Seismic Swarm S20210319.1: Analysis of Activity Near Calipatria, California

Seismic swarm S20210319.1 was recorded in the Imperial Valley region of Southern California. The sequence began at 23:03 UTC on 18 March 2021 and concluded at 13:53 UTC on 19 March 2021, spanning 14 hours and 49 minutes. A total of 27 earthquakes were registered during this period, centered approximately 12 km west-northwest of Calipatria.

The events exhibited magnitudes ranging from 1.1 to 2.3, with the majority falling between 1.4 and 1.9. Depths were predominantly shallow, concentrated between 3 km and 6 km, although a few reached up to 11 km. The sequence featured several notable shocks, including a magnitude 2.3 event at 00:56 on 19 March at 2 km depth, alongside multiple magnitude 1.8–2.1 events clustered within the first hour after initiation. Activity tapered off after the initial intense phase, with isolated events continuing until the swarm's termination.

This swarm aligns with patterns observed in the Brawley Seismic Zone, where clusters of small-magnitude earthquakes frequently occur without a single dominant mainshock. Such swarms reflect the region's tectonic setting within the Salton Trough, a transtensional basin formed by oblique extension along the Pacific–North American plate boundary. The Imperial Valley lies at the southern terminus of the San Andreas Fault system, where right-lateral strike-slip motion transitions into the Imperial and Brawley faults. Ongoing rifting contributes to elevated heat flow and shallow seismicity, often linked to fluid migration or magmatic processes beneath the Salton Sea geothermal field.

Historically, the Imperial Valley has experienced recurrent seismic swarms. Since 2000, 87 such swarms have been documented in the area, with notable concentrations in 2009 (11 events), 2010 (13), 2012 (11), and 2013 (13). Earlier activity includes isolated swarms in 2000–2005, followed by increased frequency from 2008 onward. These episodes underscore the persistent strain accumulation and release characteristic of the region, punctuated by larger events such as the 1940 El Centro earthquake (magnitude 6.9) and the 1979 Imperial Valley earthquake (magnitude 6.4), both of which produced surface rupture along nearby faults.

The geological framework supports continued monitoring, as the combination of transform faulting and geothermal activity sustains elevated seismicity rates. Data from this swarm provide further insight into the spatiotemporal clustering typical of Imperial Valley sequences.

References

USGS Earthquake Hazards Program regional reports on the Imperial Valley and Brawley Seismic Zone.
California Geological Survey tectonic summaries for the Salton Trough.
Peer-reviewed literature on San Andreas Fault system dynamics in Southern California.