HR: 09:00h
AN: T11F-05    [Abstracts]
TI: Multi-Scale Crustal Seismic Anisotropy in the Region Surrounding the San Andreas Fault Near Parkfield, CA.
AU: * Boness, N L
EM: nboness@stanford.edu
AF: Stanford University, Department of Geophysics, Mitchell Building, Stanford University, Stanford, CA 94305 United States
AU: Zoback, M D
EM: zoback@stanford.edu
AF: Stanford University, Department of Geophysics, Mitchell Building, Stanford University, Stanford, CA 94305 United States
AB: The region surrounding the San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, CA is an ideal location to study the effect of crustal structure and the state of stress on seismic velocity anisotropy because the direction of maximum horizontal compression is at a high angle to the predominantly northwest-southeast structural trend. Data from the 2.2-km-deep pilot hole and upper section of the main SAFOD borehole provides a unique opportunity for studying the in-situ physical properties of the crust adjacent to the San Andreas Fault Zone. To study seismic anisotropy in the crust at multiple scales, we utilize a suite of geophysical logs from the SAFOD boreholes, in addition to earthquake data recorded on the Pilot Hole array and on the regional High Resolution Seismic Network (HRSN) operated by U.C. Berkeley. At the smallest scale, dipole sonic logs in the SAFOD boreholes indicate that the shear-wave velocity anisotropy of the rocks within a few feet of the wellbore is on the order of 3 to 10% and controlled by the tectonic stress field. An analysis of earthquake seismograms shows that ray paths through the crust adjacent to the fault exhibit fast shear wave polarizations aligned with the direction of maximum horizontal compression, in agreement with the SAFOD measurements, whereas ray paths along the San Andreas fault yield fault-parallel fast directions. The delay times of the lagging shear wave are also much larger when the waves have traveled along the fault zone indicating that structural fabric has a stronger influence on velocity anisotropy than the regional stress field. We conclude that within the San Andreas Fault Zone, the structural fabric is the dominant mechanism responsible for velocity anisotropy whereas in the surrounding crust, the direction of maximum horizontal compression is the most important controlling factor.
DE: 8015 Local crustal structure
DE: 8164 Stresses--crust and lithosphere
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting