Seismic Swarm S20050806.1: Analysis of Activity Near Coso Junction, California

The Coso region in eastern California lies within the Basin and Range province and the Eastern California Shear Zone, where right-lateral strike-slip faulting accommodates Pacific-North American plate motion. The area hosts the Coso Volcanic Field, characterized by Quaternary rhyolite domes, basaltic flows, and an active geothermal system driven by shallow magmatic heat sources. This geological setting produces frequent earthquake swarms linked to fluid migration and minor crustal adjustments rather than large mainshock-aftershock sequences.

SeismoSight recorded swarm S20050806.1 beginning at 01:19 UTC on 6 August 2005 and concluding at 07:08 UTC on 13 August 2005. The sequence lasted 173 hours and 49 minutes, during which 120 earthquakes were detected 15 km northwest of Coso Junction. Historical records indicate 22 prior swarms in the same area since 1 January 2000, with annual counts of 6 (2000), 5 (2001), 2 (2002), and 9 (2004). These recurring episodes underscore the persistent seismic productivity of the Coso geothermal field.

Analysis of the first 100 events reveals a rapid onset followed by a gradual decline in activity. The initial hours on 6 August included the largest event of the swarm, magnitude 4.3 at a nominal depth of -1 km, accompanied by several magnitude 3+ shocks clustered between 03:00 and 06:00 UTC. Magnitudes subsequently decreased, with most events falling between 0.5 and 2.0. Depths remained predominantly shallow, concentrated between 0 and 5 km, consistent with the brittle-ductile transition zone above the geothermal reservoir. Later events on 7–10 August showed continued low-level seismicity, punctuated by occasional magnitude 2 events, before activity tapered off.

The temporal distribution indicates swarm-like behavior driven by pore-pressure changes within the fractured volcanic and sedimentary rocks. No single dominant fault plane is evident; instead, the events appear distributed across a network of small structures associated with the regional shear zone and local caldera-related faults.

This swarm fits the established pattern of seismic unrest at Coso, where geothermal production and natural hydrothermal circulation interact with tectonic stress. Continued monitoring remains essential for understanding long-term volcanic and seismic hazards in the region.

References SeismoSight internal swarm catalog S20050806.1 USGS Earthquake Catalog (queried for Coso Junction, CA, 2005) USGS Professional Paper on Coso Volcanic Field geology and geothermal resources