Seismic Swarm S20220126.1 Near Coso Junction, California

The Coso Volcanic Field in Inyo County, California, lies within the tectonically active Basin and Range province, where extensional faulting interacts with Quaternary volcanism and geothermal systems. This setting produces frequent earthquake swarms driven by fluid migration, magmatic processes, and regional shear along the eastern California shear zone. The area hosts one of the largest geothermal fields in the United States, with surface manifestations including fumaroles and hot springs that reflect ongoing heat flow from shallow crustal intrusions.

Seismic swarms have characterized the region since at least 2000, totaling 109 events through 2021. Annual counts varied significantly, with notable increases in 2019 (29 swarms) and 2020 (23 swarms), followed by a decline to six in 2021. These episodes typically involve hundreds of small-magnitude events clustered over hours to days, often at depths of 1–8 km, reflecting brittle failure in the shallow crust above the geothermal reservoir.

Swarm S20220126.1 began at 17:22 on 25 January 2022 and concluded at 01:45 on 28 January 2022, spanning 56 hours and 22 minutes. It was centered 15 km east of Coso Junction and comprised 59 earthquakes. Activity initiated with low-magnitude events (0.3–0.4) at depths of 3–4 km. A rapid escalation occurred around 05:00 on 26 January, when magnitudes reached 2.8 and 2.0 at 1 km depth, followed by additional events of 2.5 and 2.0 within minutes. Peak activity continued through midday, including two magnitude-3.0 events at 09:59 and a magnitude-3.1 event at 10:38, all at shallow depths near 1 km. Subsequent events remained predominantly below magnitude 2.0, with depths fluctuating between 1 km and 8 km. The final recorded event was a magnitude-1.1 quake at 8 km depth on 28 January.

Most events clustered at depths of 1–2 km during the highest-rate period, consistent with fluid-driven seismicity in the geothermal zone. Shallower foci suggest involvement of the brittle-ductile transition influenced by hydrothermal circulation, while occasional deeper events (6–8 km) may indicate minor fault slip on surrounding structures. Magnitudes stayed below 3.2, producing no reported damage but highlighting persistent background unrest in the volcanic field.

This swarm aligns with the historical pattern of short-duration, low-magnitude clusters typical of Coso, where cumulative energy release remains modest yet provides valuable data on subsurface dynamics. Continued monitoring supports hazard assessment for nearby infrastructure and geothermal operations.

References

USGS Earthquake Catalog
California Geological Survey regional reports
SeismoSight internal swarm classification database