Critically Stressed Crust in Southern California: A Model of Crustal Stress from Plate Driving, Topography, and Fault Loading, with Geodetic and Seismic Constraints

Abstract:

Active plate boundaries are an excellent environment for studying the relative importance of stress sources because their relatively high strain rates and frequent earthquakes provide a wealth of information on the orientation of the stress field and the spatio-temporal variations in crustal strain accumulation. Using southern California as a natural laboratory, we combine models of stress accumulation (fault loading) along the various segments of the southern San Andreas Fault System (SAFS), constrained by geodetic observations, with a uniform horizontal stress field representing a regional tectonic driving stress. The loading time on individual faults within the system, as well as the magnitude and orientation of the principal driving stresses, are adjusted so that the orientation of the total stress tensor is in alignment with the observed in situ stress orientation, as determined from an inversion of earthquake focal mechanisms. Because stress tensor orientation can only constrain the relative magnitude of stress contributions, an additional stress contribution from topography supported in the crust is included so that the resulting total stress field model includes both orientation and a lower bound on magnitude. The optimal stress model, comprised of contributions from topography, fault loading, and regional driving stress, predicts a maximum horizontal principal stress (SHmax) that agrees with focal mechanisms to within the 15º across 75% of the region. The optimal model requires 1) the regional driving stress to be closely aligned to the observed SHmax direction with a differential stress of at least 42 MPa at seismogenic depth, and 2) a loading time of at least 2000 years on the main segments of the SAFS, reflecting a temporal signal that spans multiple earthquake cycles. This suggests earthquakes on these fault segments achieve somewhat less than complete release of the accumulated stress from fault loading, such that the shallow crust is near-critically stressed throughout the earthquake cycle. This has important implications for understanding intraplate seismicity because it supports the assertion that relatively small stresses from unconventional sources could play an important role in determining the timing and location of seismic activity in a critically stressed upper crust.