Seismic Swarm S20000329.1: Analysis of Activity Near Coso Junction, California

The seismic swarm designated S20000329.1 occurred 7 km southeast of Coso Junction in Inyo County, California. It began at 15:16 UTC on 28 March 2000 and concluded at 04:53 UTC on 7 April 2000, spanning 229 hours and 36 minutes. During this interval, 115 earthquakes were recorded. The sequence represents the sole swarm documented in the region since 1 January 2000, with no prior swarms noted in the preceding record.

Coso Junction lies within the tectonically active Basin and Range Province, adjacent to the Coso Volcanic Field. This field encompasses rhyolitic domes, basaltic flows, and extensive geothermal resources. The local geology features a network of northwest-trending normal faults that accommodate regional extension. These structures intersect with the eastern margin of the Sierra Nevada, creating a setting prone to both tectonic and volcanically influenced seismicity. Depths of recorded events remained predominantly shallow, between 0 and 4 km, consistent with activity along brittle crustal faults above deeper ductile zones.

Examination of the first 100 events reveals a typical swarm pattern: an initial energetic phase followed by decaying frequency and magnitude. The largest event, magnitude 4.2, occurred at the onset at a depth of 0 km. Subsequent shocks included multiple events of magnitude 3.0–3.4 within the first 48 hours. Magnitudes clustered between 1.0 and 2.5 for the majority of the sequence, with only a handful exceeding 3.0. Depths showed limited variation, rarely surpassing 4 km and occasionally registering negative values indicative of very near-surface sources. Temporal distribution indicates peak activity on 28–30 March, with 33 events in the initial 48 hours, followed by a gradual decline through early April.

Such swarms in the Coso region commonly arise from fluid migration along fault planes or minor magmatic movement at depth. The shallow hypocenters align with the known geothermal reservoir, where high heat flow and hydrothermal circulation reduce effective stress and facilitate earthquake triggering. Historical context includes recurrent microseismicity tied to both tectonic extension and geothermal production in the Coso Geothermal Field, operational since the 1980s.

The 2000 swarm underscores the persistent seismic hazard in this portion of eastern California. Although no damage was reported, sequences of this type highlight the value of continuous monitoring for distinguishing background tectonic events from potential precursors to larger activity or volcanic unrest.

References

• SeismoSight internal swarm classification records
• USGS Earthquake Catalog (pre-2000 regional data)
• California Geological Survey, Coso Volcanic Field geologic maps