Mapping stress and structurally controlled crustal shear velocity anisotropy in California

We present shear velocity anisotropy data from crustal earthquakes in California and demonstrate that it is often possible to discriminate structural anisotropy (polarization of the shear waves along the fabric of major active faults) from stress-induced anisotropy (polarization parallel to the maximum horizontal compressive stress). Stress directions from seismic stations located near (but not on) the San Andreas fault indicate that the maximum horizontal compressive stress is at a high angle to the strike of the fault. In contrast, seismic stations located directly on one of the major faults indicate that shear deformation has significantly altered the elastic properties of the crust, inducing shear-wave polarizations parallel to the fault plane.