Stress orientations in northern and central California: Evidence for the evolution of frictional strength along the San Andreas plate boundary system

We determined orientations of principal stresses around the San Andreas fault (SAF) system in the greater San Francisco Bay Area and regions farther north along the strike-slip plate boundary. Stress orientations, as well as a ratio between principal stress magnitudes, were determined by inversions of similar to6000 earthquake fault plane solutions, divided into similar to100 groups based on the spatial distribution of seismicity with respect to major regional fault strands (i.e., the SAF, the Hayward-Rodgers Creek-Maacama fault zone, and the Calaveras-Green Valley-Bartlett Springs fault zone). The stress orientations, while spatially variable, reveal several features; to describe them, it is useful to distinguish between groups of events occurring on and off the major fault strands. First, for off-fault groups, the angle between major fault strands and the maximum horizontal compression S-H decreases systematically to the north. This contrasts with the high angles of S-H (similar to80degrees) found immediately adjacent to, as well as farther from, the creeping segment of the SAF in central California by Provost and Houston [2001]. Second, for on-fault groups, the angle that S-H makes with major fault strands changes little along strike, averaging 50degrees to 55degrees in the creeping segment, the Bay area, and the northernmost part of our study area. Third, as in the vicinity of the creeping segment of the SAF in central California, the majority of both the off-fault groups, and the on-fault groups, are in a strike-slip, rather than thrust, tectonic regime. Finally, anomalous east-west S-H orientations are seen in the vicinity of Sutter Buttes. In the north, multiple strands of the strike-slip fault system have accumulated little slip, dip relatively shallowly and are composed of short, complex en echelon segments, suggesting that they originated as thrust faults in the accretionary prism associated with the Farallon subduction and have been subsequently reactivated in a strike-slip sense following the northward passage of the Mendocino triple junction. Our results, together with the geological context, suggest that the fault system is mechanically stronger with a greater effective frictional strength in the northern portion than in the creeping section of the SAF, with the Bay Area in an intermediate state. We interpret this situation to result from the evolution of the plate boundary toward lower effective frictional strength with increasing slip.