Seismic Swarm VS20120219.1 Near Calipatria, California

The seismic swarm designated VS20120219.1 occurred 11 km west-northwest of Calipatria in Imperial County, California. It began at 05:54 on 19 February 2012 and concluded at 00:52 on 21 February 2012, encompassing 86 earthquakes over 42 hours and 57 minutes. This event took place within the tectonically active Salton Trough, a pull-apart basin formed by the interaction of the San Andreas Fault system and the Imperial Fault. The region experiences frequent seismicity due to right-lateral strike-slip faulting and associated geothermal activity linked to the underlying crustal spreading.

Earthquake depths in the swarm ranged primarily from 0 to 5 km, with the majority clustered at shallow levels of 0–3 km. Magnitudes varied from 0.2 to 3.0, indicating a typical swarm pattern of numerous small events without a single dominant mainshock. The largest event reached magnitude 3.0 at 19:37 on 20 February, followed closely by a magnitude 2.7 shock minutes later. Activity peaked during the afternoon and evening of 19 February and again on 20 February, with bursts of events separated by quieter intervals.

The Imperial Valley, including the area around Calipatria and the Salton Sea, lies at the southern terminus of the San Andreas Fault. Geological studies confirm ongoing extension and subsidence, with the Salton Trough accumulating sediments from the Colorado River while accommodating Pacific-North America plate motion. Historical records document recurrent earthquake swarms in this zone, often associated with fluid migration in geothermal reservoirs and minor fault adjustments.

Since 1 January 2000, 43 swarms have been recorded in the region. Yearly counts include one each in 2000, 2001, 2002, and 2004; three each in 2003 and 2005; four in 2008; eleven in 2009; thirteen in 2010; four in 2011; and one in 2012 up to the VS20120219.1 event. These swarms underscore the persistent seismic character of the Brawley Seismic Zone, where clusters of events commonly occur without producing significant surface rupture.

Analysis of VS20120219.1 reveals a compact spatial footprint and rapid onset followed by gradual decay, consistent with swarm behavior driven by localized stress changes rather than a classic foreshock-mainshock-aftershock sequence. Depths remained within the upper crust, aligning with the brittle-ductile transition zone influenced by elevated geothermal gradients in the Salton Trough.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• California Geological Survey, Imperial Valley regional reports
• SeismoSight internal swarm classification data