Earthquake Swarm S20101125.1 Near Coso Junction, California

The Coso region in eastern California lies within the Coso Volcanic Field, a geologically active area shaped by extensional tectonics at the boundary between the Sierra Nevada and the Basin and Range province. This setting features right-lateral shear and elevated heat flow driven by shallow magmatic processes, resulting in one of the most productive geothermal fields in the United States. The volcanic field includes rhyolite domes and basaltic flows from the Pleistocene and Holocene, with the most recent surface eruptions occurring tens of thousands of years ago. Ongoing geothermal activity sustains frequent microseismicity and occasional earthquake swarms as fluids migrate through fractured rock.

Swarm S20101125.1 began at 03:53 UTC on 25 November 2010 and concluded at 17:28 UTC on 26 November 2010, lasting 37 hours and 35 minutes. The events were centered 15 km east of Coso Junction. During this period, 36 earthquakes were recorded. Magnitudes ranged from 0.4 to 3.7, with the largest event (magnitude 3.7) occurring at 04:01:15 on 25 November at a depth of 1 km. A secondary peak reached magnitude 2.7 at 05:04:35 the same day. Depths were predominantly shallow, between 0 and 4 km, consistent with fluid-driven seismicity in the geothermal reservoir.

The sequence displayed typical swarm characteristics: a rapid onset, multiple events clustered in time, and no single dominant mainshock-aftershock pattern. On 25 November, activity concentrated in the morning hours, with 22 events before noon, followed by a lull and a smaller cluster in the evening. The final event on 26 November registered magnitude 0.7 at 17:28:32. Most events remained below magnitude 1.0, indicating low-energy release overall.

Since 1 January 2000, the Coso area has experienced 31 earthquake swarms. Yearly counts include four in 2000, four in 2001, two in 2002, seven in 2004, two in 2005, six in 2006, one in 2009, and five in 2010. This recurrence reflects persistent tectonic and hydrothermal conditions that periodically trigger swarms rather than isolated large events.

Such swarms provide valuable data for monitoring geothermal reservoir dynamics and assessing seismic hazard in the region. Continued observation helps distinguish between background tectonic activity and fluid-induced episodes.

References
SeismoSight internal swarm catalog (S20101125.1 parameters and historical counts).
USGS Earthquake Catalog for regional seismicity context.
California Geological Survey reports on Coso Volcanic Field geology and geothermal systems.