Seismic Swarm S20010408.1 Near Coso Junction, California

The Coso region of eastern California occupies a tectonically active transition zone between the Sierra Nevada and the Basin and Range Province. The area is underlain by the Coso Volcanic Field, a Pleistocene-to-Holocene volcanic center featuring rhyolite domes, basaltic flows, and an active geothermal system driven by shallow crustal heat sources. Normal and strike-slip faults accommodate regional extension, producing frequent microseismicity often linked to fluid migration within the geothermal reservoir. Between 22:12 UTC on 7 April 2001 and 14:53 UTC on 9 April 2001, a swarm of 29 earthquakes was recorded 12 km east of Coso Junction. The sequence lasted 40 hours and 41 minutes. Magnitudes ranged from 0.6 to 3.7, with the largest event occurring at 01:25 UTC on 8 April at 2 km depth. Most hypocenters clustered between 0 and 4 km depth, consistent with shallow brittle failure above the geothermal production zone. The initial events on 7 April were small (magnitudes 1.4–2.5) and very shallow. Activity intensified overnight, culminating in the magnitude-3.7 mainshock followed by a rapid succession of aftershocks. Later events on 8 and 9 April remained modest in size and again concentrated at depths of 0–3 km. No events exceeded magnitude 4.0, and the swarm exhibited the classic pattern of numerous small events without a dominant mainshock-aftershock decay. This sequence constitutes one of four swarms recorded in the Coso area since 1 January 2000, the first of which occurred in 2000. Such swarms are characteristic of the region and are commonly attributed to pore-pressure changes within the fractured geothermal reservoir rather than magmatic intrusion. The April 2001 swarm provides a representative example of background seismic release in the Coso Volcanic Field. Continued monitoring by regional networks remains essential for distinguishing geothermal-related swarms from tectonic events of greater hazard potential.