Seismic Swarm S20061127.1 in the Coso Region, California

The Coso region, located approximately 22 km east of Coso Junction in Inyo County, California, lies within the tectonically active Basin and Range province. This area features the Coso Volcanic Field, characterized by Pleistocene rhyolitic domes, basaltic flows, and an underlying geothermal reservoir. The field’s geology reflects ongoing extension along the Walker Lane belt, with heat flow enhanced by shallow magmatic intrusions. The Coso Geothermal Power Plant, operational since the 1980s, exploits this system, and microseismicity frequently occurs due to fluid migration through fractured crystalline basement rocks.

Seismic swarm S20061127.1 began at 10:09 on 27 November 2006 and concluded at 21:17 on 4 December 2006, spanning 179 hours and 7 minutes. During this interval, 109 earthquakes were recorded. The first 100 events displayed magnitudes ranging from 0.1 to 3.8, with the largest shocks (3.8 and 3.4) occurring within the first day. Depths clustered between 0 and 4 km, indicating shallow crustal processes typical of geothermal environments. Early activity concentrated near the surface, with multiple events at 0 km depth, while later events showed slightly greater average depths around 2–3 km. Temporal distribution revealed an initial intense phase on 27 November, followed by declining frequency over subsequent days, consistent with swarm behavior driven by fluid pressure changes rather than mainshock-aftershock sequences.

Analysis of the initial 100 events highlights a predominance of magnitudes below 2.0, underscoring the swarm’s low-energy character. Clusters of events separated by minutes, such as those on 27 November between 21:14 and 21:37, suggest episodic fluid pulses. Depths remained consistently shallow, aligning with the known brittle-ductile transition in the Coso geothermal reservoir.

Since 1 January 2000, the region has experienced 22 documented swarms. Annual counts include 4 in 2000, 4 in 2001, 1 in 2002, 7 in 2004, 1 in 2005, and 5 in 2006. This pattern reflects persistent hydrothermal activity and tectonic stressing in the volcanic field.

Such swarms provide insights into geothermal reservoir dynamics and contribute to hazard assessment for nearby infrastructure. Continued monitoring supports understanding of long-term volcanic and seismic hazards in this geothermally active zone.

References

USGS Earthquake Catalog
Coso Geothermal Project reports
California Geological Survey regional summaries