Seismic Swarm S20250507.1: Analysis of Earthquake Activity Near West Yellowstone

On 7 May 2025, a seismic swarm designated S20250507.1 was recorded 6 km north-northeast of West Yellowstone, Montana. The sequence began at 07:46 and concluded at 16:18, encompassing 8 hours and 31 minutes during which 37 earthquakes occurred. Magnitudes ranged from 0.1 to 1.7, with focal depths primarily between 8 km and 12 km. Events clustered in the morning hours, including peaks at 10:22 (magnitude 1.5), 11:17 (magnitude 1.5), and 11:35 (magnitude 1.7). Later activity included a magnitude 1.2 event at 16:18 that marked the swarm's end.

This swarm aligns with patterns typical of the region, where low-magnitude events occur in rapid succession without a dominant mainshock. Depths remained consistent around 9–10 km for most events, suggesting a shallow crustal source influenced by local tectonic and hydrothermal processes.

The West Yellowstone area lies along the western margin of the Yellowstone Caldera within the Intermountain Seismic Belt. This zone experiences ongoing seismicity driven by the Yellowstone hotspot, a mantle plume responsible for the region's volcanism, geothermal features, and crustal deformation. The caldera formed through massive eruptions approximately 2.1 million, 1.3 million, and 631,000 years ago, leaving a dynamic subsurface environment prone to fluid migration and stress changes that trigger swarms.

Historical records indicate frequent swarm activity here. Since 2000, 97 swarms have been documented in the vicinity, with notable annual counts including 10 in 2000, 7 in 2008, 7 in 2013, 7 in 2022, and 6 each in 2006 and 2014. These episodes often correlate with magma chamber adjustments or hydrothermal fluid movement beneath the caldera, as monitored by regional seismic networks.

Insights from the 2025 swarm reveal a typical distribution of small-magnitude events concentrated over a short duration. Early morning activity featured multiple 0.7–1.1 magnitude quakes at 9–10 km depth, transitioning to slightly deeper events (11–12 km) in the afternoon. The absence of larger events underscores the swarm's non-destructive nature, consistent with background seismicity levels averaging hundreds of events annually in the greater Yellowstone region.

Such swarms provide valuable data on subsurface dynamics without indicating imminent volcanic unrest. Continuous monitoring by agencies like the U.S. Geological Survey helps distinguish routine activity from potential precursors to larger geophysical changes.

References

U.S. Geological Survey, Yellowstone Volcano Observatory reports on regional seismicity.
Montana Bureau of Mines and Geology, Intermountain Seismic Belt earthquake catalogs.
Smith et al., Geological Society of America publications on Yellowstone Caldera evolution.