Seismic Swarm S20120622.1: Activity Near Anza, California
Seismic swarm S20120622.1 was recorded 16 km east-southeast of Anza in Riverside County, California. The sequence began at 16:12 on 21 June 2012 and concluded at 11:28 on 25 June 2012, spanning 91 hours and 15 minutes. During this interval, 39 earthquakes were registered.
The events displayed magnitudes between 0.0 and 2.8, with the majority falling below 1.0. Depths ranged primarily from 5 km to 15 km. The largest event, magnitude 2.8, occurred on 22 June 2012 at 15:11:48 at a depth of 13 km. Other notable shocks included a magnitude 1.4 event on 23 June at 15:54:33 (10 km depth) and several magnitude 1.0–1.2 events distributed across the period. Activity was concentrated in the first two days, with a gradual decline thereafter.
This swarm exemplifies typical clustered seismicity in the region, where numerous small events occur without a dominant mainshock. Such sequences are characteristic of the area’s fault systems and often reflect fluid migration or stress adjustments along fault segments.
The Anza region lies within the Peninsular Ranges geomorphic province of Southern California, at the transition between the San Jacinto Fault Zone and adjacent strands of the San Andreas system. The San Jacinto Fault Zone accommodates a significant portion of the Pacific–North American plate boundary slip and exhibits one of the highest rates of seismicity in the state. Historical records document recurrent moderate earthquakes and frequent microseismicity driven by right-lateral strike-slip motion.
Since 2000, seventeen swarms have been identified in the immediate vicinity, occurring in 2001 (1), 2002 (2), 2003 (1), 2005 (2), 2009 (2), 2010 (5), 2011 (3), and 2012 (1). These episodes underscore the persistent, distributed nature of strain release along the fault network.
Ongoing monitoring by regional seismic networks continues to provide high-resolution data on such swarms, contributing to improved understanding of fault mechanics and hazard assessment in this tectonically active corridor.
References:
USGS Earthquake Catalog
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center records