Seismic Swarm SVS20210716.1: Analysis of Activity East-Southeast of West Yellowstone, Montana

A notable earthquake swarm, classified internally as SVS20210716.1, occurred in a tectonically active zone 64 km east-southeast of West Yellowstone, Montana. The sequence began at 21:42 on 15 July 2021 and concluded at 02:06 on 23 July 2021, spanning 172 hours and 24 minutes. During this period, 820 earthquakes were recorded, highlighting the region's persistent seismic character.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.4 to 2.6, with the majority falling between 1.0 and 2.0. Depths were shallow, typically 1–9 km, consistent with crustal processes in volcanic and hydrothermal settings. Early events on 15–16 July included several around magnitude 2.0 at depths of 6–7 km, followed by clusters of smaller events. Notable peaks occurred at magnitudes 2.6 on 16 July at 06:06 and 06:26, both at 2 km depth. This pattern of rapid succession of small events with occasional larger ones is characteristic of swarm behavior rather than mainshock-aftershock sequences.

The broader geological context of the area lies within the Yellowstone volcanic system and the Intermountain Seismic Belt. This region experiences frequent seismicity driven by the interaction of the Yellowstone hotspot, crustal extension, and fluid migration in the subsurface. The Yellowstone Plateau features a large caldera formed by massive eruptions, with ongoing deformation, geysers, and hot springs indicating active magmatic and hydrothermal systems. Earthquakes in this zone often occur in swarms due to stress changes from fluid movement or minor magmatic intrusions, rather than large tectonic ruptures.

Historical records from the site indicate 24 swarms since 1 January 2000. These events cluster in specific years: 2002 (5 swarms), 2004 (1), 2006 (1), 2008 (1), 2009 (4), 2010 (1), 2011 (1), 2013 (2), 2015 (1), 2016 (1), 2018 (2), 2019 (1), and 2020 (3). Such recurrence underscores the area's ongoing dynamic state, with swarms serving as indicators of subsurface unrest but rarely leading to significant surface impacts.

Overall, SVS20210716.1 aligns with established patterns of Yellowstone-region seismicity. Continued monitoring remains essential for understanding potential links to volcanic or hydrothermal changes.

References

USGS Earthquake Hazards Program – Yellowstone Seismic Data
SeismoSight Internal Swarm Classification Records
Yellowstone Volcano Observatory Geological Summaries