Complex Fault Geometries May Account for Discrepancies Between Geologic and Geodetic Slip Rates

Three-dimensional numerical models of active faulting in southern California reveal that slip rates of non-planar and interacting faults can vary significantly along the trace of the faults. For such faults, which are prevalent in southern California, the geologic slip rates collected from specific sites may not represent the overall slip rate of the fault. Similarly, geodetic slip rates may not accurately represent the overall fault slip rates because the models used to infer slip rates often simplify the observed fault geometries. For example, connecting segmented faults and simplifying complex fault geometry produces artificially high slip rates from geodetic data that may disagree with geologic slip rates. Our three-dimensional models of geologically-constrained active fault surfaces within the Los Angeles and Ventura basins produce lower slip rates than existing geodetic models, which overly simplify fault configurations. Our models, which use geodetic data to establish tectonic boundary conditions, produce slip rates that match the geologic slip rates at specific sites along many faults. Within the models, non-planar faults, such as the San Cayetano, Oak Ridge and Palos Verdes faults, have site-specific slip rates that generally agree with the geologic estimates but differ significantly from the average slip rates for the modeled fault. Similarly, our three-dimensional model of the southern San Andreas fault, which includes several non-vertical strands, matches the seemingly conflicting geologic slip rate data available for the San Bernardino strand. Although our models have lower fault slip rates than previous geodetic models, three-dimensional interseismic models with our fault slip rates and locking depth of ~8 km match well the GPS station velocities within the Los Angeles and Ventura basins.