Interactions of Major Faults with the Regional Stress Field Across the Creeping Zone of the SAF and in the Bay Area: Implications for Fault Weakening Processes Element II: Research on Earthquake Occurrence and Effects

Technical Abstract We determined orientations of principal stresses around the San Andreas Fault (SAF) system in the greater San Francisco 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 approximately 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 SH decreases systematically to the north. This contrasts with the high angles of SH 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 SH makes with major fault strands changes 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 SH 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 échelon 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. Non-Technical Abstract We determined the spatial distribution of stress in the greater San Francisco Bay area and in Northern California using the geometry of earthquake fault slip. The …