Seismic Swarm S20010610.1: Activity Near Coso Junction, California

Seismic swarm S20010610.1 was recorded in the Coso region of eastern California, beginning at 23:22 on 9 June 2001 and concluding at 02:25 on 14 June 2001. Over this 99-hour period, 51 earthquakes were detected, with the majority occurring within the first 48 hours. Epicenters were located approximately 6 km east-southeast of Coso Junction, at shallow depths predominantly between 0 and 5 km.

The sequence featured low to moderate magnitudes, with a peak event of M3.0 recorded early on 10 June. Subsequent activity remained below M2.6, consistent with typical swarm behavior in which numerous small events occur without a dominant mainshock. Depths clustered near 2–3 km, suggesting involvement of shallow crustal processes.

The Coso area lies within the western Basin and Range province, where extensional tectonics interact with a long-lived volcanic system. The Coso Volcanic Field has produced rhyolitic eruptions over the past several hundred thousand years and hosts an active geothermal reservoir exploited for power generation since the 1980s. Elevated heat flow and fluid circulation within fractured crystalline rocks create conditions favorable for earthquake swarms driven by pore-pressure changes rather than purely tectonic stress.

Historical records indicate recurrent swarms in the Coso Volcanic Field. Since the start of 2000, seven such episodes have been documented, with four occurring in 2000 and three in 2001. These events are commonly attributed to hydrothermal fluid migration or minor magmatic unrest beneath the volcanic field.

Analysis of S20010610.1 shows a rapid onset followed by a gradual decline in rate, with most events clustered in the initial two days. The shallow focal depths and lack of a clear magnitude progression align with fluid-induced triggering mechanisms observed in other geothermal regions. No surface rupture or significant damage was reported, reflecting the modest energy release.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the Coso area, providing data that improve understanding of geothermal reservoir dynamics and volcanic hazard assessment.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey regional seismicity reports
• Geothermal Resources Council publications on Coso Volcanic Field