Seismic Swarm S20130611.1 Near Coso Junction, California

The seismic swarm designated S20130611.1 occurred approximately 10 km east-northeast of Coso Junction in California. It began at 11:07 on 10 June 2013 and concluded at 05:32 on 14 June 2013, spanning 90 hours and 25 minutes. During this interval, 43 earthquakes were recorded. Event magnitudes ranged from 0.0 to 1.2, with the majority below 1.0 and focal depths predominantly between 0 and 6 km. The sequence featured clusters of activity, including multiple events on 10 June between 23:49 and 23:58, followed by additional activity on 11 June around 05:38–05:51 and later on 12–13 June.

This swarm reflects typical patterns in the Coso region, where small-magnitude events often occur in rapid succession without a dominant mainshock. Depths remained shallow throughout, consistent with activity influenced by local crustal structures. No events exceeded magnitude 1.2, indicating low energy release overall.

The Coso Volcanic Field lies within the eastern California shear zone, a tectonically active area characterized by dextral strike-slip faulting and extensional features associated with the broader Basin and Range province. The field encompasses Pleistocene rhyolitic domes and flows, with the most recent volcanic activity occurring tens of thousands of years ago. Ongoing geothermal activity supports commercial power production at the Coso Geothermal Field, where fluid circulation interacts with fault systems to generate microseismicity. Regional geology includes fractured crystalline basement rocks intruded by younger volcanic materials, creating pathways for both hydrothermal fluids and seismic energy release.

Seismic swarms have been recurrent in this setting since at least 2000, with 42 documented episodes through 2013. Yearly occurrences varied, including four in 2000, four in 2001, two in 2002, nine in 2004, two in 2005, six in 2006, one in 2009, seven in 2010, one in 2011, four in 2012, and two in 2013. These episodes underscore persistent strain accumulation along local faults and possible contributions from geothermal reservoir dynamics.

Monitoring of such swarms aids in distinguishing tectonic from geothermal-induced events. The 2013 sequence aligns with the area's established background rate of low-magnitude activity, providing data for refined hazard assessments in this geothermal-tectonic transitional zone.

References

USGS Earthquake Hazards Program catalog data for California events.
California Geological Survey reports on the Coso Volcanic Field and geothermal systems.
Peer-reviewed studies on eastern California shear zone tectonics published in journals such as Journal of Geophysical Research.