HR: 0800h
AN: S11A-1002    [Abstracts]
TI: The Observation of Fault Finiteness and Rapid Velocity Variation in P$_n$$_l$ Waveforms for the Mw 6.5, San Simeon, California Earthquake
AU: * Konca, A O
EM: ozgun@gps.caltech.edu
AF: A. Ozgun Konca, Caltech Seismo Lab, 252-21, Pasadena, CA 91125 United States
AU: Ji, C
EM: jichen@gps.caltech.edu
AF: A. Ozgun Konca, Caltech Seismo Lab, 252-21, Pasadena, CA 91125 United States
AU: Helmberger, D V
EM: helm@gps.caltech.edu
AF: A. Ozgun Konca, Caltech Seismo Lab, 252-21, Pasadena, CA 91125 United States
AB: We observed the effect of the fault finiteness in the P$_n$$_l$ waveforms from regional distances ($4\deg$ to $12\deg$) for the M$_w$6.5 San Simeon Earthquake on 22 December 2003. We aimed to include more of the high frequencies (2 seconds and longer periods) than the studies that use regional data for focal solutions (5 to 8 seconds and longer periods). We calculated 1-D synthetic seismograms for the $P_n_l$ portion for both a point source, and a finite fault solution. The comparison of the point source and finite fault waveforms with data show that the first several seconds of the point source synthetics have considerably higher amplitude than the data, while finite fault does not have a similar problem. This can be explained by reversely polarized depth phases overlapping with the P waves from the later portion of the fault, and causing smaller amplitudes for the beginning portion of the seismogram. This is clearly a finite fault phenomenon; therefore, can not be explained by point source calculations. Moreover, the point source synthetics, which are calculated with a focal solution from a long period regional inversion, are overestimating the amplitude by three to four times relative to the data amplitude, while finite fault waveforms have the similar amplitudes to the data. Hence, a moment estimation based only on the point source solution of the regional data could have been wrong by half of magnitude. We have also calculated the shifts of synthetics relative to data to fit the seismograms. Our results reveal that the paths from Central California to the south are faster than to the paths to the east and north. The P wave arrival to the TUC station in Arizona is 4 seconds earlier than the predicted Southern California model, while most stations to the east are delayed around 1 second. The observed higher uppermost mantle velocities to the south are consistent with some recent tomographic models. Synthetics generated with these models significantly improves the fits and the timing at most stations. This means that regional waveform data can be used to help locate and establish source complexities for future events.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting