Implications of an elastic analysis of in situ stress measurements near the San Andreas Fault

Twenty‐nine measurements of in situ stress obtained with the hydraulic fracturing technique near Palmdale, California, are the basis of an elastic analysis of the state of stress in the Mojave Desert adjacent to the San Andreas fault. The measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km. The elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7.9 MPa/km. Extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 MPa, a result that is higher than estimates of the average shear stress on the San Andreas fault based on the analysis of heat flow data. This finding is consistent, however, with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of San Andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone. If so, then the coefficient of friction of the San Andreas fault zone inferred from the stress field results is about 0.45. The state of stress does not appear to vary systematically with distance from the San Andreas fault although considerable localized variation is observed. The observations suggest an upper bound of about 0.1 MPa/km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the San Andreas fault, a result that implies a corresponding limit of about 1.4 MPa on the shear traction applied to the base of the seismogenic layer. Finally, we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip, which could be released in a large earthquake. We show that on the basis of a model involving a locked fault, extending to about 22 km, the total fault slip below the locked portion is less than 13 m. A more comprehensive set of stress data could permit the estimation of an even lower bound.