Earthquake Swarm S20121127.1: Seismic Activity Near Coso Junction, California
An earthquake swarm designated S20121127.1 was recorded in the Coso region of California. The sequence began at 04:38 on 26 November 2012 and concluded at 05:14 on 20 December 2012. Located approximately 10 km east-northeast of Coso Junction, the swarm persisted for 576 hours and 35 minutes, during which 802 earthquakes were detected.
The Coso Volcanic Field lies within the eastern California shear zone, a tectonically active area characterized by extensional faulting and young volcanic activity. This setting promotes frequent earthquake swarms driven by hydrothermal fluid movement and minor magmatic processes rather than large tectonic displacements. Shallow seismicity is typical, reflecting the influence of geothermal reservoirs that extend several kilometers beneath the surface.
Analysis of the first 100 events reveals predominantly low-magnitude earthquakes. Magnitudes ranged from 0.0 to 3.6, with the majority below 1.5. Depths clustered between 0 and 6 km, indicating activity within the upper crustal layers associated with the local geothermal system. Notable events included a magnitude 2.5 quake at 00:16 on 27 November at 1 km depth and a magnitude 3.6 event at 10:40 on 30 November at 1 km depth. Temporal clustering was evident, with heightened activity during late November, particularly on 26–27 and 30 November.
Such patterns align with historical observations in the Coso area, where swarms often feature rapid sequences of small events without a dominant mainshock. The 2012 swarm fits within a broader record of seismic unrest since 2000, during which 39 swarms occurred. Yearly counts include 4 in 2000, 4 in 2001, 2 in 2002, 9 in 2004, 2 in 2005, 6 in 2006, 1 in 2009, 7 in 2010, 1 in 2011, and 3 in 2012. These episodes underscore the persistent volcanic and geothermal drivers in the region.
Overall, swarm S20121127.1 exemplifies the characteristic low-magnitude, shallow seismicity of the Coso Volcanic Field, providing further data on its dynamic subsurface environment.
References
SeismoSight internal swarm classification records.
USGS Earthquake Catalog (historical context for Coso Volcanic Field).