Seismic Swarm S20110416.1: Analysis of Activity Near Anza, California

Seismic swarms represent clusters of earthquakes occurring in rapid succession without a dominant mainshock, often linked to fluid migration or stress changes along fault systems. Swarm S20110416.1 was recorded from 11:39 on 15 April 2011 to 18:11 on 22 April 2011, centered 21 km east-southeast of Anza, California. Over 174 hours and 31 minutes, 90 events were detected, with magnitudes ranging from -0.1 to 3.1 and focal depths primarily between 3 km and 13 km.

The sequence began with two events exceeding magnitude 3.0 within the first ten minutes on 15 April, followed by a steady decline in event rate and size. Notable later activity included a magnitude-2.6 event at 2 km depth on 18 April and a magnitude-2.1 event at 11 km depth on 22 April. Depths remained shallow throughout, consistent with activity in the upper crust. The temporal distribution showed the highest frequency on the first day, tapering over subsequent days, with no single event dominating the energy release.

The Anza region 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 boundary motion in Southern California. This zone connects with the Elsinore Fault to the southwest and interacts with the San Andreas Fault to the northeast. The area experiences frequent small-magnitude seismicity due to its position along multiple fault strands and subsidiary structures. Historical records indicate persistent swarm activity, with 15 documented swarms since 2000, occurring in 2001 (1 swarm), 2002 (2), 2003 (2), 2005 (2), 2009 (1), 2010 (5), and 2011 (2). These episodes highlight the zone’s tendency toward clustered rather than isolated events.

Such swarms contribute to ongoing strain release without producing large destructive earthquakes in recent decades. Depths in the 3–13 km range align with the brittle-ductile transition typical for this portion of the fault zone. The 2011 swarm’s spatial concentration near Anza reinforces patterns observed in prior episodes, where activity remains localized along fault segments prone to fluid-assisted triggering.

This event underscores the value of dense seismic networks in monitoring low-magnitude activity that may precede larger ruptures. Continued observation of swarm statistics in the San Jacinto Fault Zone aids in refining probabilistic hazard assessments for the region.

References

USGS Earthquake Hazards Program – San Jacinto Fault Zone overview
California Geological Survey – Regional fault maps and seismicity summaries
SeismoSight internal swarm classification records