Seismic Swarm S20060222.1 Near West Yellowstone, Montana

On February 22, 2006, a notable earthquake swarm designated S20060222.1 was recorded approximately 20 km southeast of West Yellowstone, Montana. The sequence began at 06:19 UTC and concluded by 22:00 UTC the same day, encompassing a total of 44 events over 15 hours and 40 minutes. This swarm occurred within the seismically active Yellowstone region, part of the broader Intermountain Seismic Belt influenced by the underlying Yellowstone hotspot.

The Yellowstone area lies atop a continental hotspot that has driven volcanic activity for millions of years, forming the Yellowstone Caldera through massive eruptions. Current seismicity reflects ongoing adjustments from magmatic intrusions, fluid migration in hydrothermal systems, and regional tectonic stresses. Earthquake swarms are common here, often lacking a single dominant mainshock and instead featuring clustered events of similar magnitudes driven by non-tectonic processes.

Event magnitudes in the swarm ranged from 0.0 to 1.9, with the majority falling between 0.4 and 1.4. Depths were predominantly shallow, between 2 km and 14 km, consistent with the brittle upper crust overlying the volcanic system. Several entries recorded at -9.9 likely represent data artifacts or unassigned values rather than physical measurements. Notable peaks included a 1.9 magnitude event at 07:24 UTC at 9 km depth and a 1.7 magnitude shock at 08:46 UTC at 8 km depth. The temporal distribution showed highest activity in the initial hours, tapering after midday.

Historical records indicate 22 swarms have occurred in the region since January 1, 2000. Prior activity includes eight swarms in 2000, four in 2001, five in 2002, three in 2003, one in 2004, and one in 2005. These patterns underscore the persistent seismic character of the Yellowstone volcanic field, where swarms frequently accompany subtle ground deformation or hydrothermal changes.

Such events provide valuable data for monitoring volcanic unrest. Although most swarms remain minor, they contribute to understanding stress regimes and fluid dynamics beneath the caldera. Ongoing instrumentation by seismic networks supports real-time analysis and hazard assessment in this geologically dynamic setting.

References

USGS Earthquake Hazards Program – Yellowstone Seismicity Reports
Yellowstone Volcano Observatory – Geological Background and Swarm Characteristics
SeismoSight Internal Classification Database – Swarm S20060222.1 Data