HR: 16:00h
AN: S54A-01    [Abstracts]
TI: Earthquake source properties in southern California from stacking P-wave spectra
AU: * Shearer, P
EM: pshearer@ucsd.edu
AF: IGPP/SIO, U.C. San Diego, La Jolla, CA 92093-0225 United States
AU: Hauksson, E
EM: hauksson@gps.caltech.edu
AF: Seismological Laboratory, Caltech, Pasadena, CA 91125 United States
AU: Prieto, G
EM: gprieto@ucsd.edu
AF: IGPP/SIO, U.C. San Diego, La Jolla, CA 92093-0225 United States
AB: We compute P-wave spectra from over 250,000 earthquakes using a multi-taper method on both the P-wave and a pre-event noise window for all available stations within 200 km of each event, using data from the Southern California Seismic Network from 1989 to 2003. Requiring an average signal-to-noise ratio of five or greater between 1 and 10 Hz results in a total of over 2 million individual P-wave spectra for subsequent processing. We apply a stacking method to isolate the source, receiver, distance and depth dependent parts of the spectra. Our observed distance dependence of spectral falloff between 0 and 160 km implies a mid-crustal $Q_P$ of about 500, in approximate agreement with Schlotterbeck and Abers (2001). Observed source spectra exhibit spatially coherent patterns consistent with variations in stress drop or near-source attenuation differences. For example, events in the Imperial Valley are relatively depleted in high frequencies, indicating either low-corner-frequency, low-stress-drop (slow) events, or large near-source attenuation. In other areas, rapid spectral variations are likely caused by variations in source properties because the required changes in $Q_P$ at short distances would be unrealistically large. For example, aftershocks of the 1992 Mw 7.3 Landers earthquake exhibit along-strike variations in their frequency content that suggest differences in corner frequencies and stress drops along the rupture. Comparisons of these results to focal mechanisms of aftershocks and static stress change predictions from Landers mainshock rupture models should improve our understanding of how aftershocks respond to stress changes.
DE: 7230 Seismicity and seismotectonics
DE: 8164 Stresses--crust and lithosphere
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting