Influence of Fault Geometry on the Spatial Distribution of Long‐Term Slip with Implications for Determining Representative Fault‐Slip Rates

Abstract

Determining representative slip rates of active faults is essential to seismic hazard assessment and tectonic analysis. Here, we take a two-pronged approach to determine how spatially variable slip over many earthquake cycles impacts the sliprate record at potential sites of geologic investigation near releasing stepovers. First, we use 2D parametric models to estimate the probability that a point measurement is representative of the average slip-rate for continuous strike-slip faults and those with a range of releasing stepover geometries and friction values. All models result in skewed distributions for which a randomly selected site has a higher probability of sampling a slip rate that exceeds the mean. For most configurations, individual point measurements are unlikely (p < 0.5) to yield a slip rate within +/- 1 mm/yr of the mean. The probability can be notably improved (15%-300%) by summing slip rates of overlapping segments. Second, we use 3D mechanical models of a well-studied releasing stepover along the San Jacinto fault to investigate the impact of specific fault geometries on slip-rate distribution and the implications for existing slip-rate estimates. The model-calculated dextral-slip rates are consistent with the abundant geologic slip-rate data available for this system. Although summing the slip rates at geologic sites across the stepover may produce a nonrepresentative slip rate, the summed slip rates from the model are compatible with the representative slip rate. Thus, the complete along-trace slip-rate distribution produced from the geologically consistent models provides a way to select sites for determining slip rates or interpreting isolated geologic slip-rate data. The site-specific models also illustrate how to assess the uncertainty of slip-rate data due to the spatial variability of slip rates in geometrically complex fault systems.