Seismic Swarm SVS20220905.1 Near West Yellowstone, Montana

On 5 September 2022, a seismic swarm designated SVS20220905.1 was recorded 35 km east-southeast of West Yellowstone, Montana. The sequence began at 01:52 UTC and concluded at 05:25 UTC, encompassing 40 earthquakes within a span of three hours and thirty-two minutes. Magnitudes ranged from –0.1 to 2.5, with focal depths predominantly between 3 km and 7 km. The largest event, magnitude 2.5, occurred at 02:10 UTC at 7 km depth.

This swarm unfolded within the tectonically active Yellowstone region, situated along the eastern margin of the Intermountain Seismic Belt. The area overlies the Yellowstone Caldera, a large volcanic system shaped by repeated caldera-forming eruptions over the past 2.1 million years. Ongoing crustal deformation, hydrothermal fluid migration, and regional extension contribute to elevated seismicity. Earthquake swarms—clusters of events without a single dominant mainshock—are a characteristic feature of the Yellowstone volcanic field, often linked to fluid movement rather than magmatic intrusion.

Since 1 January 2000, 75 swarms have been documented in the broader region. Annual counts include seven swarms each in 2000 and 2008, six in 2002, 2014, 2020, and 2021, and four in 2022. These episodes typically comprise dozens to hundreds of small-magnitude earthquakes occurring over hours to days, consistent with the 40-event, sub-four-hour duration observed in SVS20220905.1.

Event timing within the swarm showed two phases of elevated activity: an initial cluster between 01:52 and 02:28 UTC containing the magnitude-2.5 and magnitude-2.3 events, followed by a later burst between 04:59 and 05:25 UTC. Depths remained shallow, with 80 percent of hypocenters between 3 km and 7 km, aligning with the brittle–ductile transition zone typical of the caldera’s upper crust. The absence of felt reports or damage is consistent with the modest magnitudes recorded.

Such swarms provide valuable data on subsurface stress conditions and fluid pathways. Continued monitoring by regional seismic networks supports ongoing assessment of volcanic and tectonic hazards in this geologically dynamic setting.

References
United States Geological Survey Earthquake Catalog
Yellowstone Volcano Observatory Annual Reports
Montana Bureau of Mines and Geology Seismicity Database