Seismic Swarm S20060713.1 Near Coso Junction, California

The Coso Volcanic Field in eastern California occupies a tectonically active zone within the Basin and Range Province, where northwest-directed dextral shear along the Walker Lane accommodates part of the Pacific–North America plate motion. The field hosts Pleistocene rhyolite domes, basaltic flows, and an active geothermal system driven by shallow magmatic heat sources. Seismicity is common, often occurring as swarms linked to fluid migration or minor magmatic unrest rather than large mainshock-aftershock sequences.

Swarm S20060713.1 began at 08:17 UTC on 12 July 2006 and ended at 14:17 UTC on 18 July 2006, centered 4 km northeast of Coso Junction. Over 149 hours and 59 minutes, 67 earthquakes were recorded. Magnitudes ranged from 0.3 to 4.0, with most events shallower than 6 km. Depths clustered between 2 km and 5 km, consistent with the brittle–ductile transition in the geothermal reservoir.

The sequence opened with small events (M 1.4–1.5) on 12 July. The largest shock, M 4.0 at 22:20 UTC on 12 July, occurred at 2 km depth and was followed within minutes by an M 3.6 event at the same depth. Subsequent activity included an M 2.7 event on 13 July and an M 3.0 event on 14 July. After 15 July, magnitudes remained below 1.2, and the swarm tapered to background levels by 18 July.

Event depths showed modest variation, with a few events reaching 7–8 km early in the sequence before stabilizing near 4–5 km. The temporal pattern—rapid onset, clustered larger events within the first 48 hours, then rapid decay—matches typical swarm behavior in the Coso field, where fluid pressure transients trigger distributed failure on preexisting fractures.

Since 1 January 2000 the Coso region has experienced 22 swarms. Annual counts were: 2000 (4), 2001 (4), 2002 (2), 2004 (8), 2005 (2), and 2006 (2). These recurrent swarms reflect the field’s high permeability and ongoing hydrothermal circulation above cooling intrusions.

The July 2006 swarm produced no reported damage or felt shaking beyond the immediate geothermal lease area. Continued seismic monitoring supports both geothermal reservoir management and regional hazard assessment in this volcanically youthful terrain.

References

• USGS Earthquake Hazards Program, Coso Volcanic Field overview
• California Geological Survey, Quaternary volcanic map of eastern California
• SeismoSight internal swarm catalog (S20060713.1 parameters)