Seismic Swarm S20120630.1: Analysis of Activity Near Anza, California

Seismic swarm S20120630.1 occurred 19 km east-southeast of Anza, California, within the tectonically active Peninsular Ranges province. The sequence began at 08:44 on 29 June 2012 and concluded at 08:58 on 1 July 2012, spanning 48 hours and 14 minutes. During this interval, 31 earthquakes were recorded, with magnitudes ranging from -0.1 to 2.7 and focal depths primarily between 7 km and 16 km.

The largest events reached magnitude 2.7 at depths of 13 km on 30 June 2012 at 03:20 and 03:21 UTC. Most events clustered at depths of 13–14 km, consistent with activity along shallow crustal structures. Smaller events exhibited a range of depths, including several at 7 km, indicating minor vertical migration within the fault zone. The temporal distribution showed an initial cluster on 29 June followed by a peak in frequency and magnitude on 30 June, after which activity declined sharply.

This swarm aligns with the broader seismotectonic setting of the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of Pacific–North American plate motion. The Anza region experiences recurrent earthquake swarms due to fluid migration and stress transfer along fault segments. Depths observed in S20120630.1 fall within the typical seismogenic zone for this area, where brittle failure occurs above the brittle-ductile transition.

Historical records indicate 20 swarms in the vicinity since 1 January 2000. Prior episodes occurred in 2001 (1 swarm), 2002 (2), 2003 (2), 2005 (2), 2009 (2), 2010 (5), 2011 (4), and 2012 (2). These sequences underscore the persistent swarm-prone character of the local fault network, often without a single dominant mainshock.

References

USGS Earthquake Catalog
Southern California Seismic Network archives
SeismoSight internal swarm classification database