Seismic Swarm Near Anza, California: December 2013–January 2014

The Anza region in Riverside County, Southern California, sits within the San Jacinto Fault Zone, a major component of the Pacific–North American plate boundary. This right-lateral strike-slip system accommodates roughly 20 percent of the relative plate motion and has produced numerous moderate to large earthquakes historically. The zone trends northwest–southeast through rugged terrain of the Peninsular Ranges, where crystalline basement rocks are cut by several active strands, including the Coyote Creek, Clark, and Buck Ridge faults. Seismicity here is frequent and often occurs as clusters or swarms rather than isolated mainshock–aftershock sequences.

Between 19:39 UTC on 30 December 2013 and 13:48 UTC on 8 January 2014, a swarm of 91 earthquakes was recorded approximately 19 km east-southeast of Anza. The sequence lasted 210 hours and 9 minutes. Event magnitudes remained modest, with the largest reaching 3.0 on 31 December; most events fell between 0.0 and 1.6. Focal depths clustered between 5 km and 15 km, consistent with the brittle seismogenic zone along the San Jacinto Fault. Activity began with a single magnitude-0.7 event at 8 km depth, remained low for several hours, then accelerated on 31 December with the magnitude-3.0 shock and numerous smaller after-events. Subsequent days showed episodic bursts, including a magnitude-2.5 event late on 5 January and a magnitude-1.6 event on 7 January, before the sequence tapered off.

Such swarms are characteristic of the Anza segment. Since 1 January 2000 the area has hosted 21 documented swarms, with elevated rates in 2010 (five swarms), 2011 (three), 2012 (three), and 2013 (three). Earlier clusters occurred in 2001, 2002, 2003, 2005, and 2009. These episodes typically involve hundreds of microearthquakes over days to weeks without a dominant mainshock, suggesting transient increases in pore-fluid pressure or aseismic slip that trigger distributed failure on fault segments.

The 2013–2014 swarm fits this pattern. Its spatial footprint remained compact, with events distributed along a short section of the fault zone. Depth distribution showed a slight tendency for deeper events (12–14 km) during the middle of the sequence, possibly indicating progressive failure downward along a steeply dipping structure. No surface rupture or significant damage was reported, consistent with the low magnitudes involved.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the Anza gap, a section of the San Jacinto Fault that last ruptured in a major event more than 200 years ago. Swarms like S20131231.1 provide valuable data on fault-zone properties and may serve as indicators of changing stress conditions, although they do not reliably forecast larger earthquakes.

References
USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Southern California Seismic Network annual reports
California Geological Survey fault-zone maps