Seismic Swarm S20220420.1: Analysis of Activity Near Brawley, California

The Brawley region in California's Imperial Valley sits within the tectonically active Salton Trough, a pull-apart basin formed by the interaction of the San Andreas Fault to the north and the Imperial Fault to the south. This area experiences frequent earthquake swarms due to extensional stresses and right-lateral shear along the Pacific-North American plate boundary. Depths of events in the zone typically range from 2 to 11 km, reflecting activity in the brittle upper crust overlying ductile materials influenced by high geothermal gradients.

Swarm S20220420.1 began at 16:32 on 19 April 2022 and concluded at 18:18 on 22 April 2022, approximately 7 km south of Brawley. Over 73 hours and 46 minutes, 71 earthquakes were recorded. Magnitudes ranged from 1.1 to 3.7, with the largest events occurring early in the sequence on 19 April. Depths clustered between 3 and 11 km, consistent with regional patterns where shallower events often accompany fluid migration or aseismic slip.

The sequence initiated with a magnitude 1.4 event at 4 km depth. Within hours, activity intensified, including back-to-back magnitude 3.6 and 3.7 shocks at 11 km depth around 20:22–20:30 on 19 April. Subsequent events showed a gradual decline in both frequency and magnitude, though clusters persisted into 21 April, highlighted by a magnitude 3.6 at 9 km depth. By 22 April, activity tapered to smaller events below magnitude 2.1. This temporal evolution—rapid onset followed by decay—aligns with typical swarm behavior in the Brawley Seismic Zone, where events often lack a dominant mainshock.

Historical records since 2000 indicate 54 swarms in the region, underscoring its persistent seismicity. Annual counts vary: 2013 recorded the highest at 11 swarms, while multiple years including 2000, 2002, 2005, 2008, and 2018 each saw one. Elevated activity occurred in 2010 (6 swarms), 2009 and 2020 (5 each), and 2021 (5). These patterns reflect ongoing tectonic loading and occasional triggering from distant events or local fluid dynamics.

Such swarms pose minimal immediate hazard but contribute to understanding fault interactions in this high-strain zone. Monitoring continues to refine models of swarm recurrence and potential links to larger earthquakes along the Imperial Fault system.

References

USGS Earthquake Hazards Program: Imperial Valley seismicity reports
California Geological Survey: Salton Trough tectonic summaries
Southern California Earthquake Data Center: Regional fault and swarm catalogs