HR: 1340h
AN: S13D-1077    [Abstracts]
TI: Low-Velocity Damaged Structure on the San Andreas Fault at Seismogenic Depths near the SAFOD Drilling Site, Parkfield, CA from Fault-Zone Trapped Waves
AU: * Li, Y
EM: ygli@usc.edu
AF: University of Southern California, University Park, Los Angeles, CA 90089 United States
AU: Cochran, E S
EM: cochran@moho.ess.ucla.edu
AF: University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095 United States
AU: Vidale, J E
EM: vidale@moho.ess.ucla.edu
AF: University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095 United States
AB: We deployed dense linear arrays of PASSCAL REFTEK130s across and along the San Andreas Fault near the SAFOD drilling site at Parkfield to record fault-zone trapped waves in the fall of 2003. We acquired the data for 114 local earthquakes with M0-2.3 including 2 SAFOD target events (located by Thurber and Roecker; Nadeau), five USGS explosions in a fan geometry, and 47 small shots (Hole; Catchings and Rymer). Prominent fault-zone trapped waves with large amplitudes and long duration at 2-5 Hz were observed for events within or close to the fault zone. The data from ~25 on-fault events with the raypath incidence angles smaller than 10 degrees from vertical show that the time duration of the dominant trapped wave energy increases from ~0.8 s to ~2.0 s as the event depth increases from ~2.5 km to ~10 km progressively. These observations show the existence of a low-velocity waveguide on the SAF that likely extends to seismogenic depths in this region, although the velocity reduction in the deeper part of fault zone becomes smaller due to the larger confined stress at greater depths. This is consistent with the low-velocity waveguide in the rupture zone of the 1992 M7.4 Landers earthquake, which has been mapped across seismogenic depths [Li et al., 1994; 2000]. 3-D finite-difference waveform simulations of the data delineate the internal damaged structure of the SAF near SAFOD drilling site using a depth-dependent model with the low-velocity waveguide, which is 150 m wide at the top and narrower at 10 km depth, and has shear velocities reduced by 30-45% from wall-rock velocities and Q of 10-50. The model parameters are consistent with those obtained on the SAF near Parkfield in previous studied using fault-zone trapped waves [Li et al., 1990; 1997; 2004], showing that the low-velocity zone extends along the SAF strike from the rupture of the 1966 M6 earthquake to the NW creeping segment. We interpret that the trapped wave inferred low-velocity waveguide on the SAF is a remnant of repeated damage due to M6 earthquakes and other large historical earthquakes on the principal slip plane at Parkfield. We also find some seismic energy trapped in the Buzzard Canyon fault that is approximately 1 km SW to the SAF main trace and dips toward the NE to connect to the SAF at a shallow depth. Among many other methods, fault-zone trapped waves can be used to document fine-scale damaged fault zones with high-resolution.
DE: 6982 Tomography and imaging
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting