REGIONAL-SCALE MODELS OF CRUSTAL STRESS ALONG THE PACIFIC-NORTH AMERICA PLATE BOUNDARY, WITH IMPLICATIONS FOR HETEROGENEOUS TECTONIC LOADING AND IN SITU STRESS MAGNITUDE

The in situ crustal stress field fundamentally governs, and is affected by, the active tectonic processes in plate boundary regions, yet first order questions remain about the characteristics of that field and the implications for active faults. We investigate the nature of this stress field in southern California by combining previously developed models of stress orientation, stress from topography, and stress accumulation rate on major locked faults into two synthesis models. In the first, we estimate the magnitude of the in situ stress field by balancing in situ orientation indicated by earthquake focal mechanisms against the stress imposed by topography, which tends to resist the motion of strike-slip faults. Our results based on the most rugged topography along the San Andreas fault system indicate that differential stress at seismogenic depth must exceed 62 MPa, consistent with differential stress estimates from complimentary methods. In the second, we develop a forward model of in situ stress based on topography, stress accumulation on locked faults, and a simple 2-D tectonic driving stress, in order to assess driving stress orientation and the relative importance of locked faults for the in situ stress state. We consider twelve independent segments of the San Andreas Fault System from Imperial Valley through Parkfield and find that observed heterogeneity in stress orientation cannot be accounted for by variations in fault geometry, earthquake cycle loading, or nearby topography. Instead we determine that the horizontal azimuth of driving stress varies from ~15º EofN south of the Coachella segment to ~5º WofN along the San Bernardino and Mojave segments, and from 8º EofN to 6º WofN along the San Jacinto segments. Using these results, our forward model is able to match the in situ stress orientation of ~60% of the near-fault strike-slip areas to within ±15º and >90% of the near-fault strike-slip areas to within ±30º, comparable to the errors associated with focal mechanism determination. Our results suggest that in situ stress heterogeneity at the regional scale is more influenced by deep driving processes acting on a laterally heterogeneous crust than by perturbations to the stress field associated with major locked faults in the upper crust.