Seismic Swarm S20020225.1: Analysis of Activity Near Calipatria, California
Seismic swarm S20020225.1 was recorded 9 km west of Calipatria in California's Imperial Valley. The sequence began at 20:01 on 24 February 2002 and concluded at 22:02 on 25 February 2002, spanning 26 hours and encompassing 31 earthquakes.
The events clustered at shallow depths, predominantly between 4 km and 11 km, with a few occurrences near the surface. Magnitudes ranged from 1.1 to 3.2, indicating low-to-moderate energy release typical of swarm behavior rather than a single mainshock-aftershock sequence. The largest event, magnitude 3.2, occurred at 01:28 on 25 February at 7 km depth. A notable cluster of four magnitude 2.4 events took place within minutes around 18:20 on 25 February at depths of 0–2 km. Earlier activity featured repeated magnitude 1.7 shocks at consistent 5 km depths, suggesting localized fault slip along minor structures.
This swarm represents the third such episode in the region since 2000, following single swarms in both 2000 and 2001. Such recurrent patterns align with the tectonic setting of the Imperial Valley, situated within the Salton Trough—a tectonically active pull-apart basin formed by right-lateral shear between the Pacific and North American plates. The area lies along the southern extension of the San Andreas fault system, including the Brawley Seismic Zone and nearby branches of the San Jacinto fault. Frequent small-magnitude seismicity here is driven by both tectonic strain accumulation and geothermal fluid migration associated with the Salton Sea Geothermal Field.
Historical records show that Imperial Valley swarms often exhibit similar characteristics: short durations, shallow focal depths, and magnitudes below 4.0, reflecting distributed deformation across a network of immature faults rather than rupture on a major through-going structure. Depths in this swarm, mostly 5–8 km, correspond to the brittle-ductile transition zone influenced by elevated geothermal gradients in the region.
The 2002 sequence provides additional evidence of ongoing microseismicity that helps accommodate plate-boundary motion without producing damaging ground shaking. Continued monitoring remains essential given the proximity to critical infrastructure and populated areas in the Imperial Valley.
References
- United States Geological Survey Earthquake Catalog
- California Geological Survey Regional Fault Maps
- Southern California Seismic Network Reports