Seismic Swarm S20061105.1 Near West Yellowstone, Montana

Seismic swarm S20061105.1 occurred approximately 20 km south-southeast of West Yellowstone, Montana, within the Yellowstone volcanic region. The sequence began at 21:58 on 4 November 2006 and concluded at 12:25 on 6 November 2006, spanning 38 hours and 26 minutes. During this interval, 46 earthquakes were recorded.

The events exhibited magnitudes ranging from -0.1 to 2.6, with the largest shock measuring 2.6 at a depth of 11 km on 5 November at 02:08. Depths predominantly clustered between 8 and 13 km, consistent with typical activity in the upper crust beneath the Yellowstone Plateau. Smaller events, including several below magnitude 1.0, dominated the sequence, reflecting the swarm's characteristic lack of a single dominant mainshock.

This swarm aligns with the broader tectonic and volcanic setting of the Yellowstone area. The region overlies a major silicic volcanic system driven by a mantle hotspot, featuring a large caldera formed by massive eruptions over the past 2.1 million years. Ongoing crustal deformation, hydrothermal fluid circulation, and magma movement at depth contribute to frequent earthquake swarms. Seismicity here often occurs in clusters rather than isolated events, with activity concentrated along ring-fracture zones and resurgent domes within the caldera.

Historical records maintained by SeismoSight indicate 12 swarms in the area since 1 January 2000. These occurred in the following years and quantities: 2000 (2), 2001 (2), 2002 (1), 2003 (3), and 2006 (4). Such recurrent swarms underscore the persistent dynamic processes beneath Yellowstone, where episodic unrest is common.

Analysis of the 2006 sequence shows a gradual onset with low-magnitude events, followed by a peak in activity during the afternoon and evening of 5 November. Depths remained relatively stable, suggesting a consistent source volume. No surface deformation or volcanic signals accompanied the swarm, indicating it was likely driven by fluid migration or minor stress adjustments rather than significant magmatic intrusion.

The Yellowstone region continues to be monitored closely due to its potential for both seismic and volcanic hazards. Swarms like S20061105.1 provide valuable data for understanding subsurface processes and refining hazard assessments.

References

SeismoSight internal swarm classification records.
USGS Yellowstone Volcano Observatory reports on regional seismicity.
Smith, R. B., et al., "Geodynamics of the Yellowstone hotspot and mantle plume," Journal of Volcanology and Geothermal Research (updated syntheses through 2023).