Earthquake Swarm S20040915.1: Geological Context and Analysis Near Coso Junction, California

The Coso region in eastern California lies within the Basin and Range Province, a tectonically active extensional environment characterized by normal faulting and crustal thinning. This area hosts the Coso Volcanic Field, with volcanic activity spanning from the Pliocene to the late Pleistocene, the most recent eruptions occurring approximately 40,000 years ago. The presence of rhyolitic domes and basaltic flows reflects a long history of silicic and mafic volcanism driven by mantle upwelling and crustal extension. The Coso Geothermal Field, developed since the 1980s, exploits high heat flow associated with this volcanic system and represents one of the largest geothermal electricity producers in the United States.

Swarm S20040915.1 occurred 9 km east-northeast of Coso Junction. It initiated at 08:27 on 14 September 2004 and concluded at 04:33 on 23 September 2004, spanning 212 hours and 6 minutes. During this interval, 86 earthquakes were recorded. Magnitudes ranged from -0.1 to 3.1, with the two largest events both measuring 3.1 and occurring on 14 September. Focal depths were predominantly shallow, between 0 km and 7 km, consistent with activity in the brittle upper crust above the geothermal reservoir.

Event timing showed clustering on 14 and 15 September, followed by a gradual decline through 23 September. Early high-magnitude events coincided with the swarm onset, while later activity consisted mainly of smaller events below magnitude 1.5. Depths exhibited minor variation, with most events between 1 km and 4 km, suggesting rupture within a limited vertical interval possibly influenced by local fluid pathways or fault structures.

Regional swarm frequency since 2000 includes 15 documented episodes through 2004, with yearly counts of 4 in 2000, 5 in 2001, 2 in 2002, and 4 in 2004. This pattern indicates recurring seismic unrest linked to the area's volcanic and geothermal setting, where fluid migration and stress perturbations can trigger episodic earthquake sequences without producing a single mainshock.

Such swarms contribute to understanding seismic hazard in geothermal areas, where low-magnitude events can indicate permeability changes or pressure variations within the hydrothermal system. Continued monitoring supports both hazard assessment and sustainable resource management at the Coso Geothermal Field.

References

• USGS Earthquake Catalog (historical data for Coso region)
• California Geological Survey, Regional Geologic Maps
• Geothermal Resources Council, Coso Field Technical Reports