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LATERAL VARIATIONS OF STRESS DROP NEAR PARKFIELD

ALLMANN,B.P., Scripps Institution of Oceanography, UCSD,La Jolla, CA, 92093, ballmann@ucsd.edu; SHEARER, P.M., Scripps Institution of Oceanography, UCSD,La Jolla, CA, 92093, pshearer@ucsd.edu.

We find significant lateral variations in Brune-type stress drop of small-magnitude earthquakes along the Parkfield segment of the San Andreas Fault (SAF). We obtain spectral source parameter estimates by employing a method of separating source, receiver and path contributions based on a convolutional model. In addition, we apply a spatially averaged multi-event empirical Green's function (EGF) deconvolution to correct for possible near-source attenuation effects. Our results indicate that earthquakes from this segment of the SAF are self-similar in the M0.5 to M3.5 range. We also observe no depth dependence of Brune-type stress drop if the source parameters are corrected by a depth-varying shear wave velocity. The observed lateral changes reveal lower stress drops in the Middle Mountain asperity and along the creeping section to the North. The hypocentral region of the 2004 M6.0 Parkfield earthquake shows higher stress drops than its surrounding. We test the resolution of our analysis method with a synthetic test. Comparing data before and after the M6.0 mainshock reveals small but statistically significant temporal changes of source parameters in some areas of the investigated fault segment. The observed spectral changes can be caused by near-source attenuation changes as well as by changes in source parameters. We are able to separate the two effects by interpreting our data both ways and carefully comparing the respective results. We observe evidence for an increase in attenuation in the fault zone after the main shock. This observation is independently confirmed from analysis of repeating microearthquake clusters.

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Last Modified: 2011 Aug 10

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