Seismic Swarm S20060813.1: Analysis of Activity Near Coso Junction, California

The seismic swarm designated S20060813.1 occurred in the Coso region of eastern California, centered 7 km east-southeast of Coso Junction. Registered by SeismoSight monitoring, the sequence began at 02:20 UTC on 12 August 2006 and concluded at 15:22 UTC on 20 August 2006, spanning 205 hours and 1 minute. During this interval, 97 earthquakes were recorded, with the majority exhibiting magnitudes below 2.0 and focal depths predominantly between 0 and 5 km.

Event timing shows peak activity concentrated in the first three days, followed by a gradual decline. The largest events reached magnitude 2.5 on 12 August at 02:25:06 UTC (depth 0 km) and magnitude 2.4 on 15 August at 07:00:45 UTC (depth 1 km). Numerous smaller events clustered at depths of 2–3 km, consistent with shallow crustal processes. Negative depth values in a few records reflect standard location uncertainties near the surface. Overall, the swarm displayed typical swarm characteristics: rapid onset, lack of a single dominant mainshock, and sustained low-magnitude seismicity without clear aftershock decay.

This episode forms part of a longer record of swarm activity in the area. Since 1 January 2000, 27 swarms have been identified in the Coso region. Yearly counts include seven in 2000, four in 2001, two in 2002, nine in 2004, two in 2005, and three in 2006. Such recurrent swarms indicate persistent tectonic and hydrothermal influences rather than isolated volcanic unrest.

The Coso area lies within the eastern California shear zone at the western margin of the Basin and Range province. It hosts the Coso Volcanic Field, a Quaternary volcanic center featuring rhyolitic domes, basaltic lava flows, and an active geothermal reservoir. Heat flow exceeds 100 mW/m² in the central field, driven by shallow magmatic intrusions and fault-controlled fluid circulation. The region experiences dextral strike-slip faulting along the Little Lake and Airport Lake fault zones, which accommodate Pacific–North American plate motion. Seismicity frequently occurs at depths less than 10 km, reflecting brittle failure in fractured crystalline basement and volcanic rocks.

Geothermal production at the Coso Geothermal Field, operational since the 1980s, involves extraction from a fractured reservoir at 1–3 km depth. Fluid injection and production can modulate pore pressures, contributing to swarm triggering alongside regional tectonic stress. Historical volcanic activity includes eruptions as recent as 40,000 years ago, though current monitoring shows no surface deformation indicative of magma ascent.

The 2006 swarm aligns with established patterns of shallow, low-magnitude earthquake clusters that punctuate the background seismicity of the field. No damage or felt reports were associated with the sequence, underscoring its microseismic nature.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey regional fault maps
• SeismoSight internal swarm classification database