Seismic Swarm S20001224.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20001224.1 was recorded beginning at 19:47 on 23 December 2000 and concluding at 06:57 on 28 December 2000. The sequence occurred 22 km ENE of West Yellowstone, Montana, within the tectonically active Yellowstone Plateau. Over 107 hours and 10 minutes, 64 earthquakes were registered. Magnitudes remained low throughout, with the majority between 0.0 and 2.4 and one anomalous reading of -9.9. Focal depths ranged from 0 to 9 km, consistent with shallow crustal processes.

The swarm exhibited a typical clustered pattern, with events concentrated on 24 December. Early activity included micro-earthquakes at depths of 1–4 km, followed by slightly larger events reaching magnitude 2.4 at 7 km depth. Subsequent days showed continued low-level seismicity tapering toward the end of the sequence on 28 December. Such patterns reflect episodic stress release rather than a single mainshock-aftershock series.

The Yellowstone region overlies a continental hotspot that has driven volcanic activity for millions of years. The current Yellowstone Caldera formed approximately 631,000 years ago during the most recent supereruption. Ongoing hydrothermal circulation and magma movement at depth generate frequent earthquake swarms. These swarms differ from tectonic mainshock sequences by their diffuse spatial distribution and lack of a dominant event. Historical records indicate that earthquake swarms have been documented in the area since systematic monitoring began.

Since 1 January 2000, nine swarms have been identified in the broader region, with S20001224.1 representing the earliest. This statistic underscores the recurrent nature of swarm activity driven by the underlying volcanic system. Depths recorded in the present swarm align with the shallow brittle crust above deeper ductile zones influenced by heat from the hotspot.

Monitoring by the U.S. Geological Survey and the Yellowstone Volcano Observatory provides continuous data on such sequences. No surface deformation or changes in hydrothermal features were associated with this particular swarm, consistent with its modest energy release.

References

SeismoSight internal swarm classification records
U.S. Geological Survey Earthquake Hazards Program data for the Yellowstone region