Seismic Swarm VS20201127.1: Analysis of Activity Near Calipatria, California
A seismic swarm designated VS20201127.1 occurred 10 km northwest of Calipatria, California, from 20:22 on 26 November 2020 to 06:43 on 28 November 2020. Over 34 hours and 20 minutes, the event registered 27 earthquakes, with magnitudes ranging from 0.9 to 3.5 and focal depths predominantly between 3 and 5 km. One outlier event reached 10 km depth.
The swarm began with a pair of magnitude 1.9 events at 3 km depth within the first 15 minutes. Activity continued with events clustered in the low-to-mid magnitude range, culminating in the largest shock of magnitude 3.5 at 22:59 on 26 November. Subsequent events remained below magnitude 2.2, indicating a rapid decay in energy release after the peak. Depths stayed shallow throughout, consistent with the tectonic regime of the region.
The Imperial Valley lies within the Salton Trough, a tectonically active pull-apart basin formed by the interaction of the San Andreas Fault system and the Imperial Fault. This setting produces frequent microseismicity and swarm sequences driven by right-lateral strike-slip motion and localized geothermal fluid migration. The Brawley Seismic Zone, immediately north of the swarm epicenter, is recognized for similar clustered activity linked to both tectonic strain and hydrothermal processes.
Historical records since 1 January 2000 document 81 swarms in the immediate area. Annual counts show notable variability, with peaks of 13 events each in 2010 and 2013, followed by lower activity in intervening years. The five swarms recorded in 2020, including VS20201127.1, align with the long-term pattern of episodic swarm behavior rather than isolated mainshock-aftershock sequences.
Shallow depths and modest magnitudes in this swarm reflect typical background strain release along the Imperial-Brawley fault network. No surface rupture or significant infrastructure impact was associated with the sequence.
References
USGS Earthquake Catalog
California Geological Survey Regional Fault Maps
Southern California Seismic Network Bulletins