A mechanical model of the San Andreas fault and SAFOD Pilot Hole stress measurements

Stress measurements made in the SAFOD pilot hole provide an opportunity to study the relation between crustal stress outside the fault zone and the stress state within it using an integrated mechanical model of a transform fault loaded in transpression. The results of this modeling indicate that only a fault model in which the effective friction is very low (\u3c0.1) through the seismogenic thickness of the crust is capable of matching stress measurements made in both the far field and in the SAFOD pilot hole. The stress rotation measured with depth in the SAFOD pilot hole (~28 ° ) appears to be a typical feature of a weak fault embedded in a strong crust and a weak upper mantle with laterally variable heat flow, although our best model predicts less rotation (15 ° ) than observed. Stress magnitudes predicted by our model within the fault zone indicate low shear stress on planes parallel to the fault but a very anomalous mean stress, approximately twice the lithostatic stress