Seismic Swarm S20110202.2 Near Anza, California

A seismic swarm designated S20110202.2 occurred approximately 10 km NNW of Anza, California, from 19:02 on 1 February 2011 to 13:02 on 3 February 2011. Over this 42-hour period, 43 earthquakes were recorded. The events clustered tightly in time and space, with the majority occurring on 2 February between roughly 17:00 and 18:00 UTC. Magnitudes ranged from 0.2 to 2.8, and focal depths remained shallow, mostly between 12 km and 17 km.

The sequence began with a magnitude 1.4 event at 15 km depth. Activity intensified the following day, featuring a magnitude 2.8 earthquake at 12 km depth at 17:15:26, accompanied by numerous smaller events within minutes. Subsequent shocks included magnitudes of 1.9, 1.6, and 1.8, gradually tapering through the night and into 3 February. Depths showed modest variation, with several events at 17 km late in the swarm, indicating activity distributed across a narrow crustal volume typical of swarm behavior.

This swarm aligns with the broader seismic character of the region. Anza lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the Pacific-North American plate motion. The fault zone exhibits frequent small-to-moderate earthquakes and occasional swarms driven by fluid migration or aseismic slip transients rather than a single mainshock-aftershock sequence.

Historical records since 2000 document seven prior swarms in the immediate area: one each in 2002, 2003, and 2009, two in 2005, and two in 2010. These recurrent episodes underscore the persistent low-level seismic productivity of the San Jacinto Fault near Anza, where strain accumulation and release occur through both isolated events and clustered activity.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Southern California Seismic Network (scsn.org)
California Geological Survey Fault Activity Map