Implications of earthquake focal mechanisms for the frictional strength of the San Andreas fault system

Abstract

Analysis of stress orientation data from earthquake focal plane mechanisms adjacent to the San Andreas fault in the San Francisco Bay area and throughout southern California indicates that the San Andreas fault has low frictional strength. In both regions, available stress orientation data indicate low levels of shear stress on planes parallel to the San Andreas fault. In the San Francisco Bay area, focal plane mechanisms from within 5 km of the San Andreas and Calaveras fault zones indicate a direction of maximum horizontal compression nearly orthogonal to both subvertical, right-lateral strike-slip faults, a result consistent with those obtained previously from studies of aftershocks of the 1989 Loma Prieta earthquake. In southern California, the direction of maximum horizontal stress near the San Andreas fault is nearly everywhere at a high angle to it, similarly indicating that the fault has low frictional strength. Thus, along these two major sections of the San Andreas fault (which produced great earthquakes in southern California in 1857 and central and northern California in 1906), the frictional strength of the fault is much lower than expected for virtually any common rock type if near-hydrostatic pore pressure exists at depth, and so low as to produce no discernible shear-heating anomaly. Our findings in southern California are in marked contrast to recent suggestions by Hardebeck & Hauksson that stress orientations rotate systematically within c. 25 km of the fault, which prompted a high frictional strength model of the San Andreas fault. As we utilize the same stress data and inversion technique as Hardebeck & Hauksson, we interpret the difference in our findings as being related to the way in which we group focal plane mechanisms to find the best-fitting stress tensor. We suggest that the Hardebeck & Hauksson gridding scheme may not be consistent with the requisite a priori assumption of stress homogeneity for each set of earthquakes. Finally, we find no evidence of regional stress changes associated with the occurrence of the 1992 M7.4 Landers earthquake, again in apparent contradiction with the findings of Hardebeck & Hauksson.