2001 and April-August 2002. A total of approximately 500 events are located and recorded by 70 stations that are within 5 km of the SAF strike. We obtained accurate picks of head and direct P wave arrival times, with FZHW identified at 26 stations on the slow (NE) side of the fault. For stations on the slow block that are about 1-5 km from the fault there is a continuous head wave propagation for approximately 30 km along strike, from the creeping section NE of the SAFOD site to approximately 15 km SE of Middle Mountain. This encompasses the entire study region and implies a velocity contrast that is geometrically coherent throughout at least this portion of the fault. Stations within a few hundred meters of the fault show head waves only from events to the NW in the creeping section of the SAF. These stations are likely affected by small-scale structural complexities associated with the fault, such as damage zones, multiple fault branches or step-over. The average values of the velocity contrast, estimated from the arrival time moveouts between the head and direct P waves at the different stations, fall in the range of ~3-8%.
DE: 7203 Body waves
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: 2007 Fall Meeting