Seismic Swarm S20091231.1: Analysis of Activity Near Coso Junction, California

Seismic swarm S20091231.1 occurred approximately 15 km east of Coso Junction in California, beginning at 15:53 UTC on 31 December 2009 and concluding at 10:10 UTC on 2 January 2010. Over 42 hours and 16 minutes, the event sequence produced 54 earthquakes, with magnitudes ranging from -0.2 to 3.1 and focal depths predominantly between 0 and 3 km.

The swarm exhibited a rapid onset with multiple events clustered within the first several hours. The largest earthquake, magnitude 3.1, struck at 19:57:42 on 31 December at a depth of 1 km. Subsequent activity included events of magnitude 2.8 at 03:25:30 on 1 January and 1.8 at 19:59:39 on 31 December. Depths remained shallow throughout, with many events recorded at 1–2 km, consistent with activity influenced by near-surface geological structures.

Coso Junction lies within the Coso Volcanic Field, a region of Quaternary volcanism situated at the western margin of the Basin and Range province. The area features rhyolitic domes, basaltic flows, and an active geothermal system driven by shallow crustal heat sources. Tectonic extension along the Walker Lane belt interacts with local magmatic processes, producing frequent small-magnitude seismicity. Geothermal energy production in the Coso field further modulates stress through fluid extraction and injection, contributing to swarm-type earthquake sequences.

Historical records since 2000 document 23 swarms in the immediate vicinity. Annual counts include four swarms each in 2000 and 2001, one in 2002, seven in 2004, one in 2005, and six in 2006. These episodes typically last from hours to days and involve low-magnitude events at shallow depths, mirroring the characteristics of S20091231.1.

Analysis of the 2009–2010 sequence shows a high rate of microearthquakes in the initial 24 hours, followed by a gradual decline. Over 70 percent of events registered magnitudes below 1.0, indicating limited energy release beyond the mainshock. The concentration of activity at depths of 1 km or less suggests involvement of hydrothermal fluids within fractured volcanic rock.

Such swarms are common in the Coso region due to its combination of active tectonics and geothermal circulation. Monitoring by regional networks helps distinguish these sequences from potential volcanic unrest, as no surface deformation or gas emissions were associated with this event.

References

USGS Earthquake Catalog
California Geological Survey regional reports
Coso Operating Company geothermal field documentation