Seismic Swarm S20100302.1: Analysis of Activity Near Coso Junction, California

Seismic swarm S20100302.1 occurred in the Coso Volcanic Field region of eastern California, centered 8 km east-northeast of Coso Junction. The sequence began at 09:06 on 1 March 2010 and concluded at 20:48 on 6 March 2010, spanning 131 hours and 42 minutes. During this interval, 44 earthquakes were recorded, providing a representative example of the episodic swarm behavior typical of the area.

The Coso Volcanic Field lies within the Basin and Range extensional province, where crustal thinning and faulting interact with residual magmatic heat. The field features Quaternary rhyolite domes and flows, with the most recent eruptions dated to approximately 40,000 years ago. An active geothermal system, exploited by the Coso Geothermal Power Plant, drives fluid circulation that influences seismicity. Earthquakes in this setting commonly occur at shallow depths due to brittle failure along fracture networks above the brittle-ductile transition.

Analysis of the swarm reveals predominantly shallow focal depths between 0 and 4 km. The largest event, magnitude 3.7, occurred at the onset on 1 March at a depth of 0 km. Subsequent activity included multiple events of magnitude 2.6–2.9, with the second-highest magnitude (3.4) recorded early on 2 March. Depths clustered near the surface or within the upper 3 km, consistent with hydrothermal or magmatic fluid migration along pre-existing faults. Event frequency was highest during the first two days, then declined steadily, a pattern observed in prior Coso swarms.

Historical records since 2000 indicate 29 swarms in the immediate region. Annual counts vary, with notable concentrations in 2002 (9 swarms) and 2006 (6 swarms). The 2010 sequence represents the second swarm documented that year, underscoring the recurrent nature of clustered seismicity driven by the local tectonic and geothermal regime.

Such swarms differ from mainshock-aftershock sequences by lacking a dominant triggering event and instead reflecting distributed stress changes. In the Coso area, these episodes are frequently linked to pore-pressure fluctuations within the geothermal reservoir or minor magmatic intrusions at depth. Continued monitoring by regional seismic networks supports hazard assessment and geothermal reservoir management.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
California Geological Survey, Geologic Map of California
USGS Open-File Report on Coso Volcanic Field Geothermal System