HR: 1340h
AN: S43A-1048    [Abstracts]
TI: Seismic Documentation for Rock Damage and Heal on the San Andreas Fault Involved in the 2004 M6 Parkfield Earthquake
AU: Malin, P M
EM: pmalin@duke.edu
AF: Dept. of Earth and Ocean Sciences, Duke Univ., Durham, NC 27708, United States
AU: * Li, Y
EM: ygli@usc.edu
AF: Dept. of Earth Sciences, USC, Los Angeles, CA 90089, United States
AU: Chen, P
EM: pochen@ldeo.columbia.edu
AF: Lamont, Univ. of Columbia, New York, NY 10964, United States
AU: Cochran, E M
EM: elizabeth.cochran@ucr.edu
AF: Dept. of Earth Science, UCR, Riverside, CA 92521, United States
AU: Vidale, J E
EM: seismoguy@mac.com>
AF: PNSN, Univ. of Washington, Seattle, WA 98195, United States
AB: After the M6 Parkfield earthquake that occurred on 28 September 2004, we deployed a dense seismic array at the same sites as used in our experiment in the fall of 2002. The measurements using moving-window cross- correlation of waveforms for the repeated explosions and microearthquakes recorded in 2002 and 2004 show a decrease in shear velocity of at least ~2.5% within a ~200-m-wide zone across the San Andreas main fault trace most likely owing to co-seismic damage of fault rocks caused by dynamic rupture in this M6 earthquake. The width of the damage zone characterized by larger velocity changes is consistent with the low-velocity waveguide model on the SAF near Parkfield derived from fault-zone trapped waves [Li et al., 2004]. The estimated ratio between the P and S wave traveltime changes is 0.57 within the rupture zone and ~0.65 in the surrounding rocks, indicating wetter cracks within the damaged fault zone, probably due to the ground water percolating into the cracks opened in the mainshock. The measurements of traveltime changes for repeated aftershocks in 21 clusters, with a total of ~130 events, located at different depths along the rupture in 2004 show that the maximum shear velocity increased by ~1.2% within the damage zone in 3.5 months starting a week after the mainshock, indicating that the fault heals in the post-seismic stage due to the closure of cracks in the damaged rock. The data recorded at a seismograph installed in the SAFOD mainhole passing the San Andreas fault zone at ~3-km depths for repeated aftershocks in December of 2004 and later show that seismic velocities within the damage zone were changed by ~0.3% in a month, but no changes were registered at seismographs installed in the vertical pilot borehole drilled ~1.8 km away from the main fault trace for the same repeated events. We find that the healing rate is logarithmically decreasing through time with greater healing rate in the earlier stage after the mainshock. The magnitude of fault damage and healing varies along the rupture zone and with depth, but is most prominent at depths above ~7 km and roughly correlating with the slip distribution on the SAF in the 2004 M6 Parkfield earthquake [Johanson et al, 2006]. Observations of fault zone damage and healing associated with this M6 event illuminate the faulting-healing progression on an active fault in the major earthquake, in general consistent with previous observations of velocity evolution owing to damage and healing for Lander and Hector Mine earthquakes [Li et al., 1998; Vidale and Li, 2003]. However, the magnitude of damage and healing observed near Parkfield on the SAF is smaller than those on the Landers and Hector Mine rupture zones, probably related to the smaller magnitude mainshock, and smaller slip, and possibly differences in stress drop, pore-pressure, and rock type.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics (1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8034 Rheology and friction of fault zones (8163)
SC: Seismology [S]
MN: 2007 Fall Meeting