HR: 0800h
AN: T21A-0448    [Abstracts]
TI: Seismic Evidence for Rock Damage and Healing on the San Andreas Fault Associated with the 2004 M6 Parkfield Earthquake
AU: * Li, Y
EM: ygli@usc.edu
AF: University of Southern California, Department of Earth Sciences Science Building, Los Angeles, CA 90089 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
AU: Chen, P
T21A-0448 AF: University of Southern California, Department of Earth Sciences Science Building, Los Angeles, CA 90089 United States
AU: Cochran, E S
T21A-0448 AF: University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095 United States
AU: Burdette, T
T21A-0448 AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AB: We deployed a dense seismic array of 45 seismometers deployed across and along the San Andreas fault near Parkfield immediately after the M6 Parkfield earthquake on September 28, 2004 to record fault-zone seismic waves from aftershocks and explosions detonated within the rupture zone. Seismic stations and explosions were co-sited in our previous experiment in the fall of 2002. The data from repeated shots in the two surveys show ~1.0-1.5% decreases in seismic velocity within the ~150-m-wide zone along the fault trace and less changes beyond this zone, most likely due to the coseismic damage of rocks during dynamic rupture of the latest M6 event. The width of this zone characterized by greater velocity changes is consistent with the low-velocity waveguide model on the San Andreas fault, Parkfield we derived from fault-zone trapped waves (Li et al., 2004). The damage zone is not symmetric with the main fault trace but broader on the southwest side of the fault. Waveform cross-correlations for repeated aftershocks in 21 clusters, total ~130 events show ~0.7-1.1% increases in S-wave velocity within the fault zone in 3 months starting a week after the mainshock, indicating that the damaged rock has been healing and regaining the strength with time, most likely due to the closure of cracks opened in the mainshock. The healing rate was greater in the earlier stage of post-mainshock healing. We estimate the velocities within the fault zone decreased by up to ~2.5%, most likely associated with the 2004 M6 event at Parkfield. The magnitude of fault healing is not uniform along the ruptured segment; it is slightly larger beneath Middle Mountain in accordance with larger slip mapped there. The fault healing is seen at depth above ~6-7 km and is likely to be depth-dependent, too. The damage and healing progression observed on the SAF associated with the 2004 M6 Parkfield earthquake are consistent with our previous observations at rupture zones of the 1992 M7.4 Landers and 1999 M7.1 Hector Mine earthquakes. However, the fault-zone damage degree is smaller and the healing process is shorter on the SAF at Parkfield than those on at Landers and Hector Mine.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics (1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
SC: Tectonophysics [T]
MN: Fall Meeting 2005