January 9, 2007 marked the passing of 150 years of relative seismic quiescence along California’s central and southern San Andreas Fault System (SAFS). For an active fault system with earthquake repeat times averaging roughly 150 years, the SAFS is likely primed for another major, and potentially devastating, seismic event. In an effort to identify inherent stress behaviors of individual fault segments of the SAFS, a 3-D time-dependent deformation model is used to simulate stress evolution throughout the earthquake cycle. The model spans the last 1000 years of earthquake history and combines an up-to-date set of paleoseismic, geologic, and geodetic constraints. Interseismic Coulomb stress rates generated by the model range from 0.2 to 7.2 MPa/100yrs and reflect variations in slip rate, fault orientation, and locking depth. Assuming a prescribed slip history based on paleoseismic evidence and the historical earthquake record, the model can also be used to estimate the magnitude of accumulated Coulomb stress on each fault segment spanning multiple earthquake cycles. These simulations reveal an evolving stress field through time, and in particular, a significant level of accumulated stress (~7.8 MPa) along the southernmost portion of the San Andreas (Coachella segment) at present day, where major earthquake activity has been absent for over 300 years. While models of this nature are highly dependent on an assumed time and slip history, they provide a quantitative foundation for advancing our ability to recognize zones of elevated seismic risk.
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