Variations in Coulomb Stress Accumulation Along the San Andreas Fault System

In order to test the hypothesis that shallow fault creep and earthquake recurrence intervals along the San Andreas Fault (SAF) system are related to stress accumulation rate, we investigate the accumulation of Coulomb stress due to relative motion between the Pacific and North American plates. Using a Fourier Transform method, we solve for the 3-dimensional vector displacement due to an arbitrary body force distribution in an elastic half space. We then apply this method to fault plane segments along the trace of the SAF system that extend from a shallow locking depth (9-40km) to infinite depth. We require that deep slip rates on parallel fault strands sum to a constant 47 mm/yr across the fault in order to constrain our far-field displacements to that of rigid-block rotations. We have constrained our model using GPS-derived velocities, both the Southern California Earthquake Center (SCEC) velocity field and measurements made along the northern SAF system, and find that our model requires slip depth variations in order to reproduce the behavior of the GPS data sets. This simple model also reveals an appropriate sensitivity to fault geometry. The model predicts 2-4 mm/yr of vertical uplift in the Transverse Ranges, corresponding to the compressional component of the fault as it bends westward. The main advantage of this approach over previously published Green's function models is that we are now capable of computing very large arrays, such as a 1000x2000 km grid of the SAF system, in a remarkably short amount of time. Our primary purpose for developing this deep slip model is to estimate Coulomb stress accumulation in the seismogenic depth range along the various strands of the SAF system. Because Coulomb stress is a function of both fault geometry and locking depth, we find large variations in accumulated Coulomb stress along the SAF system. At shallow depths (2km), high Coulomb stress accumulation of 3-6 MPa/100yr occurs along the Imperial and Central segments of the SAF system where shallow fault creep occurs regularly. Low accumulation rates occur along sections where stress is partitioned on multiple strands. At mid-seismogenic depths (7 km), we find Coulomb stress accumulation as high as 14 MPa/100yr along the Imperial section, while accumulation along the main strand of the SAF system is typically 2-4 MPa/100yr. The relatively high Coulomb stress accumulation rate along the Imperial and Central segments may explain the short recurrence time for major earthquakes along these segments.