Seismic Swarm S20101213.1 Near Brawley, California

Seismic swarm S20101213.1 was recorded on 13 December 2010, beginning at 07:34 and concluding at 20:18 local time. The sequence occurred 6 km NNW of Brawley in Imperial County, California, and comprised 49 earthquakes over a span of 12 hours and 44 minutes. Magnitudes ranged primarily between 1.1 and 2.2, with the largest events reaching 2.2 at a depth of 9 km. Focal depths varied from the surface to 15 km, though most clustered between 4 km and 10 km. The events exhibited a typical swarm pattern of closely spaced occurrences without a dominant mainshock, reflecting distributed strain release along local fault structures.

The Brawley area lies within the Imperial Valley portion of the Salton Trough, an active rift zone influenced by the transition between the San Andreas Fault and the Imperial Fault. This setting features right-lateral strike-slip tectonics combined with extensional components and geothermal activity that facilitate fluid migration and swarm triggering. Earthquake swarms are a recurring phenomenon here, often linked to aseismic slip and pore-pressure changes rather than classic mainshock-aftershock sequences.

Historical records maintained since 1 January 2000 document 37 swarms in the region. Yearly counts include two in 2000, one in 2001, one in 2002, four in 2003, one in 2004, three in 2005, five in 2008, eleven in 2009, and nine in 2010. These episodes underscore the persistent seismic productivity of the Brawley Seismic Zone, where swarms can occur multiple times per year without producing damaging ground motion.

Analysis of the 13 December 2010 swarm shows a rapid onset with multiple events in the first hour, followed by sustained activity through midday and a gradual decline toward evening. Depths remained shallow to moderate, consistent with the brittle upper crust in this transtensional environment. No events exceeded magnitude 3.0, aligning with the low-to-moderate energy release characteristic of Imperial Valley swarms.

Such sequences contribute to ongoing monitoring efforts aimed at understanding fault interactions and precursory signals in tectonically complex zones. Continued observation supports regional hazard assessment for the Imperial Valley, where proximity to critical infrastructure and the Salton Sea adds importance to detailed swarm characterization.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional tectonic summaries for the Imperial Valley.