Seismic Swarm S20210208.1: Analysis of Activity Near Ocotillo, California
An earthquake swarm designated S20210208.1 was recorded 17 km east of Ocotillo, California, beginning at 23:49 UTC on 7 February 2021 and concluding at 03:16 UTC on 12 February 2021. Over 99 hours and 26 minutes, the sequence produced 49 earthquakes. The events clustered in a compact area within the tectonically active Salton Trough, a region shaped by the interaction between the Pacific and North American plates along the southern San Andreas Fault system and subsidiary faults of the Imperial Valley.
The swarm exhibited typical characteristics of swarm behavior, with no single dominant mainshock followed by aftershocks. Magnitudes ranged from 0.4 to a peak of 3.4, recorded at 08:00 UTC on 8 February at a depth of 11 km. Subsequent notable events included a magnitude 2.8 quake at 07:55 UTC on 10 February (depth 10 km) and a magnitude 2.5 event at 13:03 UTC on 8 February (depth 11 km). Depths throughout the sequence varied between 1 km and 14 km, with many events occurring between 7 km and 11 km, consistent with shallow crustal faulting in the region.
Geologically, the Ocotillo area lies within the Imperial Valley portion of the Salton Trough, an extensional basin formed by oblique rifting. The local fault network includes strands of the Elsinore Fault Zone to the west and the Coyote Creek Fault, alongside influences from the Brawley Seismic Zone to the southeast. This setting promotes episodic swarm activity driven by fluid migration and aseismic slip rather than large locked-fault ruptures. Historical records indicate elevated microseismicity, with the region experiencing frequent low-magnitude sequences that release strain without producing damaging earthquakes.
Swarm activity in this locale has been documented repeatedly. Since 1 January 2000, 23 swarms have occurred in the broader area. Prior episodes took place in 2006 (three swarms), 2007 (one), 2009 (three), 2010 (thirteen), 2011 (one), 2019 (one), and 2021 (one, encompassing the present sequence). These recurrent patterns underscore the persistent tectonic stress and structural complexity of the Imperial Valley.
The 2021 swarm remained modest in scale, with the largest event below the threshold for felt damage in nearby communities. Such sequences provide valuable data for refining models of fault interaction and strain accumulation along the plate boundary. Continued monitoring supports improved understanding of seismic hazards in Southern California’s southernmost fault systems.