Seismic Swarm S20040925.1 Near Coso Junction, California

Seismic swarm S20040925.1 occurred approximately 10 km east-northeast of Coso Junction in California. The sequence began at 18:04 on 24 September 2004 and concluded at 03:21 on 29 September 2004, spanning 105 hours and 16 minutes. During this interval, 57 earthquakes were recorded.

The Coso region lies within the eastern California shear zone at the transition between the Sierra Nevada and Basin and Range provinces. It is underlain by Mesozoic granitic rocks intruded by a Quaternary volcanic field that includes rhyolite domes and basaltic flows last active within the past 10,000 years. Active geothermal systems driven by shallow heat sources produce persistent microseismicity, often manifesting as swarms rather than mainshock-aftershock sequences.

Event depths ranged from 0 km to 5 km, with the majority occurring between 1 km and 3 km. Magnitudes remained modest, peaking at 1.9 on 25 September 2004; most events fell between 0.4 and 1.6. The temporal distribution showed highest activity on 25 September, followed by a gradual decline through 28 September and only two events on the final day.

Since 1 January 2000 the area has hosted 16 swarms. Annual counts include four in 2000, four in 2001, two in 2002, and six in 2004. This pattern reflects the structurally complex crust beneath the Coso Volcanic Field, where fluid migration and minor magmatic processes repeatedly trigger clustered seismicity.

The 2004 swarm fits the established regional style of low-magnitude, shallow, swarm-like activity. No damage or felt reports were associated with these events, consistent with their small sizes and depths. Continued monitoring by regional networks supports ongoing assessment of geothermal and tectonic hazards in this part of eastern California.

References:
USGS Earthquake Catalog
California Geological Survey, Coso Volcanic Field reports
SeismoSight internal swarm classification database