Seismic Swarm S20120106.1: Analysis of Activity East of Coso Junction, California

Seismic swarm S20120106.1 was recorded in the Coso region of California, beginning at 00:41 on 6 January 2012 and concluding at 19:18 on 8 January 2012. Over 66 hours and 37 minutes, a total of 48 earthquakes were detected at a location 16 km east of Coso Junction. This event aligns with the area's established pattern of clustered microseismicity.

The swarm exhibited predominantly low-magnitude events, with depths concentrated between 2 km and 7 km. The largest earthquake reached magnitude 3.2 at a depth of 4 km on 6 January at 06:50:10, followed by a magnitude 2.7 event minutes earlier at 5 km depth. Other notable shocks included magnitudes of 2.2, 2.1, and 1.9, all occurring at shallow depths of 2–4 km. The majority of events registered below magnitude 1.0, consistent with typical swarm behavior driven by fluid migration or stress adjustments rather than a single mainshock-aftershock sequence.

Event timing showed peak activity on 6 January, with 30 earthquakes distributed across morning and afternoon hours. Activity declined on 7 January (seven events) and remained sparse on 8 January (six events). Depths remained stable near 4–5 km for most shocks, though a few outliers reached 7 km or registered negative values indicative of near-surface or location uncertainties.

The Coso Junction area forms part of the tectonically active Basin and Range Province in eastern California. It hosts the Coso Volcanic Field, characterized by Quaternary rhyolite domes, basaltic flows, and an active geothermal system. This setting promotes frequent earthquake swarms through hydrothermal fluid circulation and minor crustal extension along normal faults. Geothermal energy production in the Coso field further influences local seismicity by altering pore pressures.

Historical records indicate 35 swarms in the region since 1 January 2000. Yearly distribution includes four swarms in 2000, four in 2001, two in 2002, eight in 2004, two in 2005, six in 2006, one in 2009, seven in 2010, and one in 2011. These recurrent episodes underscore the persistent seismic character of the volcanic and geothermal terrain.

Swarm S20120106.1 contributes to the long-term monitoring of this dynamic zone, where low-magnitude clusters rarely produce significant surface effects but provide valuable data on subsurface processes.

References SeismoSight internal swarm classification records USGS Earthquake Hazards Program regional seismicity summaries California Geological Survey notes on Coso Volcanic Field tectonics