Seismic Swarm S20101019.1 Near Coso Junction, California

Seismic swarm S20101019.1 was recorded 16 km east of Coso Junction in Inyo County, California. The sequence began at 22:52 on 18 October 2010 and concluded at 23:07 on 20 October 2010, spanning 48 hours and 15 minutes. During this interval, 40 earthquakes were registered.

The events were characterized by low magnitudes and shallow focal depths. The largest shocks reached magnitude 1.9, while most remained below magnitude 1.0. Depths ranged from 0 to 3 km, with the majority occurring between 1 and 2 km. Activity clustered in two main pulses on 19 October, one in the early morning and another in the evening, followed by a gradual decline on 20 October.

Such swarms reflect the tectonic and volcanic setting of the Coso region. The area lies within the eastern California shear zone at the transition between the Basin and Range province and the Sierra Nevada. Active normal and strike-slip faults accommodate regional extension, while the underlying Coso Volcanic Field supplies heat that drives hydrothermal circulation. Geothermal production at the Coso facility further perturbs the shallow crust, lowering effective stress and promoting swarm-like sequences.

Historical records maintained since 2000 document 27 swarms in the immediate vicinity. Yearly counts include four events in 2000, four in 2001, one in 2002, seven in 2004, one in 2005, six in 2006, one in 2009, and three in 2010. These episodes demonstrate persistent seismic unrest linked to the same geological framework.

The 2010 swarm fits this established pattern. Its brief duration, small magnitudes, and shallow depths are typical of fluid-driven or thermally influenced sequences observed repeatedly in the Coso geothermal field. No damage or felt reports were associated with the activity.

Continued monitoring by regional seismic networks remains essential for distinguishing background swarm behavior from any potential escalation that could signal changes in the underlying magmatic or hydrothermal system.

References

• United States Geological Survey Earthquake Catalog
• California Geological Survey Regional Geologic Maps
• Geothermal Resources Council publications on Coso field operations