Seismic Swarm S20200522.1: Analysis of Activity Near West Yellowstone, Montana
On 22 May 2020, a seismic swarm designated S20200522.1 was recorded 25 km east-northeast of West Yellowstone, Montana. The sequence began at 08:02 and concluded at 10:29, lasting 2 hours and 27 minutes. During this interval, 36 earthquakes were registered, with magnitudes ranging from -0.9 to 2.0. Depths varied between 2 km and 9 km, indicating shallow crustal activity typical of the region.
The swarm exhibited a rapid onset with several events clustered in the first 30 minutes, including two magnitude-2.0 earthquakes at 08:02:47 and 08:08:40. Subsequent activity showed a gradual decline, with smaller events persisting until the final recorded tremor at 10:29:41. Depths remained predominantly in the 2–5 km range, consistent with fluid-driven processes in the upper crust.
This event occurred within the Yellowstone volcanic field, where the underlying hotspot fuels ongoing geological dynamism. The area lies along the Intermountain Seismic Belt, a zone of extensional tectonics that accommodates regional strain. Earthquake swarms here often arise from hydrothermal fluid migration or minor magmatic movements rather than large tectonic ruptures.
Since 2000, 79 swarms have been documented in the vicinity, demonstrating a recurring pattern. Annual counts include 10 in 2000, 7 each in 2008, 2013, and 2014, and lower frequencies in intervening years, with 2 recorded in 2020 prior to this sequence. Such episodes underscore the persistent low-level seismicity that characterizes the caldera margins.
Monitoring networks provide continuous observation of these patterns, aiding in distinguishing routine swarm behavior from potential precursors to larger unrest. The 2020 swarm aligns with established norms for the locale, where events rarely exceed magnitude 3 yet contribute to a cumulative understanding of subsurface dynamics.
References
SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional summaries for the Yellowstone area.
Yellowstone Volcano Observatory geological overviews.