Seismic Swarm S20191201.1: Analysis of Activity Near Coso Junction, California

The Coso Volcanic Field in eastern California represents one of the most seismically active volcanic regions in the western United States. Located within the Basin and Range Province, the field features extensive rhyolitic volcanism, geothermal manifestations, and frequent earthquake swarms driven by both tectonic extension and magmatic processes. The area hosts a major geothermal energy facility that taps into shallow heat sources associated with Quaternary volcanic activity.

Swarm S20191201.1 initiated at 01:23 on 30 November 2019 and concluded at 04:19 on 27 December 2019. Centered 11 km northeast of Coso Junction, the sequence lasted 650 hours and 56 minutes, recording 353 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude seismicity. The largest event reached magnitude 3.3 on 6 December 2019 at a depth of 2 km. Most events clustered between magnitudes 0.0 and 2.7, with the majority occurring at depths of 1–4 km and occasional deeper occurrences up to 7 km. Temporal distribution showed initial activity concentrated within the first 48 hours, followed by episodic bursts, including notable clusters on 4 December and 6 December.

This swarm aligns with the long-term pattern of seismic unrest in the Coso region. Since 1 January 2000, 77 swarms have been documented. Annual counts vary considerably, with elevated activity in 2004 (9 swarms), 2010 (7 swarms), 2013 (6 swarms), and especially 2019 (20 swarms). Earlier years such as 2000, 2001, and 2016 each recorded four swarms, while several years featured only one or two events. These recurrent swarms reflect the interplay between regional tectonics and local volcanic-hydrothermal systems.

Geologically, the Coso Volcanic Field formed through episodic eruptions spanning the Pleistocene to Holocene epochs. Dominant rock types include rhyolite domes and flows, with associated basaltic components. The field lies at the intersection of northwest-trending strike-slip faults and northeast-trending normal faults, facilitating fluid migration and pressure changes that trigger swarm sequences. Depths of seismicity typically remain shallow, consistent with the influence of geothermal reservoirs.

The 2019 swarm underscores the persistent hazard potential of the region. Although individual events remained small, the high event count and rapid succession highlight the capacity for sudden increases in activity. Ongoing geothermal operations require continuous monitoring to distinguish between natural swarm behavior and any induced components.

References

USGS Earthquake Hazards Program – Coso Volcanic Field regional reports
California Geological Survey – Quaternary volcanic field mapping
Coso Geothermal Project technical summaries