Investigation of Large Event Clusters and Aftershock Statistics in Simulated Catalogs

We employ the 3D boundary element code RSQSim with a California fault model to investigate clustering of large earthquakes. Clusters of large events, such as the Joshua Tree - Landers - Hector Mine sequence, indicate that the occurrence of a large earthquake may increase the probability of additional large earthquakes. However, the California catalog is too short to define the statistics of large event clusters. RSQSim generates synthetic catalogs with ~200,000 M>4 events, spanning ~10,000 years, which allows us to investigate the characteristics of long-term earthquake probabilities. The simulations incorporate rate-state fault constitutive properties, and the synthetic catalogs include foreshocks, aftershocks and large event clusters. The interevent-time and space-time statistics of smaller earthquakes in the RSQSim catalogs are in good agreement with California catalogs. Therefore, we believe that the RSQSim catalogs provide an appropriate basis for investigation of large-event clustering. One such catalog contains 177 M≥7 events with 23 large event clusters. The vast majority of clusters are event pairs consisting of only two events with occasional three and four event clusters. Here we define a large event cluster as two or more M≥7 events within four years of each other. Most clustered events are closely grouped in space as well as time. Among potential indicators of large event clusters are highly productive aftershock sequences where the aftershock locations of the first events in a cluster appear to correlate with the location of the next large event in the cluster. We quantify the aftershock productivity, including the timescale of decay and magnitude-frequency dependence of all large events, in order to compare characteristics of clustered to non-clustered events.