Anisotropy in the Shallow Crust Observed around the San Andreas Fault Before and After the 2004 M 6.0 Parkfield Earthquake

This is the authors' abstract. We don't add key points for this paper.

Local seismic arrays were deployed at two locations along the San Andreas fault (SAF) near Parkfield, California, before and after the 2004 M 6.0 Parkfield earthquake. Using local earthquakes we determine the anisotropic field within 1– 2 km of the main trace of the SAF at the two array locations separated by 12 km. The initial array, near the SAFOD site, was deployed for six weeks in October and November 2003, and the second array, located near the town of Parkfield, was deployed for 3 months following the 28 September 2004 M 6.0 Parkfield earthquake. We find the fast shear-wave polarization direction nearly fault-parallel (N40�W) for stations on the main fault trace and within 100 m to the southwest of the SAF at both array locations. These fault-parallel measurements span the 100- to 150-m-wide zone of pervasive cracking and damage interpreted from fault-zone-trapped waves associated with the main fault core (Li et al., 2004, 2006). Outside of this zone, the fast orientations are scattered with some preference for orientations near N10�E, roughly parallel to the regional maximum horizontal compressive stress direction (r h). In addition, fast directions are preferentially oriented parallel to a northern