HR: 1340h
AN: S13B-0198    [Abstracts]
TI: Spatial and Temporal Stress Drop Variations in the Vicinity of the M6.0 2004 Parkfield Earthquake
AU: * Allmann, B P
EM: ballmann@ucsd.edu
AF: Scripps Institution of Oceanography, Institute of Geophysics and Planetary Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: Scripps Institution of Oceanography, Institute of Geophysics and Planetary Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Lin, G
EM: gulin@ucsd.edu
AF: Scripps Institution of Oceanography, Institute of Geophysics and Planetary Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AB: In this study we investigate earthquake source parameters in the Parkfield segment of the San Andreas Fault (SAF) by analyzing P-wave spectra from 34316 earthquakes that occurred between 1984 and June 2005. We are focusing our analysis on a fault segment of about 70~km length that ranges from the southernmost part of the creeping section over the Middle Mountain region beneath the M6.0 1966 hypocenter into the rupture zone of the M6.0 2004 Parkfield event. We apply a method that selects high signal--to--noise records and isolates source, receiver and path dependent terms. Resulting source spectra are corrected for attenuation using an empirical Green's function (EGF) method. We use the EGF-corrected spectra to estimate event source parameters and Brune-type stress drops based on a Madariaga source model. The resulting stress drop estimates show little correlation with estimated moment. Computed stress drops generally increase with depth but do not show a correlation with S-wave velocity perturbations in existing tomographic models of the area. Here, we examine the spatial changes in estimated stress drop within the Parkfield segment as well as the temporal changes before and after the M6.0 2004 Parkfield earthquake. By applying a seismicity-based median filter to the calculated stress drop values and gridding the result over the vertical fault surface, we observe coherent lateral variations in stress drop, which we compare to b-value variations that have been observed in the area. South of the San Andreas Fault Observatory at Depth (SAFOD), aftershocks of the 2004 M6.0 earthquake clearly show reduced high frequencies compared to earlier events in the same region. Preliminary results suggest that this change is primarily a source effect consistent with reduced earthquake stress drops rather than a change in attenuation structure. We compare these spatial and temporal variations in estimated stress drop with preliminary slip models for the 2004 M6.0 mainshock.
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7250 Transform faults
SC: Seismology [S]
MN: Fall Meeting 2005