Earthquake Swarm S20110727.1: Seismic Activity Near Anza, California
An earthquake swarm designated S20110727.1 occurred 19 km east-southeast of Anza, California, between 04:13 on 26 July 2011 and 05:55 on 30 July 2011. Over 97 hours and 41 minutes, the sequence produced 42 events. Depths ranged primarily from 4 km to 15 km, with the majority clustered between 7 km and 14 km. Magnitudes remained low overall, peaking at 3.0 for the event recorded at 17:42:13 on 26 July. Other notable shocks included a 1.5 magnitude event at 18:31:50 on the same day and several events between 1.3 and 1.4. The swarm exhibited typical characteristics of clustered, low-magnitude activity without a dominant mainshock-aftershock pattern.
The Anza region sits within the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the Pacific-North American plate boundary motion. This fault zone features multiple strands and step-overs that create localized stress concentrations, promoting swarm-type seismicity. Historical records indicate frequent small-to-moderate earthquakes, reflecting the zone's high strain rate and structural complexity. Depths observed in the 2011 swarm align with the seismogenic zone in this area, where brittle failure occurs in the upper crust above the transition to ductile behavior.
Since 2000, the Anza area has experienced 17 documented swarms. Yearly counts include one in 2001, two each in 2002, 2003, 2005, and 2009, five in 2010, and three in 2011. These episodes underscore the persistent background of swarm activity driven by fault interactions rather than isolated large ruptures. The 2011 swarm fits this pattern, with events distributed across shallow to mid-crustal depths and no escalation toward higher magnitudes.
Such sequences provide insight into fault mechanics, as the spatial and temporal clustering often traces fluid migration or aseismic slip transients along fault segments. In the San Jacinto system, these processes occur amid ongoing tectonic loading, contributing to the region's overall seismic hazard profile.
References
USGS Earthquake Hazards Program catalog data for southern California.
Southern California Earthquake Center fault database for San Jacinto Fault Zone characteristics.