Physical properties and multi-scale seismic anisotropy in the crust surrounding the San Andreas Fault near Parkfield, California

In this thesis I present my contribution to understanding the physical nature of the crust in and near the San Andreas Fault by studying the physical processes controlling shear velocity anisotropy at a variety of scales.; To study shear velocity anisotropy at the scale of the brittle crust (∼15km), I study crustal earthquakes occurring beneath high quality three-component seismic stations throughout western California. Seismic stations located away from major faults exhibit fast shear polarizations aligned with the direction of maximum horizontal stress, SHmax. In contrast, seismic stations located along major faults show that the structural fabric of the fault zone controls velocity anisotropy. With knowledge of the seismic wavelengths, I deduce there is a zone ∼200-500 m wide of anomalous physical properties.; At a more local scale (∼3 km), dipole sonic logs in the SAFOD (San Andreas Fault Observatory at Depth) boreholes located 1.8 km to the southwest of the San Andreas Fault, indicate stress-induced shear velocity anisotropy in granitic rocks and unbedded sandstones. In bedded shales, structurally-controlled anisotropy is dominant. To substantiate this interpretation, I developed a theoretical model of structural anisotropy that may be applied to an arbitrarily oriented borehole in a transversely isotropic formation.; At the finest scale investigated in this thesis (< 1 km) petrophysical data acquired in SAFOD indicates the deformation is concentrated in a clay-rich fault core about 20 m wide with unique physical properties, embedded within a ∼250 m wide damage zone. Stress-induced anisotropy indicates that the maximum horizontal compressive stress, SHmax, rotates from being approximately fault-normal to approximately north-south within the fault core as predicted by the model of Rice [1992].; At all scales, the orientation of SHmax a regional scale is found to be at a high angle to the San Andreas Fault trace. The evidence presented in this thesis supports the hypothesis that the San Andreas Fault is slipping at low levels of resolved shear stress.