Seismic Swarm VS20150521.1 Near Calipatria, California

The seismic swarm designated VS20150521.1 occurred 12 km west-northwest of Calipatria in Imperial County, California. It began at 03:15 on 21 May 2015 and concluded at 16:22 on 22 May 2015, spanning 37 hours and 6 minutes. During this interval, 38 earthquakes were recorded. The largest event reached magnitude 4.1 at a depth of 5 km on 21 May at 03:15:29 UTC. Subsequent activity included a magnitude 3.3 event at 3 km depth on 21 May at 05:06:12 and a magnitude 3.2 event at 3 km depth on 21 May at 03:51:01. Most events clustered between 1 km and 5 km depth, with a few reaching 8 km.

This swarm unfolded within the tectonically active Salton Trough, a pull-apart basin formed by the interaction of the Pacific and North American plates along the San Andreas Fault system. The area lies in the Brawley Seismic Zone, characterized by right-lateral strike-slip faulting and occasional normal faulting. Frequent earthquake swarms here often relate to fluid migration in the crust, influenced by the geothermal fields near the Salton Sea. Depths typically remain shallow, reflecting the thin crust and high heat flow in the Imperial Valley.

Historical records indicate 69 swarms in the region since 1 January 2000. Annual counts show variability, with peaks in 2010 (13 swarms) and 2013 (13 swarms), followed by lower activity in 2014 (1 swarm). Earlier years included 11 swarms each in 2009 and 2012. These patterns suggest episodic clustering tied to regional stress accumulation and release along local faults.

Key events within the swarm progressed rapidly after the initial magnitude 4.1 shock. A magnitude 2.8 followed within minutes at 3 km depth, succeeded by multiple events between magnitude 1.3 and 2.7 over the first several hours. Activity continued at lower magnitudes through the afternoon and evening of 21 May, with isolated events on 22 May tapering to a final magnitude 0.5 at 5 km depth. Depths remained predominantly under 5 km, consistent with shallow crustal processes in this geothermal setting.

The Imperial Valley experiences elevated seismicity due to its position at the southern terminus of the San Andreas Fault and proximity to the Cerro Prieto Fault in Mexico. Swarm sequences like VS20150521.1 contribute to understanding strain release without producing a single dominant mainshock. No significant surface rupture or damage was associated with this event, aligning with the modest magnitudes observed.

References

USGS Earthquake Catalog California Geological Survey regional reports SeismoSight internal swarm classification data