Seismic Swarm VS20201113.1: Activity Near Calipatria, California
A seismic swarm designated VS20201113.1 occurred 11 km west-northwest of Calipatria in Imperial County, California. The sequence began at 05:41 UTC on 13 November 2020 and concluded at 04:54 UTC on 14 November 2020, spanning 23 hours and 12 minutes. During this interval, 38 earthquakes were recorded.
The events ranged in magnitude from 0.3 to 2.6, with the largest shock measuring 2.6 at a depth of 5 km. Most hypocenters clustered between 3 km and 5 km depth, though a few reached 8 km. Activity was concentrated in the first several hours, with diminishing frequency after midday on 13 November. The final recorded event was a magnitude 1.1 earthquake at 8 km depth.
This swarm took place within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the divergent boundary between the Pacific and North American plates. The Imperial Valley hosts numerous northwest-trending strike-slip faults, including the Brawley Seismic Zone, which accommodates right-lateral shear and produces frequent earthquake swarms. Geothermal gradients in the region, linked to shallow magmatic intrusions beneath the Salton Sea, further contribute to episodic seismic unrest through fluid migration and pore-pressure changes.
Imperial Valley seismicity has been documented for decades. Since 1 January 2000, 84 swarms have been identified in the broader area. Annual counts show notable variation, with peaks in 2010 (13 swarms) and 2013 (13 swarms), and lower activity in intervening years such as 2014 and 2015 (one swarm each). The 2020 total reached four swarms prior to VS20201113.1.
Swarm sequences in this setting typically lack a single dominant mainshock and instead reflect distributed slip along multiple small fault segments. Depths recorded in VS20201113.1 align with the shallow seismogenic zone characteristic of the Imperial Valley, where brittle failure occurs above approximately 10 km. The modest magnitudes and rapid onset followed by decay are consistent with fluid-driven triggering observed in prior regional episodes.
Ongoing monitoring by regional networks continues to track microseismicity in the Brawley Seismic Zone. Such data contribute to refined models of strain accumulation and release along the southern San Andreas Fault system, supporting improved hazard assessments for nearby communities and infrastructure.
References
SeismoSight internal swarm catalog VS20201113.1
USGS Earthquake Catalog (regional Imperial Valley events)
California Geological Survey, Quaternary Fault and Fold Database