HR: 1340h
AN: T13A-1344    [Abstracts]
TI: Depth-Dependent Low-Velocity Structure of the San Andreas Fault near the SAFOD Drilling Site at Parkfield from Fault-Zone Seismic Waves
AU: Alvarez, M
EM: alvarez@passcal.nmt.edu
AF: PASSCAL Instrument Center, New Mexico Institute of Mining and Technology, 100 East Road, Socorrp, NM 87801 United States
AU: * Li, Y
EM: ygli@usc.edu
AF: University of Southern California, University Park, Los Angeles, CA 90089 United States
AU: Vidale, J
EM: vidale@ucla.edu
AF: University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095 United States
AU: Cochran, E
AF: University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095 United States
AB: Coordinated by the SAFOD PIs, we used 96 PASSCAL short-period three-component seismometers in linear arrays deployed across and along the San Andreas fault (SAF) near the town of Parkfield and the SAFOD drilling site in 2002 and 2003, respectively. The data recorded for near-surface explosions detonated in the experiments (Li and Vidale), PASO project (Thurber and Roecker) and refraction profiling (Hole), and local earthquakes show fault-zone trapped waves clearly for the source and receivers located close to the fault. The time duration of the dominant trapped energy after S-arrivals increases with the event-to-array distance and focal depth progressively. Using a finite-difference code, we first synthesize fault-zone trapped waves generated by explosions to determine the shallowest 1 or 2 km fault zone structure with the velocity constraints from seismic profiling of the shallow SAF at Parkfield [Catchings et al., 2002]. We then strip shallow effects to resolve deeper structure of the fault zone, and synthesize trapped waves from earthquakes at depths between 2.5 and 11 km to complete a model of the SAF with depth-variable structure in 3-D. We also use the P-first arrivals and polarity as additional information in modeling of velocities and location of the material interface with the structural constraints from seismic tomography at Parkfield [Thurber et al., 2004] to the bed-rock velocities. In grid-search modeling, we tested various values for fault zone depth, width, velocity, Q, and source location. The best-fit model parameters from this study show evidence of a damaged core zone on the main SAF, which likely extends to seismogenic depths. The zone is marked by a low-velocity waveguide ~150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-45% from wall-rock velocities. We also find some seismic energy trapped partitioned in the branching faults that connect to the San Andreas main fault at a shallow depth near Parkfield.
DE: 7230 Seismicity and seismotectonics
DE: 8100 TECTONOPHYSICS
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1242)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting