Seismic Swarm S20120521.3 Near Anza, California: Geological Context and Event Analysis

The seismic swarm designated S20120521.3 occurred 15 km southeast of Anza, California, in the San Jacinto Fault Zone. It began at 14:37 on 20 May 2012 and concluded at 16:43 on 24 May 2012, spanning 98 hours and 6 minutes. During this period, 45 earthquakes were recorded, with magnitudes ranging from -0.2 to 3.3 and focal depths primarily between 7 and 15 km.

This swarm exemplifies typical microseismicity in the region. The largest event, magnitude 3.3, occurred on 21 May at 06:19:58 UTC at a depth of 13 km. Subsequent activity included multiple events above magnitude 1.0 clustered within the first 48 hours, followed by a gradual decline. Depths remained consistent around 10–14 km for most events, indicating activity within the seismogenic zone of the fault system.

The Anza area lies within the Peninsular Ranges of Southern California, where the San Jacinto Fault Zone forms a major right-lateral strike-slip system. This zone accommodates a significant portion of the Pacific–North American plate boundary motion, with slip rates estimated at 10–15 mm per year. The local geology features Mesozoic granitic and metamorphic rocks overlain by Quaternary alluvial sediments. High heat flow and fluid migration along fault planes contribute to swarm behavior by promoting episodic slip on small fault patches.

Historically, the Anza segment has hosted numerous swarms. Since 2000, 15 such sequences have been documented, with notable activity in 2010 (five swarms) and 2011 (three swarms). Earlier clusters occurred in 2001, 2002, 2003, 2005, and 2009. These recurrent swarms reflect the fault zone’s tendency toward distributed, low-magnitude seismicity rather than large single ruptures.

Analysis of S20120521.3 reveals a classic swarm pattern: rapid onset, peak activity within 24 hours, and exponential decay without a clear mainshock-aftershock sequence. Most events clustered at depths of 12–13 km, consistent with the brittle-ductile transition in this tectonically active setting. No surface rupture or significant ground deformation was associated with the sequence.

Such swarms provide valuable data for understanding fault mechanics and improving seismic hazard models in Southern California. Continued monitoring by regional networks supports refined assessments of the San Jacinto Fault’s potential for larger events.

References

USGS Earthquake Hazards Program – San Jacinto Fault Zone overview
Southern California Seismic Network – Regional fault characteristics and swarm documentation
California Geological Survey – Peninsular Ranges geology and seismicity reports