SPATIAL VARIATIONS IN SLIP ALONG COMPLEX FAULT SURFACES MAY ACCOUNT FOR DISCREPANCIES BETWEEN PERMANENT GPS DERIVED AND GEOLOGIC SLIP RATES

The southern Big Bend of the San Andreas fault incorporates numerous non-vertical, non-planar, and intersecting fault surfaces that cannot adequately be represented by rectangular fault segments. Numerical models that neglect the complex geometry of faults in this area are unable to incorporate the spatial variability of fault slip. Additionally, models with simple fault geometry may infer erroneous slip rates if they use only a few GPS data to constrain slip. Differences between such GPS-derived slip rates and longer-term geologic slip rates have been attributed to temporal variations in fault slip rate but instead may owe to inadequate fault complexity within the models and/or lack of GPS constraints near the sites of the geologic slip rates. For example, GPS inversion studies have suggested 12 ± 2 mm/yr of accumulated shear strain across the northern section of the Eastern California Shear Zone in disagreement with geologic slip rates across the central Mojave of ≤6.2± 1.9 mm/yr. Data from new permanent array GPS stations show significantly slower strain rates across the central Mojave in agreement with geologic slip rates. Meanwhile, discrepancies between geologic and GPS-derived slip rates along the San Bernardino strand of the San Andreas fault may owe to over simplified fault configurations. Our 3D models with complex fault configurations show significant spatial variation in both fault slip rates and interseismic velocities along the Earth’s surface. We are able to simulate both fault slip rates similar to the geologic rates and horizontal velocities that closely match permanent GPS station velocity data without evoking temporal variations in fault slip rates. We also test the sensitivity of fault slip to the orientation and magnitude of regional tectonic loading.