Analysis of Earthquake Swarm S20200604.2 Near Coso Junction, California

The Coso Volcanic Field in eastern California lies within the Basin and Range province, where extensional tectonics interact with young volcanic systems. This region features rhyolitic domes, basaltic flows, and active geothermal reservoirs that drive frequent seismic activity. Seismicity here often manifests as swarms linked to fluid migration in fractured crust rather than large mainshock-aftershock sequences.

Swarm S20200604.2 began at 07:01 on 3 June 2020 and concluded at 02:47 on 9 June 2020. Centered 13 km NNW of Coso Junction, the sequence lasted 139 hours and 46 minutes, recording 148 earthquakes. Depths remained predominantly shallow, between 1 km and 10 km, consistent with geothermal and magmatic fluid processes in the upper crust.

The first 100 events displayed a characteristic swarm pattern of clustered low-to-moderate magnitudes without a dominant mainshock. Magnitudes ranged from negative values to a peak of 4.1, with most events below 2.0. Notable larger events included a magnitude 3.1 on 3 June at 15:33, a 3.4 on 3 June at 23:13, and the 4.1 on 4 June at 02:27. Depths for these events stayed between 1 km and 4 km, indicating activity within the shallow hydrothermal system. Smaller events occurred at slightly greater depths up to 10 km, suggesting minor involvement of deeper structures.

Temporal distribution showed intense activity on 3 and 4 June, with dozens of events in the initial 24 hours, followed by a gradual decline. This progression aligns with historical patterns in the Coso area, where swarms typically last several days and reflect episodic fluid pressure changes.

Since 1 January 2000, the Coso region has experienced 70 earthquake swarms. Annual counts vary, with peaks in 2004 (9 swarms), 2019 (9 swarms), and multiple years recording 6 swarms each. This long-term frequency underscores the persistent tectonic and volcanic influences shaping the area's seismicity.

The 2020 swarm fits within this established record, reinforcing the Coso Volcanic Field's status as one of California's most seismically active geothermal zones. Continued monitoring supports hazard assessment for nearby infrastructure and communities.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog for regional context.
California Geological Survey reports on Coso Volcanic Field.