Seismic Swarm S20140321.1 Near Coso Junction, California

A seismic swarm designated S20140321.1 occurred 10 km northeast of Coso Junction in California between 02:45 UTC on 20 March 2014 and 22:37 UTC on 21 March 2014. The sequence lasted 43 hours and 51 minutes and included 35 earthquakes. Events were recorded with magnitudes ranging from -0.3 to 1.8 and focal depths primarily between 0 and 8 km, indicating shallow crustal activity typical of the region.

The Coso Volcanic Field lies within the eastern California shear zone, where right-lateral strike-slip faulting accommodates part of the Pacific-North America plate motion. The area features Quaternary rhyolite domes, basaltic flows, and an active geothermal system driven by shallow heat sources. Seismicity frequently clusters in swarms linked to fluid migration along faults and fractures associated with the geothermal reservoir. Depths recorded during this swarm align with the brittle-ductile transition zone influenced by elevated temperatures from the underlying magmatic system.

Historical records document 42 swarms in the vicinity since 1 January 2000. Annual counts include four in 2000, four in 2001, two in 2002, seven in 2004, two in 2005, six in 2006, one in 2009, seven in 2010, one in 2011, four in 2012, and four in 2013. These recurrent swarms reflect persistent tectonic and hydrothermal stressing rather than isolated mainshock-aftershock sequences.

Within the 2014 swarm, the largest event reached magnitude 1.8 at 4 km depth on 21 March at 00:05 UTC. Most events remained below magnitude 1.0, with the majority occurring at depths of 0–1 km. The temporal distribution showed peak activity on 20 March, followed by a gradual decline. Such patterns are consistent with episodic fluid pressure changes in a geothermal setting.

The Coso region continues to experience low-level seismicity monitored by regional networks. Ongoing geothermal production and tectonic loading maintain the potential for future swarms. Analysis of event rates and depths provides insight into subsurface fluid pathways and stress conditions within the volcanic field.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Regional Geologic Maps
SCEC Community Fault Model Documentation