Seismic Swarm S20071231.1 Near West Yellowstone, Montana

An earthquake swarm designated S20071231.1 was recorded 18 km northeast of West Yellowstone, Montana, from 04:00 on 30 December 2007 to 14:54 on 2 January 2008. Over 82 hours and 53 minutes, the swarm produced 61 events. Magnitudes ranged from -0.4 to 3.6, with the largest shock occurring at 03:29 on 31 December at a depth of 10 km. Most events clustered between 4 km and 11 km depth, consistent with shallow crustal processes in the region.

This swarm exemplifies typical seismic behavior within the Yellowstone volcanic system. The area lies on the northeastern margin of the Yellowstone Plateau, underlain by the active Yellowstone Caldera formed approximately 631,000 years ago during the most recent supereruption. Ongoing uplift and subsidence, driven by a crustal magma reservoir and hydrothermal fluids, generate frequent earthquake swarms. These swarms often result from fluid migration along faults rather than direct magma movement, producing sequences of small-magnitude events without a single dominant mainshock.

The 2007–2008 swarm fits established patterns. Depths predominantly between 6 km and 9 km align with the brittle-ductile transition zone influenced by elevated geothermal gradients. The distribution of events, including several above magnitude 2.0 on 31 December and 1 January, reflects episodic stress release along pre-existing fractures in the volcanic basement.

Regional monitoring since 2000 has documented 34 swarms through the end of 2007. Annual counts show variability: 10 in 2000, 5 in 2001, 6 in 2002, 3 in 2003, 2 in 2004, 1 in 2005, 5 in 2006, and 2 in 2007. Such activity underscores the persistent tectonic and magmatic influences that maintain elevated seismicity rates compared with surrounding stable continental crust.

No damage or felt reports were associated with this swarm, as expected for events below magnitude 4.0 in a sparsely populated area. Continued surveillance by regional networks remains essential for distinguishing background swarm activity from potential precursors to larger volcanic or tectonic events.

References
SeismoSight internal swarm classification records
USGS Earthquake Hazards Program – Yellowstone region seismicity summaries
USGS Volcano Hazards Program – Yellowstone Caldera geologic framework