Seismic Swarm VS20210209.1: Analysis of Activity Near Calipatria, California

An earthquake swarm designated VS20210209.1 was recorded 12 km west-northwest of Calipatria, California, in the Imperial Valley. The sequence began at 08:33 UTC on 8 February 2021 and concluded at 18:27 UTC on 11 February 2021, spanning 81 hours and 54 minutes. During this period, 117 earthquakes were detected.

Analysis of the first 100 events reveals a typical swarm pattern dominated by small-magnitude shocks. Magnitudes ranged from 0.5 to 3.8, with the majority below 2.0. Depths concentrated between 3 km and 7 km, indicating shallow crustal activity. A notable escalation occurred early on 10 February, when multiple events exceeded magnitude 3.0, including peaks of 3.8 and 3.7. These larger shocks clustered within minutes of one another, accompanied by numerous aftershocks of magnitude 1.0–2.0 at similar depths. The temporal distribution shows initial sparse activity on 8 February, increasing frequency and intensity through 9 February, followed by the peak cluster on 10 February before gradual decay.

This swarm occurred within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas and Imperial fault systems. The region experiences frequent swarm sequences due to transtensional stress, geothermal fluid migration, and volcanic processes beneath the Salton Sea. Historical records document 85 swarms in the area since 2000, with notable concentrations in 2009 (11 swarms), 2010 (13), 2012 (11), and 2013 (13). Such episodic activity reflects the ongoing accommodation of Pacific–North American plate motion across a complex network of strike-slip and normal faults.

The 2021 swarm aligns with established patterns in the Brawley Seismic Zone, where shallow seismicity often occurs without a single dominant mainshock. Depths and magnitudes observed here are consistent with prior sequences driven by fluid pressure changes rather than large tectonic rupture. No surface rupture or significant damage was associated with these events.

Continued monitoring remains essential given the Imperial Valley’s proximity to critical infrastructure and the Salton Sea geothermal field. Understanding swarm dynamics contributes to improved seismic hazard assessment in this high-strain-rate corridor.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Imperial Valley regional reports
SCEC Community Fault Model documentation