Seismic Swarm SVS20021111.1: Analysis of Activity Near West Yellowstone, Montana

The seismic swarm designated SVS20021111.1 was recorded 44 km east-southeast of West Yellowstone, Montana, within the Yellowstone volcanic region. The sequence began at 20:17 on 10 November 2002 and concluded at 12:04 on 11 November 2002, encompassing 30 earthquakes over 15 hours and 46 minutes. All events occurred at shallow depths between 1 and 7 km, with magnitudes predominantly between 0.7 and 2.9. The largest event reached magnitude 2.9 at 10:28 on 11 November.

This swarm reflects typical background seismicity in the Yellowstone area, driven by the underlying volcanic and hydrothermal systems. The Yellowstone Plateau lies at the eastern end of the Snake River Plain and forms part of the Intermountain Seismic Belt. The region hosts an active caldera system last formed approximately 631,000 years ago during the most recent major eruption. Ongoing crustal deformation, fluid migration, and tectonic stresses produce frequent earthquake swarms rather than isolated mainshock-aftershock sequences.

Magnitudes in the swarm clustered between 1.0 and 2.5 for the majority of events, with several below magnitude 1.0 and two records showing anomalous values likely representing data artifacts. Depths remained consistently shallow, consistent with activity in the brittle upper crust overlying the Yellowstone magma reservoir. The temporal distribution showed an initial sparse phase followed by a concentrated burst between 07:55 and 08:16 on 11 November, then a secondary peak around 10:27–10:58 before tapering off by midday.

Since 1 January 2000, six swarms have been identified in the same general area, with four occurring in 2002 alone. Earlier swarms took place in 2000 and 2001. This elevated rate in 2002 aligns with periods of heightened monitoring and natural variability in Yellowstone seismicity.

The Yellowstone region experiences several thousand earthquakes annually, most too small to be felt. Swarms such as SVS20021111.1 contribute to the long-term monitoring record used by the Yellowstone Volcano Observatory to assess volcanic unrest. No surface deformation or gas emission anomalies were associated with this particular sequence, indicating it remained within normal background levels.

References

• United States Geological Survey, Yellowstone Volcano Observatory: Geology and History of Yellowstone
• United States Geological Survey Earthquake Catalog: Events near West Yellowstone, Montana, November 2002
• Intermountain Seismic Belt historical seismicity summaries (USGS)