Seismic Swarm S20040629.1 Near Coso Junction, California

A notable earthquake swarm, designated S20040629.1, occurred in the Coso region of eastern California. The sequence began at 20:53 on 28 June 2004 and concluded at 19:07 on 2 July 2004, spanning 94 hours and 14 minutes. It was centered 5 km east-northeast of Coso Junction and produced 108 earthquakes.

Analysis of the first 100 events reveals predominantly low-magnitude activity. The largest event reached magnitude 3.7 on 29 June at 10:20:43, with a focal depth of 4 km. Subsequent events included magnitudes of 2.7, 2.3, 2.2, and 2.1, all occurring within the initial 24 hours after the peak event. Depths ranged from 0 km to 9 km, with the majority clustered between 3 km and 6 km. Temporal clustering was evident, with elevated rates following the magnitude 3.7 shock and several magnitude 1.5–2.0 events later on 29 and 30 June. Shallow events (under 3 km) appeared sporadically, while deeper events (7–9 km) remained infrequent.

This swarm fits the established pattern of seismic activity in the Coso Volcanic Field. The region lies within the western Basin and Range province, where northwest-directed extension accommodates Pacific–North American plate motion. The Coso Volcanic Field itself encompasses Pleistocene rhyolite domes, basaltic flows, and an active geothermal system. Heat flow exceeds 100 mW/m² in the central field, supporting commercial geothermal power production since the 1980s. Seismicity frequently manifests as swarms linked to fluid migration along fractures rather than mainshock-aftershock sequences driven by tectonic stress release.

Historical records maintained by SeismoSight indicate 11 swarms in the area since 1 January 2000. Earlier episodes occurred in 2000 (four swarms), 2001 (four swarms), and 2002 (two swarms), with the 2004 event representing the sole swarm recorded that year up to the date of S20040629.1. These recurrent swarms underscore the persistent influence of hydrothermal and possibly magmatic processes beneath the volcanic field.

The 2004 sequence exhibited typical swarm characteristics: rapid onset, absence of a single dominant mainshock, and quick decay after the initial energetic phase. Depths align with the brittle-ductile transition zone in the geothermal reservoir, where temperatures approach 200–300 °C. No damage or felt reports beyond the immediate vicinity were associated with the events.

Continued monitoring of the Coso area remains essential for understanding the interplay between geothermal exploitation, tectonic extension, and volcanic unrest. Future swarms may provide additional constraints on subsurface fluid pathways and stress evolution within this geothermally active corridor.

References

SeismoSight internal swarm catalog (S20040629.1 parameters and historical statistics).
USGS Earthquake Catalog (regional seismicity context).
California Geological Survey, Coso Volcanic Field geologic map and geothermal reports.