Seismic Insights from Swarm S20190721.1 near Coso Junction, California

The earthquake swarm designated S20190721.1 was recorded beginning at 20:10 on 20 July 2019 and concluding at 01:22 on 30 July 2019. Centered 6 km east-northeast of Coso Junction, California, the sequence produced 474 events over 221 hours and 12 minutes. This activity aligns with the recurrent seismicity observed throughout the Coso Volcanic Field, a region shaped by extensional tectonics within the Basin and Range province. The Coso Volcanic Field features late Cenozoic rhyolitic and basaltic volcanism, with the most recent eruptions occurring in the Holocene. Geothermal manifestations, including hot springs and fumaroles, overlie a shallow heat source linked to crystallizing magma at depths of 5–10 km. Regional faulting along the Little Lake and Airport Lake fault zones accommodates dextral shear associated with the Eastern California Shear Zone, generating frequent microseismicity. Depths in the swarm remained predominantly shallow, consistent with fluid migration or minor crustal adjustments above the geothermal reservoir. Analysis of the first 100 events reveals magnitudes between 0.5 and 2.8, with the majority falling in the 0.9–1.6 range. Depths clustered between 0 and 5 km, though isolated events reached 11 km. Temporal distribution showed peak rates during the initial 48 hours, followed by gradual decay. Notable events included a magnitude 2.8 at 13:29 on 22 July and several magnitude 2.0–2.4 shocks on 21 July, all at depths of 0–4 km. These characteristics indicate swarm-like behavior driven by aseismic slip or pore-pressure changes rather than a single mainshock–aftershock sequence. Historical records document 61 swarms in the area since 1 January 2000. Yearly counts include four 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, and four in 2019. This pattern underscores persistent seismic restlessness tied to the volcanic–geothermal system. The 2019 swarm fits established trends of episodic, low-magnitude activity that rarely exceeds magnitude 3.0. Monitoring by regional networks continues to track such sequences for potential links to geothermal production or deeper magmatic processes. References
USGS Earthquake Catalog
California Geological Survey Regional Geologic Maps
SeismoSight internal swarm classification records