Seismic Swarm S20170608.1: Activity Near Coso Junction, California
Seismic swarm S20170608.1 occurred 8 km southeast of Coso Junction in California. The sequence began at 03:05 on 8 June 2017 and concluded at 06:39 on 11 June 2017, spanning 75 hours and 34 minutes. During this period, 52 earthquakes were recorded.
The events clustered at shallow depths between 0 and 4 km. Magnitudes ranged from 0.1 to 3.2, with the largest shock reaching 3.2 on 8 June at 08:30:40. Other notable events included two magnitude-2.9 earthquakes on the same day at 07:58:11 and 08:19:02. Depths remained consistently shallow, suggesting activity within the upper crustal layers.
This swarm aligns with the long-term seismic character of the Coso region. Since 2000, 55 swarms have been documented in the area. Annual counts show variability, with peaks of seven swarms each in 2004 and 2010, followed by lower activity in intervening years. The 2017 swarm represents the sole recorded sequence that year.
Geological Setting
The Coso Junction area lies within the Coso Volcanic Field, part of the eastern California shear zone. This region experiences ongoing tectonic extension associated with the Basin and Range province. The volcanic field features Pleistocene to Holocene rhyolite domes and basaltic flows, with the most recent eruptions dated to approximately 40,000 years ago. Active geothermal systems, exploited for power generation since the 1980s, indicate persistent heat flow from crustal sources.
Seismicity in the Coso Volcanic Field arises from a combination of strike-slip faulting along the Little Lake and Airport Lake fault zones and possible fluid migration linked to geothermal reservoirs. Earthquake swarms are characteristic, often occurring without a clear mainshock-aftershock pattern. Depths typically fall between 1 and 5 km, consistent with the brittle-ductile transition influenced by elevated geothermal gradients.
Historical monitoring by the USGS and regional networks has documented recurrent swarm activity tied to both tectonic stress accumulation and hydrothermal processes. The shallow focal depths observed in S20170608.1 match patterns seen in prior sequences, underscoring the area's elevated seismic hazard potential.
References
USGS Earthquake Catalog
California Geological Survey Regional Reports
SeismoSight internal swarm records