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

TitleStress-drop heterogeneity within tectonically complex regions: a case study of San Gorgonio Pass, southern California
Publication TypeJournal Article
Year of Publication2015
AuthorsGoebel T.HW, Hauksson E., Shearer PM, Ampuero J.P
JournalGeophysical Journal International
Date Published2015/07
Type of ArticleArticle
ISBN Number0956-540X
Accession NumberWOS:000360623400036
Keywordsandreas fault; Dynamics and mechanics of faulting; Dynamics: seismotectonics; Earthquake source observations; earthquakes; energy-release; focal mechanisms; Fourier analysis; frictional slip; loading velocity; recurrence interval; repeating; Seismicity and; source parameters; source spectra; strong ground motion; tectonics

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 similar to 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.

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