Seismic Swarm S20000921.1: Insights from the Coso Region, California

The Coso region in east-central California lies within the tectonically active Basin and Range province at the western margin of the Mojave Desert. This area encompasses the Coso Volcanic Field, a Quaternary volcanic complex featuring rhyolite domes, basaltic flows, and extensive geothermal systems. The field formed through episodic volcanism over the past several million years, driven by crustal extension and upwelling mantle material. Active faulting along the Little Lake fault zone and nearby structures contributes to ongoing seismicity, while the Coso Geothermal Field supports commercial energy production through hydrothermal reservoirs at shallow depths. Seismic swarms are recurrent in this setting, often linked to fluid migration in fractured volcanic and crystalline basement rocks rather than large tectonic events. The region experiences background seismicity at rates influenced by both natural tectonic strain and anthropogenic factors from geothermal operations. Swarm S20000921.1 was recorded beginning at 15:22 on 20 September 2000 and concluding at 04:33 on 6 October 2000. Centered 7 km east of Coso Junction, the sequence lasted 373 hours and 11 minutes and comprised 274 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity, with the majority between 0.5 and 2.0 and a single peak event of magnitude 3.6 occurring early in the sequence at 16:10 on 20 September. Focal depths remained shallow, concentrated between 0 and 4 km, consistent with brittle failure in the upper crust near geothermal reservoirs. Event timing showed clustering on 20–22 September, followed by a gradual decline. Magnitudes exhibited no clear escalation after the initial larger shock, and negative depth values in isolated records likely reflect minor location uncertainties typical of local network processing. Depths averaging near 2 km suggest involvement of the shallow hydrothermal system rather than deeper magmatic sources. Since 1 January 2000, three swarms have occurred in the area, with this sequence representing the earliest. Such patterns underscore the persistent, swarm-dominated character of Coso seismicity, where cumulative energy release occurs through numerous small events rather than infrequent large earthquakes.

References

USGS Earthquake Hazards Program regional seismicity reports for California. California Geological Survey bulletins on the Coso Volcanic Field. Peer-reviewed studies in Journal of Geophysical Research on Coso geothermal and volcanic processes.