Seismic Swarm S20210605.1: Analysis of Activity Near Calipatria, California
The seismic swarm designated S20210605.1 was recorded 11 km west of Calipatria in California's Imperial Valley. It began at 19:53 on 4 June 2021 and concluded at 23:55 on 23 June 2021, spanning 460 hours and 1 minute. During this interval, 2,347 earthquakes were registered, characteristic of swarm behavior in this tectonically active region.
Geological Setting
The Imperial Valley lies within the Salton Trough, a pull-apart basin formed by the interaction of the Pacific and North American plates along the San Andreas Fault system. This area overlaps the Brawley Seismic Zone, where right-lateral strike-slip faults and geothermal activity contribute to frequent earthquake swarms. Depths typically range from 1 to 10 km, reflecting shallow crustal processes influenced by both tectonic extension and fluid migration from the underlying magmatic system beneath the Salton Sea.
Swarm Characteristics and First 100 Events
Analysis of the initial 100 events reveals a rapid onset following the swarm's start. These occurred primarily between 19:53 on 4 June and 15:00 on 5 June 2021. Magnitudes ranged from 0.5 to 2.7, with the majority below 2.0. Notable peaks included events of M2.7 at 12:21:57, M2.6 at 12:18:37, and M2.5 at 12:18:58 on 5 June. Depths clustered between 3 and 7 km, indicating shallow sources consistent with the region's fault network.
The sequence showed clustered bursts, with multiple events occurring within minutes, such as four shocks between 10:29 and 10:31 on 5 June. This pattern suggests episodic stress release along interconnected faults, common in Imperial Valley swarms where fluid pressure may trigger successive ruptures.
Historical Context
Imperial Valley has experienced persistent swarm activity. Since 2000, 88 swarms have been documented, with notable annual counts including 13 in both 2010 and 2013, and 11 each in 2009 and 2012. Earlier years showed lower frequencies, such as single swarms in 2000–2002 and 2004. The 2021 total reached five swarms, underscoring the zone's ongoing productivity.
Implications
Swarm S20210605.1 aligns with established patterns in the Brawley Seismic Zone, where events rarely exceed moderate magnitudes but contribute to cumulative strain release. Monitoring such sequences aids in understanding fault interactions and potential links to larger regional events along the San Andreas system.
References
SeismoSight internal swarm classification data.
USGS Earthquake Catalog for Imperial Valley tectonics.
California Geological Survey regional fault maps.