Crustal stress field in southern California and its implications for fault mechanics

We present a new, high spatial resolution image of stress orientation in southern California based on the inversion of earthquake focal mechanisms. We use this image to study the mechanics of faulting in the plate boundary region. The stress field contains significant spatial heterogeneity, which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause. Temporal changes in the stress field are also observed, primarily related to major earthquakes. The observed 15° (±10°) rotation of the stress axes due to the 1992 M7.3 Landers mainshock implies that the deviatoric stress magnitude in the crust is low, of the order of 10 MPa. This suggests that active faults in southern California are weak. The maximum principal stress axis near the San Andreas Fault is often at ∼50° to the fault strike, indicating that the shear stress on the fault is comparable to the deviatoric stress. The San Andreas in southern California may therefore be a weak fault in a low-strength crust