Seismic Swarm S20010423.1: Analysis of Activity Near Coso Junction, California

A notable earthquake swarm, designated S20010423.1, was recorded 5 km east-southeast of Coso Junction in Inyo County, California. The sequence began at 02:53 on 23 April 2001 and concluded at 23:59 on 1 May 2001, spanning 213 hours and 6 minutes. During this period, 256 earthquakes were detected, providing a clear example of swarm behavior in a volcanically and tectonically active region.

The Coso area lies within the Basin and Range Province, characterized by extensional tectonics, normal and strike-slip faulting, and Quaternary volcanism. The nearby Coso Volcanic Field features rhyolitic domes, basaltic flows, and an active geothermal system driven by shallow crustal heat sources. Seismicity in this zone frequently occurs at depths of 1–6 km, reflecting both tectonic strain along the Walker Lane belt and fluid migration within fractured volcanic rocks. Geothermal production at the Coso facility, operational since the 1980s, has been associated with induced microearthquakes, although natural swarms also occur due to magmatic or hydrothermal processes.

Analysis of the first 100 events reveals a rapid onset followed by sustained low-to-moderate activity. The initial event measured magnitude 1.2 at 2 km depth. Within hours, a magnitude 3.5 shock occurred at 3 km depth, triggering a cluster of events with magnitudes ranging from 0.5 to 3.3. Depths remained consistently shallow (1–5 km), with the majority between 2 and 4 km. Magnitudes were predominantly below 2.5, though several events exceeded 3.0, including a 3.2 at 16:44 and a 3.3 at 19:59 on 23 April. Temporal clustering was evident, with elevated rates immediately after larger shocks, consistent with aftershock triggering modified by fluid pressure changes typical of geothermal fields.

Subsequent events through 24 April maintained similar characteristics, showing no clear migration in depth or location within the provided data. This pattern aligns with historical observations of short-lived swarms in the Coso region, where high b-values often indicate fluid involvement rather than purely tectonic mainshock-aftershock sequences.

Since 1 January 2000, five swarms have been documented in the immediate area. The 2000 swarm comprised four sequences, while 2001 recorded this single episode. Such recurrence underscores the persistent seismic productivity of the Coso geothermal and volcanic complex.

These observations contribute to understanding swarm dynamics in tectonically complex volcanic settings and support ongoing monitoring by regional seismic networks.

References

USGS Earthquake Catalog California Geological Survey regional reports Coso Operating Company geothermal monitoring summaries