Detailed Study of the 2004 Mw 6 Parkfield Earthquake Sequence Using a New Relocated Microearthquake Catalog

We present an updated earthquake catalog for the Parkfield section of the San Andreas Fault, California, spanning from December 2003 to March 2005. The new catalog was compiled using a matched filter detection technique. We use the waveforms of more than 10,000 earthquakes from high-quality location catalogs recorded at the High-Resolution Seismic Network (HRSN). For a more robust detection we included earthquakes from adjacent regions in the template catalog such as aftershocks from the 2003 M6.5 San Simeon earthquake and removed associated detections from the final catalog. Detection was performed on filtered data between 2-8 Hz and with a detection threshold of 12 times the Median Absolute Deviation to the daily cross-correlation functions. We relocated the detection catalog using the XCORLOC package performing source-specific station term and waveform cross-correlation differential time location to constrain both the absolute and relative locations. Magnitudes of the new detections were estimated comparing the L2 norms with the template events. The new catalog with 3 times more events than the local template catalog has an estimated magnitude of completeness of 0, which compares to the template catalog magnitude of completeness of 1. Using the new detailed catalog, we investigate the spatiotemporal evolution of the microearthquakes. Aftershocks expand along-strike for the first 12 days after the 2004 M6 Parkfield mainshock with log-time, consistent with previous inferences that aftershocks are driven by propagating afterslip. A b-value decrease can be identified in the months before the mainshock, suggesting a preparation mechanism. However, no clear foreshocks were detected in the last 24 hours before the M6 mainshock. Additional results will be presented at the meeting.