Seismic Swarm S20210616.1 Near Coso Junction, California

A seismic swarm designated S20210616.1 was recorded 16 km east of Coso Junction, California, beginning at 18:15 on 15 June 2021 and concluding at 14:58 on 20 June 2021. Over 116 hours and 42 minutes, the sequence comprised 83 earthquakes. Events were distributed across depths of 0–9 km, with the majority occurring at shallow levels between 1 km and 5 km. Magnitudes ranged from 0.0 to 3.0, the largest event registering at 3.0 on 18 June at a depth of 5 km.

The temporal distribution showed an initial sparse phase on 15 June followed by a marked increase in activity on 16 June, when multiple events above magnitude 1.5 clustered within hours. Activity remained elevated through 17 and 18 June before tapering on 19 and 20 June. Notable clusters included a sequence of events near magnitude 2.0–2.6 on 16 June at depths of 1–2 km and a trio of events reaching 2.5–3.0 on 18 June at depths of 5–7 km. Most events stayed below magnitude 1.0, consistent with typical swarm characteristics dominated by low-energy releases.

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 rhyolite domes, basaltic flows, and extensive geothermal manifestations driven by elevated crustal heat flow. Ongoing tectonic extension combined with magmatic and hydrothermal processes produces frequent microseismicity. Geothermal energy production at the Coso facility further influences local stress conditions through fluid extraction and injection.

Seismic swarms have been a persistent feature of the Coso region. Since 1 January 2000, 102 swarms have been documented. Annual counts varied considerably, with notable concentrations in 2019 (23 swarms) and 2020 (21 swarms). Earlier years recorded lower numbers, including four swarms each in 2000 and 2001, seven in both 2004 and 2010, and six each in 2006 and 2013. The 2021 total stood at three swarms prior to the June event.

The June 2021 swarm fits the established pattern of short-duration, spatially confined sequences that recur throughout the volcanic field. Shallow focal depths and the absence of a single dominant mainshock underscore the swarm-like nature of the activity. Continued monitoring of such episodes provides insight into the interplay between regional tectonics, volcanic structures, and hydrothermal circulation in this geothermally active setting.