Impact of Variable Fault Geometries and Slip Rates on Earthquake Catalogs From Physics-Based Simulations of a Normal Fault

Physics-based earthquake simulators have been developed to overcome the relatively short duration and incompleteness of historical earthquake and paleoseismic records, respectively. These simulators have the potential to be a useful addition to seismic hazard assessment as they produce millions of synthetic earthquakes over thousands to millions of years using predefined fault geometries and slip rates. Due to the sparsity of fault data and the computational expense of the modeling, it is common to simplify earthquake simulator input parameters. Fault surfaces are typically characterized by simplified mapped traces and constant dip with depth, with slip rates that are often assumed to be uniform over the fault plane. This study uses the well-defined 3D geometry of an active normal fault in offshore Aotearoa New Zealand, the Cape Egmont Fault, to demonstrate the impact of using nonuniform slip rates and nonplanar fault geometries on the resulting synthetic earthquakes from the earthquake simulator RSQSim. Adopting variable slip rates that decrease to zero along the fault tipline (rather than uniform slip rates) reduces unrealistically high nucleation rates of seismicity along fault edges. Introduction of complex 3D fault geometries, including fault segmentation and bends on kilometer scales, and variable slip rates produces less characteristic earthquake populations, increasing the number of M6-7 moderate-large magnitude events. Incorporation of variable fault geometries and slip rates in physics-based simulators may modify the seismic hazards estimated from synthetic earthquake catalogs. Plain Language Summary Earthquake simulators are used to study and anticipate earthquakes. These simulators produce millions of synthetic earthquakes over thousands to millions of years using predefined fault properties. We use a well-defined active normal fault in offshore Aotearoa New Zealand, to examine the impact of nonuniformly distributed fault slip rates and nonplanar fault geometries on the resulting synthetic earthquake catalogs. We show that adopting variable fault slip rates, with a gradual decrease in slip rate toward fault-surface edges, reduces the unrealistic occurrence of high concentrations of earthquakes along the fault tip line. The study also shows that utilization of complex 3D fault geometries together with variable slip rates produces populations of earthquakes in a less predictable manner, increasing the number of moderate to large magnitude events. Therefore, details of the input fault geometries and slip rate distributions can significantly affect the resulting synthetic earthquake catalogs from physics-based earthquake simulators with significant implications for seismic hazard applications.