Seismic Swarm S20010227.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20010227.1 occurred in the tectonically active region 9 km northeast of West Yellowstone, Montana. This sequence began at 14:40 on 26 February 2001 and concluded at 20:12 on 28 February 2001, spanning 53 hours and 32 minutes. During this period, 56 earthquakes were recorded, with magnitudes ranging from -0.2 to 1.8 and focal depths predominantly between 7 and 9 km.

The swarm unfolded with an initial cluster of low-magnitude events on 26 February, including several below magnitude 1.0 clustered at depths of 7–9 km. Activity intensified slightly in the evening, followed by sporadic events overnight. On 27 February, the sequence showed a gradual increase, highlighted by a magnitude 1.8 event at 05:11 at 7 km depth. Subsequent events maintained similar depth ranges, with additional notable shocks of magnitude 1.5 and 1.4 later that day. The final day, 28 February, featured fewer but comparable events, including a magnitude 1.8 quake at 07:42 at 9 km depth and one shallower event at 3 km.

This pattern reflects typical swarm behavior, characterized by numerous small events without a dominant mainshock. Magnitudes remained modest throughout, with most below 1.0, indicating distributed stress release rather than a single large rupture. Depths clustered tightly around 8 km suggest activity within the brittle upper crust overlying the region's volcanic structures.

The location places the swarm within the Yellowstone volcanic system, part of the Intermountain Seismic Belt. This area overlies the Yellowstone hotspot, where a mantle plume drives crustal deformation, hydrothermal activity, and frequent seismicity. The Yellowstone Caldera, formed by supereruptions approximately 2.1 million, 1.3 million, and 640,000 years ago, continues to influence regional geology through ongoing uplift, subsidence, and earthquake swarms. Historical records document numerous such swarms linked to fluid migration and magmatic processes beneath the plateau.

Since 1 January 2000, 12 swarms have been identified in the vicinity. These include 10 events in 2000 and 2 in 2001, underscoring the recurrent nature of clustered seismicity in this volcanic setting. Such sequences provide valuable data on subsurface dynamics without producing significant surface damage.

References

USGS Earthquake Hazards Program: Yellowstone Seismic Activity
Yellowstone Volcano Observatory Annual Reports (2000–2001)
Montana Bureau of Mines and Geology: Regional Tectonic Overview