Seismic Activity Analysis: The January 2001 Earthquake Swarm Near West Yellowstone, Montana

The earthquake swarm designated S20010119.1 occurred 4 km southeast of West Yellowstone, Montana, within the Yellowstone volcanic region. It began at 17:59 on 18 January 2001 and concluded at 12:15 on 23 January 2001, spanning 114 hours and 15 minutes. During this period, 341 earthquakes were recorded. This swarm represents a typical episode of seismic unrest in an area shaped by ongoing volcanic and hydrothermal processes.

Yellowstone lies atop a large volcanic system driven by a mantle hotspot. The region features the Yellowstone Caldera, formed by massive eruptions over the past 2.1 million years, with the most recent at approximately 631,000 years ago. Seismic swarms commonly arise from fluid migration, including hydrothermal waters and magmatic gases, interacting with the brittle crust at depths of 5–12 km. These events reflect the dynamic interplay between the shallow magma reservoir and the overlying hydrothermal system rather than immediate volcanic eruption risk.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Recorded magnitudes ranged from –0.4 to 2.0, with the majority below 1.0. Depths clustered tightly between 5 and 9 km, centering around 7 km, consistent with the brittle-ductile transition zone in this tectonically active setting. A single anomalous reading of –9.9 likely represents a data artifact. Temporal distribution showed a gradual increase in event rate through 19–20 January, peaking with several events above magnitude 1.5, before tapering. Such patterns align with fluid-driven swarm behavior rather than mainshock-aftershock sequences.

Since 1 January 2000, nine swarms have been documented in the region, beginning with the initial swarm in 2000. These recurrent episodes underscore the persistent seismic vitality of the Yellowstone system, where annual earthquake counts often exceed 1,000, most too small to be felt.

This swarm contributed to the broader understanding of crustal fluid dynamics beneath Yellowstone. Continued monitoring by seismic networks supports hazard assessment and scientific study of the volcanic environment.

References

USGS Volcano Hazards Program – Yellowstone Yellowstone Caldera Chronicles, USGS SeismoSight internal swarm classification records