Seismic Swarm VS20000512.1: Analysis of Activity Near Calipatria, California

A notable earthquake swarm, designated VS20000512.1, occurred in the Imperial Valley of southern California between 18:49 UTC on 11 May 2000 and 04:51 UTC on 14 May 2000. The sequence lasted 58 hours and 2 minutes, during which 52 earthquakes were recorded 13 km west-northwest of Calipatria. Magnitudes ranged from 0.0 to 2.9, with the largest event occurring at 23:39 on 12 May. Depths were predominantly shallow, clustered between 2 km and 5 km, consistent with activity in the upper crust, although a few events reached 15–18 km.

The swarm exhibited typical characteristics of clustered seismicity, with events distributed across multiple days. On 12 May alone, more than 30 earthquakes were detected, including several above magnitude 2.0. Activity declined sharply by 14 May, with only two final events recorded. Depths remained stable in the 3–5 km range for the majority of events, suggesting a shallow source zone influenced by local crustal conditions rather than deeper tectonic processes.

The Calipatria area lies within the Brawley Seismic Zone, part of the Salton Trough—a tectonically active pull-apart basin formed by the interaction between the San Andreas Fault to the north and the Imperial Fault to the south. This region experiences frequent earthquake swarms driven by a combination of strike-slip faulting, geothermal fluid circulation, and possible magmatic intrusions beneath the Salton Sea geothermal field. Historical records show repeated swarm episodes throughout the 20th and 21st centuries, reflecting the high strain rate and elevated heat flow of the Imperial Valley.

Seismic monitoring in this zone benefits from dense instrumentation operated by the USGS and the Southern California Seismic Network. Updated assessments confirm that swarm activity remains a persistent feature, with similar clusters documented in subsequent years. Depths and magnitudes observed in VS20000512.1 align with patterns seen in other Imperial Valley swarms, where events rarely exceed magnitude 3.0 but can occur in rapid succession.

This sequence provides insight into the localized stress release and fluid-driven mechanisms typical of the Brawley zone. Continued monitoring supports improved understanding of how such swarms relate to the broader transform boundary between the Pacific and North American plates.

References

USGS Earthquake Catalog California Geological Survey Southern California Seismic Network reports