Seismic Analysis of Swarm SVS20211001.1 East of West Yellowstone, Montana

Earthquake swarm SVS20211001.1 was recorded 30 km east of West Yellowstone, Montana, beginning at 12:03 on 30 September 2021 and concluding at 04:11 on 2 October 2021. The sequence lasted 40 hours and 8 minutes, encompassing 62 events. This activity occurred within the seismically active Yellowstone volcanic region, where swarms commonly arise from interactions between tectonic stresses and subsurface magmatic or hydrothermal processes.

Magnitudes in the swarm ranged from -0.3 to 1.8, with the majority below 1.0 and only isolated events reaching 1.2–1.8. Depths clustered primarily between 2 km and 9 km, though several events extended to 10–12 km. Early activity on 30 September featured modest magnitudes around 0.5–1.3 at depths of 6–9 km. Activity intensified on 1 October, producing clusters with events up to 1.8 at 7 km depth, alongside shallower occurrences near 2–4 km. The sequence tapered on 2 October with a final event of magnitude 0.3 at 7 km.

Such patterns reflect typical swarm behavior in the area, where rapid sequences of small earthquakes occur without a dominant mainshock. The provided event list illustrates this distribution, showing repeated low-magnitude releases at mid-crustal depths consistent with fluid migration or stress adjustments along local faults.

Since 1 January 2000, the region has experienced 74 swarms. Annual counts include 9 in 2000, 4 in 2001, 7 in 2002, 3 in 2003, 1 in 2004, 7 in 2006, 2 in 2007, 6 in 2008, 2 in 2009, 1 in 2010, 1 in 2011, 6 in 2013, 6 in 2014, 2 in 2017, 4 in 2018, 3 in 2019, 6 in 2020, and 4 in 2021. This history underscores the recurrent nature of swarm activity east of West Yellowstone.

The locale forms part of the Yellowstone Plateau, underlain by a large volcanic system responsible for ongoing seismicity. Earthquake swarms here frequently correlate with the caldera’s dynamic geology, including movement of magma or geothermal fluids that trigger fault slip at shallow to moderate depths. Historical monitoring indicates that swarms of this scale rarely produce significant surface effects but contribute to understanding the interplay between volcanism and regional tectonics.

References
SeismoSight internal swarm classification SVS20211001.1
USGS Earthquake Catalog (Yellowstone region records)
Yellowstone Volcano Observatory annual seismicity summaries