Seismic Swarm S20240416.2 Near Coso Junction, California

A seismic swarm designated S20240416.2 was recorded 9 km northeast of Coso Junction, California. The sequence began at 18:33 UTC on 15 April 2024 and concluded at 09:51 UTC on 18 April 2024, spanning 63 hours and 17 minutes. During this interval, 51 earthquakes were registered. Magnitudes ranged from 0.5 to 3.9, with the largest event occurring at 15:23:26 on 16 April at a depth of 5 km. Most events clustered between depths of 5 and 8 km, consistent with shallow crustal processes.

The swarm exhibited a typical pattern of clustered, low-to-moderate magnitude events without a single dominant mainshock. Activity peaked shortly after initiation on 16 April, with several events above magnitude 2.0 occurring within the first few hours. Subsequent activity declined gradually through 17 and 18 April. Depths remained stable in the mid-crust, suggesting involvement of fluid migration or minor crustal adjustment rather than deeper magmatic intrusion.

The Coso region lies within the eastern California shear zone at the transition between the Sierra Nevada and Basin and Range provinces. It hosts the Coso Volcanic Field, a Pleistocene-to-Holocene volcanic center characterized by rhyolitic domes, basaltic flows, and extensive geothermal alteration. Active geothermal production has operated since the 1980s, with fluid extraction and reinjection known to influence local seismicity. The area experiences frequent earthquake swarms linked to both tectonic strain and hydrothermal circulation along northwest-trending faults.

Since 2000, 111 swarms have been documented in the immediate vicinity. Annual counts show considerable variability, with notable increases in 2019 (22 swarms) and 2020 (17 swarms). Earlier decades recorded lower totals, such as nine swarms in 2004 and seven each in 2000 and 2010. This long-term record indicates persistent seismic restlessness driven by the region's volcanic and geothermal setting.

The April 2024 swarm fits within established patterns of short-duration, shallow activity typical of the Coso geothermal field. No damage or felt reports beyond the immediate area were associated with the sequence. Continued monitoring by regional seismic networks remains essential for distinguishing between routine geothermal-related events and any potential changes in volcanic unrest.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional reports on the Coso Volcanic Field
SeismoSight internal swarm classification database