Data on principal stress orientation from shallow wells indicate a low magnitude shear stress on the San Andreas Fault, California. These data are consistent with heat-flow measure- ments and predictions, but are inconsistent with estimated strength of rocks. This project will attempt to test the hypo- thesis that the shallow well data are representative of the state of stress at mid-crustal depths by determining the orien- tation of the principal stresses that existed during activity on the San Gabriel fault, now deeply exhumed. Paleostress orien- tations and mechanisms of slip will be determined by petrofabric analysis of structures in the fault zone. Results will be compared to laboratory data and applied to the problem of the apparently weak fault strength.
