Seismic Swarm S20130311.1: Analysis of Activity Near Anza, California

The Anza region in Riverside County, California, lies within the San Jacinto Fault Zone, a major component of the broader San Andreas Fault system. This area experiences frequent seismic activity due to right-lateral strike-slip faulting along the San Jacinto and nearby Elsinore faults. The local geology features a complex network of faults cutting through crystalline basement rocks of the Peninsular Ranges, with sedimentary basins that amplify ground motion during events. Historical records indicate persistent microseismicity and occasional swarms, consistent with the tectonic regime of southern California where the Pacific and North American plates interact.

Swarm S20130311.1 began at 18:43 on 10 March 2013 and concluded at 21:01 on 21 May 2013. Located 21 km east-southeast of Anza, the sequence produced 2118 earthquakes over 1730 hours and 18 minutes. Depths for events in the initial phase ranged primarily between 5 and 13 km, with the majority clustered around 9–12 km, aligning with the brittle-ductile transition zone typical of this fault system.

Analysis of the first 100 events reveals a rapid escalation in activity following the initial small-magnitude shocks. Early events on 10–11 March registered magnitudes below 1.0 at depths of 5–13 km. A notable increase occurred around 16:35 on 11 March, with multiple events exceeding magnitude 2.0 within minutes. The sequence peaked with a magnitude 4.7 earthquake at 16:56:06 on 11 March at 10 km depth, followed by a magnitude 3.0 event shortly after. Subsequent events maintained magnitudes mostly between 0.5 and 2.3, with depths stabilizing near 10–11 km. This pattern shows a mainshock-aftershock dynamic embedded within the swarm, where the larger event triggered a dense cluster of smaller quakes over the following hours.

Since 2000, 19 swarms have occurred in the region, with annual counts as follows: 2001 (1), 2002 (2), 2003 (1), 2005 (2), 2009 (2), 2010 (5), 2011 (3), and 2012 (3). These recurrent swarms underscore the area's propensity for episodic seismic clusters rather than isolated large events.

Such sequences provide valuable data for understanding fault behavior and stress transfer in the San Jacinto zone. Monitoring by regional networks continues to refine models of seismic hazard for nearby communities.

References USGS Earthquake Hazards Program Southern California Seismic Network reports California Geological Survey fault zone maps