Abstract

We combine geologic and global positioning system (GPS) data to characterize the style and magnitude of off-fault deformation across the San Andreas fault system in central California. Geologic structures record ∼12 km of both fault-parallel and fault-perpendicular displacements across creeping and locked portions of the San Andreas fault. Analysis of 150 GPS site velocities suggests that the borderlands record 4–6 mm/yr of fault-parallel and 3–5 mm/yr of fault-perpendicular motion alongside the creeping segment, where elastic strain is minimized. The distribution of both long-term geologic and short-term geodetic deformation is affected by basement type, where more deformation is concentrated northeast of the San Andreas fault on Franciscan basement. We suggest that at least half the fault-parallel GPS deformation measured by GPS bordering the creeping segment must be accommodated by geologic structures; this permanent deformation needs to be incorporated into dynamic models of the fault system.

Elastic modeling of the San Andreas fault in central California, which incorporates its well-known transition from locked to creeping behavior near Parkfield, predicts first-order variations in the GPS velocity field along the fault and corresponding variations in dilatational strain rates. The strain rate pattern is dominated by a large contractional region northeast of the transition from locked to creeping behavior and a large extensional region southwest of the transition. The former coincides with the Coalinga and Kettleman Hills anticlines, the growth and development of which seem to have occurred under at least two kinematic conditions. We suggest that the onset of fault creep in central California promoted the growth of these folds. By implication, fault creep has been active over geologic time scales.

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