Seismic Swarm Near Calipatria, California: February 2024 Analysis

A seismic swarm designated S20240214.3 began at 02:56 on 14 February 2024 and concluded at 14:58 on 15 February 2024, centered 9 km west of Calipatria in Imperial County, California. Within 36 hours and 2 minutes, 31 earthquakes were recorded. Magnitudes ranged from 1.0 to 4.0, with the largest event (magnitude 4.0) occurring at 07:53 on 14 February at a depth of 9 km. Most events clustered at depths of 7–9 km, consistent with shallow crustal activity in the region, though a few reached 17 km.

The sequence initiated with a magnitude 1.3 event at 3 km depth, followed by a rapid increase in activity. Within the first hour after the magnitude 4.0 shock, multiple events of magnitude 2.0–2.5 occurred at similar depths around 7–9 km. Activity continued through the afternoon and evening of 14 February, with events such as a magnitude 2.0 at 13:03 and a magnitude 1.6 at 19:11. On 15 February, seismicity tapered off, ending with a magnitude 1.0 event at 5 km depth.

This swarm unfolded within the Brawley Seismic Zone, part of the Salton Trough in the Imperial Valley. The area forms a tectonically active pull-apart basin at the southern terminus of the San Andreas Fault system, where the Pacific and North American plates interact along a network of strike-slip and normal faults. High heat flow linked to the underlying magmatic system and geothermal fields contributes to frequent shallow seismicity. Earthquake swarms here often arise from fluid migration or aseismic slip rather than mainshock-aftershock sequences.

Imperial Valley seismicity has long been documented, with swarms reflecting the zone’s distributed faulting. Since 2000, 100 swarms have been identified in the vicinity, distributed across multiple years with peaks in 2010 and 2013 (13 each). Earlier notable periods include 2009 and 2012 with 11 swarms apiece. These episodes typically produce low-to-moderate magnitudes without a single dominant mainshock.

The February 2024 swarm aligns with this established pattern, remaining confined to small-to-moderate events at typical depths for the zone. No larger triggered earthquakes were observed during the 36-hour window.

References

USGS Earthquake Hazards Program – Imperial Valley regional tectonics
California Geological Survey – Salton Trough fault mapping
Southern California Seismic Network – Brawley Seismic Zone reports