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Published July 2015 | Supplemental Material + Published
Journal Article Open

Stress-drop heterogeneity within tectonically complex regions: a case study of San Gorgonio Pass, southern California


In general, seismic slip along faults reduces the average shear stress within earthquake source regions, but stress drops of specific earthquakes are observed to vary widely in size. To advance our understanding of variations in stress drop, we analysed source parameters of small-magnitude events in the greater San Gorgonio area, southern California. In San Gorgonio, the regional tectonics are controlled by a restraining bend of the San Andreas fault system, which results in distributed crustal deformation, and heterogeneous slip along numerous strike-slip and thrust faults. Stress drops were estimated by fitting a Brune-type spectral model to source spectra obtained by iteratively stacking the observed amplitude spectra. The estimates have large scatter among individual events but the median of event populations shows systematic, statistically significant variations. We identified several crustal and faulting parameters that may contribute to local variations in stress drop including the style of faulting, changes in average tectonic slip rates, mineralogical composition of the host rocks, as well as the hypocentral depths of seismic events. We observed anomalously high stress drops (>20 MPa) in a small region between the traces of the San Gorgonio and Mission Creek segments of the San Andreas fault. Furthermore, the estimated stress drops are higher below depths of ∼10 km and along the San Gorgonio fault segment, but are lower both to the north and south away from San Gorgonio Pass, showing an approximate negative correlation with geologic slip rates. Documenting controlling parameters of stress-drop heterogeneity is important to advance regional hazard assessment and our understanding of earthquake rupture processes.

Additional Information

© 2015 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. Accepted 2015 April 13. Received 2015 April 12; in original form 2014 October 28. We thank Yehuda Ben-Zion and Adrien Oth for detailed reviews of an earlier version of the manuscript. TG and EH were supported by NEHRP/USGS grant G13AP00047. This research was also supported by the Southern California Earthquake Center (SCEC) under contribution number 12017. SCEC is funded by NSF Cooperative Agreement EAR-0529922 and USGS Cooperative Agreement 07HQAG0008. We would also like to thank the open-source community for many of the programs utilized here (GMT, python, python-basemap, Gimp and the Linux operating system). The here utilized data were obtained from the Southern California Earthquake Data Center (Caltech.Dataset. doi:10.7909/C3WD3xH1).

Attached Files

Published - Geophys._J._Int.-2015-Goebel-514-28.pdf

Supplemental Material - SI_stressDrop_rev2.pdf


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