Seismic Swarm S20190804.1 Near Coso Junction, California

The seismic swarm designated S20190804.1 began at 03:36 on 4 August 2019 and concluded at 21:24 on 14 September 2019. Centered 10 km east-northeast of Coso Junction, California, the sequence produced 1154 earthquakes over 1001 hours and 48 minutes. This event forms part of the recurrent seismicity that characterizes the Coso Volcanic Field, a region of active geothermal and tectonic processes within the Basin and Range province. Analysis of the first 100 recorded events reveals a predominance of low-magnitude earthquakes. Magnitudes ranged from 0.4 to 2.0, with the majority falling between 0.9 and 1.5. Depths remained shallow, concentrated between 0 km and 9 km, consistent with activity in the upper crust. The initial event measured magnitude 0.9 at 3 km depth, followed by a gradual increase in event rate and occasional peaks reaching magnitude 1.9–2.0 within the first 48 hours. Temporal clustering was evident, particularly during the afternoon and evening of 4 August and early 5 August, when multiple events occurred within minutes of one another. These patterns align with fluid-driven swarm behavior rather than mainshock-aftershock sequences. The Coso Volcanic Field has hosted 61 swarms since 1 January 2000. Annual counts include four swarms each in 2000 and 2001, seven in both 2004 and 2010, and ten in 2019 alone. Earlier episodes occurred in 2002, 2005, 2006, 2009, 2011–2018, demonstrating persistent seismic productivity over two decades. Such recurrence reflects ongoing interaction between regional tectonics and local magmatic-hydrothermal systems. Geologically, the Coso region lies at the western margin of the Basin and Range extensional province. Pleistocene to Holocene rhyolitic and basaltic volcanism has produced domes, flows, and pyroclastic deposits. Active geothermal reservoirs support commercial power generation and are underlain by a partially molten crustal zone that facilitates fluid migration and induced seismicity. Earthquake swarms in this setting commonly arise from pore-pressure changes within fractured volcanic and metamorphic rocks rather than from large tectonic stress release. The 2019 swarm exhibited typical traits of Coso seismicity: shallow focal depths, modest magnitudes, and episodic clustering. These features underscore the value of continuous monitoring for distinguishing background geothermal noise from potential volcanic unrest. No surface deformation or eruptive phenomena were associated with the sequence.

References

SeismoSight internal swarm catalog S20190804.1
USGS Earthquake Catalog (historical swarm verification)
California Geological Survey, Coso Volcanic Field overview
USGS Professional Paper on Basin and Range geothermal systems