Seismic Swarm S20180618.1 Near Calipatria, California
The region 9 km west of Calipatria lies within California's Imperial Valley, part of the tectonically active Salton Trough. This pull-apart basin formed at the southern terminus of the San Andreas Fault system, where right-lateral strike-slip motion transitions between the San Andreas and Imperial faults. The Brawley Seismic Zone dominates local seismicity, producing frequent earthquake swarms driven by fluid migration, geothermal activity, and crustal extension. Depths typically range from 3 to 6 km, consistent with shallow brittle failure above a ductile lower crust influenced by high heat flow from the nearby Salton Sea geothermal field.
Swarm S20180618.1 began at 18:34 on 17 June 2018 and concluded at 03:39 on 19 June 2018, spanning 33 hours and 4 minutes. During this interval, 26 earthquakes were recorded. The sequence initiated with a magnitude 3.6 event at 5 km depth, followed by a magnitude 2.5 shock minutes later. Subsequent activity remained predominantly below magnitude 2.0, with notable events including a magnitude 2.9 at 23:20 on 17 June and a magnitude 2.3 at 03:13 on 18 June. Depths clustered between 3 and 6 km, reflecting the shallow seismogenic zone characteristic of the area.
This swarm fits the established pattern of Imperial Valley seismicity. Since 1 January 2000, 75 swarms have occurred in the region. Annual counts show marked variability: single swarms in 2000, 2001, 2002, 2004, 2014, 2015, and 2018; four each in 2003, 2011, and 2016; three in 2005; five in 2008; eleven each in 2009 and 2012; and thirteen each in 2010 and 2013. Such episodic clustering underscores the zone's tendency toward swarm-type sequences rather than isolated mainshock-aftershock patterns.
The June 2018 event produced no reported damage or felt reports beyond the immediate vicinity, aligning with the modest magnitudes involved. Ongoing monitoring by regional seismic networks continues to track microseismicity in this high-strain-rate corridor, where annual slip rates approach several millimeters along principal faults.
References
USGS Earthquake Hazards Program
California Geological Survey
Southern California Seismic Network catalog data