Temporal changes in near-surface layers and deep fault zone scatterers after the 2004 Mw6.0 Parkfield earthquake observed by the UPSAR

We systematically investigate temporal changes in the near-surface layers and deep fault zone scatterers after the 2004 Mw6.0 Parkfield earthquake using many repeating aftershocks recorded by the USGS Parkfield Dense Seismograph Array (UPSAR). The UPSAR is located about 10 km SW of the Parkfield segment of the San Andreas Fault, and consists of 14 stations within an area of about 1 km2. After aligning the P arrival recorded at each station, a sliding window waveform cross-correlation technique is used to measure the travel time differences in each cluster. We find clear delays in the S and S coda arrivals for earthquakes immediately after the mainshock relative to those at later times. The amplitudes of the time delays drop logarithmically with time since the mainshock, indicating a recovery of material strength over time. The time delays and decorrelation index show significant variations within each station in the UPSAR for the same set of repeating clusters. We also find a positive correlation between the average time delays and the peak ground accelerations (PGA) recorded during the 2004 Parkfield mainshock. These results suggest that the observed temporal changes in the UPSAR are probably induced by the dynamic shaking of the Parkfield mainshock and mostly confined in the near-surface layers beneath each station. We suggest that variations in PGA and average time delays could be mainly caused by topographic effects, because they are the only obvious differences among these stations. We also calculate the semblance coefficient as a function of time for several clusters. Our preliminary results show a few isolated picks in the semblance coefficient that are well correlated among the repeating events. Such picks could be related to scattered wavefields that are generated by deep fault zone scatterers. Our next step is to use semblance analysis to measure temporal changes associated with those deep fault zone scatterers. Updated results will be presented at the meeting.