Seismic Swarm S20150608.1: Activity Near Coso Junction, California

Seismic swarm S20150608.1 occurred 12 km northeast of Coso Junction in Inyo County, California. The sequence began at 00:27 on 8 June 2015 and concluded at 14:09 on 9 June 2015, spanning 37 hours and 42 minutes. During this period, 31 earthquakes were recorded, with magnitudes ranging from 0.0 to 2.9 and focal depths predominantly between 0 and 7 km. The largest event reached magnitude 2.9 at a depth of 1 km on 8 June at 15:28.

The Coso region lies within the western Basin and Range province, where extensional tectonics drive normal faulting along the eastern Sierra Nevada margin. This setting coincides with the Coso Volcanic Field, a Quaternary volcanic system featuring rhyolite domes, basaltic lava flows, and active geothermal reservoirs. Heat from a shallow crustal magma body sustains one of California’s largest geothermal power plants, where fluid circulation through fractured rock amplifies microseismicity. Shallow depths recorded in the swarm align with the brittle-ductile transition zone influenced by elevated geothermal gradients.

Earthquake swarms are a recurrent feature of the Coso area. Since 1 January 2000, 53 swarms have been documented, distributed across multiple years: seven in 2000, four in 2001, two in 2002, nine in 2004, two in 2005, six in 2006, two in 2009, seven in 2010, one in 2011, four in 2012, six in 2013, two in 2014, and one in 2015. These episodes typically involve low-magnitude events clustered in time and space, consistent with fluid migration or aseismic slip along pre-existing fractures rather than mainshock-aftershock sequences.

Analysis of the 2015 swarm reveals a rapid onset followed by a gradual decline in event rate, with most activity concentrated in the first 24 hours. Magnitudes remained below 3.0, indicating limited energy release and low potential for damage. Depths clustered near 1–2 km suggest involvement of the shallow hydrothermal system. Such patterns mirror earlier swarms in the catalog and underscore the persistent influence of geothermal processes on local seismicity.

Ongoing monitoring by regional networks continues to track activity in this high-temperature geothermal field. The 2015 swarm fits the established pattern of episodic, low-magnitude unrest that characterizes the Coso Volcanic Field without indicating imminent volcanic hazard.