Seismic Swarm VS20231209.1: Analysis of Activity Near Calipatria, California
A seismic swarm designated VS20231209.1 occurred 14 km west-northwest of Calipatria in California's Imperial Valley. The sequence began at 18:14 on 9 December 2023 and concluded at 12:42 on 10 December 2023, spanning 18 hours and 27 minutes. During this interval, 34 earthquakes were recorded.
The events exhibited magnitudes between 0.6 and 3.3, with the largest shock reaching 3.3 at a depth of 3 km. Depths throughout the swarm ranged from 0 to 6 km, with the majority clustered between 2 and 4 km. Activity initiated with several events above magnitude 2.0 within the first 30 minutes, followed by a peak in the evening hours that included the strongest shocks. Later stages featured smaller-magnitude events that gradually diminished until the sequence ended the following day.
The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas and San Jacinto fault systems. This setting produces frequent swarm sequences driven by transtensional stresses and shallow magmatic or hydrothermal influences associated with the nearby Salton Sea geothermal field. The Brawley Seismic Zone, which encompasses the swarm location, is recognized for episodic clusters of small to moderate earthquakes rather than isolated mainshock-aftershock sequences.
Historical records indicate persistent swarm activity in the region. Since 1 January 2000, 95 swarms have been documented. Annual counts show notable concentrations in 2009 (11 swarms), 2010 (13), 2012 (11), and 2013 (13), with additional episodes recorded in 2020 (6) and 2021 (8). The 2023 total prior to this event stood at two swarms, underscoring the area's ongoing seismic productivity.
Such swarms typically pose low to moderate hazard because individual events rarely exceed magnitude 4.0, yet they provide valuable data on fault behavior and stress transfer within the broader plate-boundary system. Continued monitoring remains essential given the proximity to populated areas and critical infrastructure in the Imperial Valley.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
Southern California Seismic Network reports
California Geological Survey, Imperial Valley regional tectonics summary