Seismic Swarm S20000911.1: Analysis of Activity Near Coso Junction, California

Seismic swarm S20000911.1 was recorded beginning at 08:51 on 10 September 2000 and concluding at 23:46 on 19 September 2000, approximately 12 km east-northeast of Coso Junction in Inyo County, California. Over 230 hours and 54 minutes, the sequence produced 140 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity clustered at shallow depths.

Magnitudes ranged from 0.2 to 2.7, with the largest event occurring on 10 September at 19:51. Depths were generally between 0 and 7 km, indicating a shallow source consistent with local crustal conditions. Event timing showed initial clustering on 10 September, followed by sustained but declining rates through 17 September. Depths fluctuated modestly without systematic migration, suggesting distributed fracturing rather than a single propagating feature.

The Coso region lies within the Basin and Range province of eastern California, where extensional tectonics interact with a long-lived volcanic system. The Coso Volcanic Field features Pleistocene rhyolite domes, basaltic flows, and extensive geothermal manifestations. Heat flow in the area exceeds regional averages due to shallow magmatic intrusions, supporting one of the largest geothermal power operations in the United States. Seismicity here arises from both regional strike-slip and normal faulting linked to the Eastern California Shear Zone and from fluid-driven processes within the geothermal reservoir.

Historical records indicate that seismic swarms have occurred intermittently in the Coso area since at least the late twentieth century. Since 1 January 2000, two such swarms have been documented, with the first being S20000911.1. These episodes typically involve hundreds of small events over days to weeks and are often associated with hydrothermal fluid movement rather than magmatic intrusion.

The 2000 swarm's characteristics align with patterns observed in other geothermal fields, where rapid sequences of microearthquakes reflect stress changes induced by natural fluid circulation. Depths mostly below 4 km coincide with the known producing interval of the Coso geothermal reservoir. No damage or felt reports were associated with the sequence, consistent with the low magnitudes recorded.

Continued monitoring of the Coso area remains important for both geothermal operations and regional seismic hazard assessment. The interplay between tectonic extension and hydrothermal activity produces persistent background seismicity punctuated by episodic swarms.

References USGS Earthquake Hazards Program catalog Coso Geothermal Field technical reports California Geological Survey regional geologic maps