HR: 14:40h
AN: T53C-05    [Abstracts]
TI: Dynamic Weakening of the San Andreas Fault by the 2004 Sumatra-Andaman Earthquake
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution of Washington, DTM, 5241 Broad Branch Rd. NW, Washington, DC 20015, United States
AU: * Taira, T
EM: taira@seis.utah.edu
AF: Department of Geology and Geophysics, University of Utah, 135 South 1460 East Room 510, Salt Lake City, UT 84112, United States
AU: Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, Houston, TX 77005, United States
AU: Nadeau, R
EM: nadeau@seismo.berkeley.edu
AF: UC Berkeley Seismological Laboratory, 215 Mccone Hall #4760, Berkeley, CA 94720, United States
AB: The time-varying properties of seismic scatterers have recently been used to probe stress-induced changes at seismogenic depth within the San Andreas Fault zone near Parkfield, CA. Temporal properties are measured by using the decorrelation index for a waveform pair from repeating earthquakes. We have been monitoring the behavior of a group of time-dependent scatterers for a 20-year period (1987-2007), using well-recorded repeating earthquake sequences. These scatterers are located within the fault zone at seismogenic (about 3 km) depth and are interpreted as fluid-filled fractures whose properties change due to the stress-induced migration of fluids. We have found three dramatic responses of the time-dependent scatterers during this 20-year period: to the 1993 Parkfield Aseismic Transient, to the 2004 Mw 6.0 Parkfield earthquake, and a third excitation about 3 months after the Parkfield earthquake. For this third excitation, the magnitude of change in decorrelation index is comparable to those observed for the 1993 Parkfield Aseismic Transient and for the 2004 Parkfield earthquake. We were able to limit the onset time of the third excitation to a 3.5-day time window between 22 and 26 December 2004, by combining the decorrelation indexes obtained from a suite of repeating earthquake sequences. This excitation is probably of tectonic origin for two reasons. First, it is unlikely to be environmentally induced, because the scatterers are deep, and there was no precipitation (the likeliest environmental influence) within the time interval of interest. Second, the excitation was also observed as a reduction in the recurrence interval and seismic moment of repeating earthquakes. This reduction, however, did not significantly change the corresponding seismic slip rate (proportional to seismic moment divided by recurrence interval), a result that is consistent with the absence of a velocity change at nearby GPS stations. This behavior suggests that there was a temporary reduction in the strength of the fault, rather than an increase in slip rate. The most dramatic event to occur during this 3.5-day time interval is the 26 December 2004 Mw 9.1 Sumatra- Andaman earthquake, which took place 7 hours before the end of the interval. This timing is strongly suggestive that a form of remotely-triggered fault weakening occurred, due to the dynamic stresses from this seismic event. We hypothesize that fault strength was temporarily reduced by a stress-induced increase in fault-zone pore pressure.
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: 2007 Fall Meeting