HR: 14:10h
AN: T23E-03    [Abstracts]
TI: Shear Velocity Anisotropy in and Near the San Andreas Fault: Implications for Mapping Stress Orientations
AU: * Boness, N L
EM: nboness@stanford.edu
AF: Stanford University, Department of Geophysics, Mitchell Building, Stanford, CA 94305 United States
AU: Zoback, M D
EM: zoback@stanford.edu
AF: Stanford University, Department of Geophysics, Mitchell Building, Stanford, CA 94305 United States
AB: We utilize a suite of geophysical logs from the SAFOD boreholes, earthquake data recorded on the SAFOD Pilot Hole array and regional seismic data from the Northern California Seismic Network (NCSN) and the Southern California Seismic Network (SCSN) to study the physical processes controlling shear velocity anisotropy in and near the San Andreas fault. Because the direction of maximum horizontal compression is at a high angle to the predominantly northwest-southeast structural trend, it is relatively easy to distinguish stress-induced from structurally-induced velocity anisotropy near the San Andreas fault. Dipole sonic logs in the SAFOD boreholes indicate that the shear-wave velocity anisotropy of the granitic rocks surrounding the wellbore is on the order of 3 to 10% and controlled by the tectonic stress field. The amount of stress-induced velocity in the granite decreases with depth because as confining pressure increases, the seismic velocity becomes less stress sensitive to stress. Within the sedimentary section found at greater depth (and closer to the San Andreas fault), both stress-induced and structurally-controlled velocity anisotropy is observed in the dipole sonic logs. At depth intervals where finely laminated shales are present, cross-dipole sonic logs indicate that structurally-induced anisotropy is dominant. However, in the well-cemented arkosic sandstones (which are not well bedded), the fast direction of the shear waves is controlled by the stress field (as in the granite) and is not consistent with theoretical models of structural anisotropy. These data provide further constraints on the orientation of the maximum horizontal compressive stress at depth. As one approaches the San Andreas fault along the trajectory of the SAFOD main hole, the direction of maximum stress rotates from approximately North-South (at relatively shallow depth) to become more fault-normal at an angle of about 80 degrees to the strike of the fault within the fault zone at seismogenic depths. This observation supports the hypothesis that the San Andreas fault is a weak fault slipping at low levels of shear stress. An analysis of earthquake seismograms shows that ray paths through the Salinian granite adjacent to the fault exhibit fast shear wave polarizations aligned with the direction of maximum horizontal compression (in agreement with pilot hole stress measurements). In contrast, ray paths along the San Andreas fault (or through fault-parallel sedimentary structures) yield fast directions consistent with the northwest-southeast structural trend. An analysis of shear waves from local crustal earthquakes recorded at regional seismic stations in California shows a similar signature, indicating that seismic anisotropy may be useful in mapping the crustal stress field. We conclude that within the San Andreas fault zone, the structural fabric is the dominant mechanism responsible for velocity anisotropy, whereas in crust without predominant structural trend, the direction of maximum horizontal compression is the most important controlling factor.
UR: http://www.earthscope.org
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8150 Plate boundary: general (3040)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005