Identification and Characterization of Earthquake Swarms in Southern California

Abstract:

Earthquake swarms are space-time clusters of seismicity that cannot easily be explained by typical aftershock behavior, and are likely triggered by external processes such as fluid migration and/or slow slip. However, swarm properties are not fully understood and how much swarm occurrence is related to the tectonic environment (e.g., heat flow, stressing rate) or source characteristics (e.g., focal mechanism, stress drop) is unclear. Systematic study of large numbers of swarms and their source properties should help to resolve these issues, but is hampered by the challenge of identifying swarms at a range of spatiotemporal scales from a large earthquake catalog. We have developed a new method to search for clusters by comparing the number of neighboring events to the background events in scalable space/time windows, similar to the idea of STA/LTA algorithms, and then discriminating swarms from aftershock clustering. We first apply this method to the San Jacinto Fault Zone (SJFZ) and find ten times more swarms than a previous study using fixed spatiotemporal windows. The most striking spatial pattern of our identified swarm events is a higher fraction of swarms at the northern and southern ends of the SJFZ than its central segment, which correlates with an increased proportion of normal faulting earthquakes. We then apply our method to search the entire southern California catalog of 433,737 events with M ≥ 1 from 1981 to 2014. Preliminary results indicate that swarms are heterogeneously distributed in space and time, but that higher swarm rates are generally found in regions of normal faulting. We will explore other swarm properties, such as event stress drops, spatial migration behavior, distribution of moment release, and relation to foreshock sequences in order to better understand the driving physical mechanisms of swarms and improve earthquake forecasts.