Seismic Swarm S20200307.1 Near Coso Junction, California

Seismic swarm S20200307.1 occurred 10 km northeast of Coso Junction in Inyo County, California. The sequence began at 10:15 on 6 March 2020 and concluded at 08:45 on 17 March 2020, spanning 262 hours and 29 minutes during which 145 earthquakes were recorded.

The Coso region lies within the Basin and Range province at the western margin of the Great Basin. It hosts the Coso Volcanic Field, a Quaternary volcanic center characterized by rhyolite domes, basaltic flows, and extensive geothermal activity. The field sits above a shallow crustal magma body that drives hydrothermal circulation and induces brittle failure through fluid migration and thermal stressing. Historical deformation and heat-flow measurements confirm ongoing magmatic influence beneath the area.

Seismic swarms have been a recurrent feature of the Coso Volcanic Field. Since 1 January 2000, 88 swarms have been documented. Annual counts include five in 2000, four in 2001, two in 2002, nine in 2004, two in 2005, six in 2006, one in 2009, seven in 2010, one in 2011, four in 2012, six in 2013, two in 2014, two in 2015, four in 2016, two in 2017, one in 2018, 24 in 2019, and six in 2020. These episodes typically comprise low-magnitude events clustered at depths of 1–5 km, consistent with the shallow brittle-ductile transition above the geothermal reservoir.

Analysis of the first 100 events in swarm S20200307.1 reveals a classic swarm signature. Magnitudes ranged from –0.3 to 3.8, with the majority below 2.0. The largest event, magnitude 3.8, occurred at 00:33 on 8 March at 2 km depth. A secondary peak of magnitude 3.6 was recorded the previous day. Depths remained shallow, predominantly 1–3 km, with occasional events at 4–5 km. Temporal clustering was pronounced on 7 and 8 March, when roughly half the listed events occurred within 24 hours. Subsequent activity through 12 March showed continued low-magnitude release without clear mainshock-aftershock decay, supporting a fluid-driven rather than tectonic origin.

The provided event list shows no systematic migration of hypocenters, indicating that activity remained confined to a localized volume above the geothermal system. Negative and near-zero magnitudes reflect the high sensitivity of the regional network and the microseismic character typical of Coso swarms.

This episode fits the long-term pattern of swarm activity driven by the interaction between magmatic heat, hydrothermal fluids, and pre-existing faults. Continued monitoring of the Coso Geothermal Field remains essential for understanding both seismic hazard and the sustainability of local energy production.

References:
SeismoSight internal swarm catalog S20200307.1
USGS Earthquake Catalog (background regional seismicity)
California Geological Survey, Coso Volcanic Field geologic map and geothermal reports