| A high-resolution image of crustal stress orientation, determined from the inversion of earthquake focal mechanisms, provides insights into the mechanics and evolution of faulting in southern California. The plate boundary region is a wide, complex zone of deformation, and the stress field also appears to be spatially heterogeneous. Temporal variations in stress orientation are observed as well. The observed stress rotations due to the 1992 Landers earthquake are used to make an order of magnitude estimate of the deviatoric stress magnitude at seismogenic depths. The inferred deviatoric stress is on the order of 100 bar, an order of magnitude less than the fault strength predicted from laboratory experiments, implying that active faults in southern California are weak. The San Andreas Fault does not appear to be weaker than other faults, as it is well-oriented for failure in the stress field of its immediate surroundings. Earthquakes both modify and respond to the stress field. The 1992 Landers and 1994 Northridge earthquakes caused observable rotations of the stress axes, and the mainshock-induced static stress changes appear to have triggered some aftershocks. The number of aftershocks consistent with static stress triggering in each sequence is significantly greater than the number that would be expected to appear consistent by chance. Since small earthquakes are a response to the stress field, one might expect their stress drops to scale with effective normal stress. However, the stress drops of a set of small events do not appear to scale with depth, and therefore normal stress, in most of the seismogenic crust. |
