Variations Of Velocity Contrast Along The Rupture Zone Of The 2004 M6 Parkfield Earthquake On The San Andreas Fault

We systematically investigate the velocity contrast along the Parkfield section of the San Andreas Fault (SAF) that ruptured during the 2004 M6 Parkfield earthquake, using fault zone head waves (FZHW) that refract along the bimaterial interface. The analysis employs a total of 322 repeating earthquakes clusters identified from 8993 earthquakes in the relocated catalog of Thurber et al. (2006). The seismic data are recorded by 13 borehole stations in the High Resolution Seismic Network (HRSN) since 1987 and 23 surface stations in the Northern California Seismic Network (NCSN) since 1984, with normal distances to the fault less than 6 km. The study is part of a larger project on imaging bimaterial interfaces in the Parkfield region with multiple seismic networks. We stack waveforms of each repeating earthquake cluster, and align the peaks or troughs of the direct P waves assuming right-lateral strike-slip focal mechanisms. Clear FZHW are observed at surface and borehole stations that are within a few kms on the NE (slow) side of the SAF. The obtained velocity contrast is about 8% north of Middle Mountain, and decreases rapidly toward Gold Hill near the epicenter of the 2004 event. This implies an abrupt change of velocity contrast along the Parkfield section of SAF near Gold Hill. The observed variation of velocity contrast is consistent with 3-D tomography models of the Parkfield section, which include a high velocity body near Gold Hill on the NE side that produces a local reversal of the velocity contrast, and geological observations of a sliver of high-velocity rock immediately to the NE of the SAF associated with the Gold Hill fault.