Earthquake relocations along the San Andreas Fault provide insight into rupture processes at intermediate depths

English Abstract:

Slip processes on the San Andreas Fault near Parkfield, California vary with depth. Recent observations of low-frequency earthquakes (LFEs) and tectonic tremor at depths of approximately 15 -35 km (Shelly and Hardebeck, 2010) suggest slow-slip earthquakes occur in the zone where the deep fault (> 35 km) transitions to stable sliding. Precise relocations of earthquakes at depths near where vigorous tremor occur would provide key insights into whether LFE and earthquake locations overlap at depth or if, instead, there is a clear depth separation between the two slip processes. Here we use new and existing waveform data and a new P- and S-wave 3D velocity model to determine the precise double-difference relocations of roughly 100 events occurring on the San Andreas Fault near Cholame, California in 2010-2011. The new data used in our study were collected using 13 broadband seismometers installed at distances < 30 km from Cholame, CA during a temporary deployment between May 2010 and July 2011. Initial results show the majority of earthquakes (~80%) occurring at depths < 10 km, but some earthquake locations extend to depths of 18 km. The double-difference relocation calculation incorporates both catalog and cross-correlation time differences, and uses a new high-resolution 3-D velocity model with improved resolution at greater depths based on LFE arrivals. The minimal depth separation between the relocated earthquakes and LFEs (~ 2 km) near Cholame suggests that the change in frictional properties on the fault surface may be gradual in nature.