Stress Orientations in Northern California Point to Variations in Frictional Strength Along Plate Boundary System and to Anomalous Stresses Near Sutter Buttes

The level of the frictional strength of the San Andreas Fault system has been a point of contention for over two decades. We determined orientations of principal stresses around the San Andreas Fault system in the greater Bay Area and regions further north along the strike-slip plate boundary. Stress orientations, as well as a ratio between stress magnitudes, were determined by inversions of ~6000 earthquake fault plane solutions, divided into ~100 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 zones, and the Calaveras-Green Valley-Bartlett Springs fault zones). The stress orientations, while spatially variable, show 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 (70° to 85° ) found immediately adjacent to, as well as farther from, the creeping segment of the SAF in central California by Provost and Houston [JGR, 2001]. Second, for on-fault groups, the angle that S_H makes with major fault strands changes but little along strike, averaging 50° to 55° 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 off-fault groups, as well as the on-fault groups, are in a strike-slip, rather than thrust, tectonic regime. Finally, anomalous east-west S_H orientations are seen for three groups in the vicinity of Sutter Buttes, possibly related to an isolated fragment of subducted slab that has been imaged below this region. Only 10 to 20 km away S_H lies in the more typical north-south orientation. In both cases, the seismicity in this region is relatively deep, between 10 and 25 km. If the far-field S_H is assumed to lie in a similar direction in northern, central, and southern California, then our results imply 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. This is consistent with the geological context in which multiple strands of the strike-slip fault system in the north 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.