Seismic Swarm S20000229.1: Analysis of Early 2000 Activity near Coso Junction, California

An earthquake swarm designated S20000229.1 was recorded beginning at 20:44 on 28 February 2000 and concluding at 18:04 on 9 March 2000. The events were located 31 km east of Coso Junction, California, with a total of 154 earthquakes registered over 237 hours and 20 minutes.

Examination of the first 100 events reveals predominantly low-magnitude activity. The largest shock reached magnitude 4.2 shortly after initiation, followed by additional events of magnitude 4.0 and 3.9. Most events fell between magnitudes 1.0 and 2.5, with numerous smaller shocks below magnitude 1.5. Depths clustered near the surface, frequently reported at 0 km or slightly negative values, while occasional readings extended to 5–7 km. Timing shows intense clustering in the initial 24 hours, with subsequent bursts of elevated rates on 29 February and 2 March.

The Coso region lies within the Coso Volcanic Field in eastern California, part of the Basin and Range province. This area features Quaternary rhyolitic domes, basaltic flows, and an active geothermal system driven by shallow heat sources. Seismicity here commonly occurs as swarms linked to fluid migration within fractured volcanic rocks and ongoing geothermal exploitation. The 2000 swarm aligns with historical patterns of episodic unrest in the field, where swarms often last days to weeks without producing surface rupture.

Geological monitoring in the Coso area benefits from dense seismic networks operated by the U.S. Geological Survey and regional partners. These instruments have documented recurrent swarm activity since the late twentieth century, supporting interpretations of hydrothermal rather than purely tectonic drivers. Depths recorded in the swarm are consistent with the shallow brittle–ductile transition influenced by elevated geothermal gradients.

The provided event catalog indicates no single dominant mainshock–aftershock sequence; instead, the distribution supports swarm classification characterized by similar-magnitude events without clear decay. Shallow focal depths reinforce association with the local geothermal reservoir rather than deeper crustal faulting.

References

• U.S. Geological Survey Earthquake Catalog (events 2000)
• Coso Volcanic Field geologic mapping, California Geological Survey
• Geothermal resource assessments, U.S. Department of Energy (updated through 2023)