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Published June 2006 | Published
Journal Article Open

Comprehensive analysis of earthquake source spectra in southern California

Abstract

We compute and analyze P wave spectra from earthquakes in southern California between 1989 and 2001 using a method that isolates source-, receiver-, and path-dependent terms. We correct observed source spectra for attenuation using both fixed and spatially varying empirical Green's function methods. Estimated Brune-type stress drops for over 60,000 M_L = 1.5 to 3.1 earthquakes range from 0.2 to 20 MPa with no dependence on moment or local b value. Median computed stress drop increases with depth in the upper crust, from about 0.6 MPa at the surface to about 2.2 MPa at 8 km, where it levels off and remains nearly constant in the midcrust down to about 20 km. However, the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop. Spatially coherent variations in median stress drop are observed, with generally low values for the Imperial Valley and Northridge aftershocks and higher values for the eastern Transverse ranges and the north end of the San Jacinto fault. We find no correlation between observed stress drop and distance from the San Andreas and other major faults. Significant along-strike variations in stress drop exist for aftershocks of the 1992 Landers earthquake, which may correlate with differences in main shock slip.

Additional Information

© 2006 American Geophysical Union. Received 4 August 2005; revised 20 December 2005; accepted 16 February 2006; published 13 June 2006. We thank the personnel of the USGS/Caltech Southern California Seismic Network who pick and archive the seismograms and the Southern California Earthquake Center (SCEC) for distributing the data; we especially thank Vikki Appel, who facilitated access to the database. We thank Susan Hough and Gregory Beroza for constructive reviews. Funding for this research was provided by NEHRP/USGS grant 03HQPA0001. This research was also supported by the Southern California Earthquake Center, which is funded by NSF Cooperative Agreement EAR-0106924 and USGS Cooperative Agreement 02HQAG0008. This is SCEC contribution 919 and contribution 9121 of Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena.

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Created:
August 22, 2023
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October 23, 2023