Seismic Swarm SVS20201207.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm SVS20201207.1 was recorded 45 km east-southeast of West Yellowstone, Montana, beginning at 20:21 on 6 December 2020 and concluding at 03:56 on 8 December 2020. Over 31 hours and 35 minutes, the swarm comprised 37 earthquakes. Magnitudes ranged from 0.1 to 1.7, with the majority occurring at shallow depths between 1 km and 6 km. The events clustered tightly in both space and time, consistent with swarm behavior rather than a mainshock-aftershock sequence.

The region lies within the Yellowstone Plateau, an area shaped by Quaternary volcanism and ongoing tectonic extension associated with the Basin and Range province. The Yellowstone Caldera, formed by massive eruptions approximately 2.1 million, 1.3 million, and 631,000 years ago, remains underlain by a partially molten crustal magma reservoir. This setting produces frequent small-magnitude earthquakes driven by fluid migration, hydrothermal pressurization, and regional stress fields. Depths recorded in the swarm align with the shallow brittle crust overlying the hydrothermal system.

Historical records indicate persistent swarm activity in the vicinity. Since 1 January 2000, 45 swarms have been documented in the area, with notable yearly totals including six events in 2002, five in both 2006 and 2013, and four each in 2008, 2009, and 2014. The 2020 swarm represents the second occurrence that year. Such recurrent swarms reflect the dynamic interplay between magmatic heat, groundwater circulation, and faulting along the caldera margins.

Analysis of the December 2020 sequence shows peak activity on 7 December, when more than two dozen events occurred within a few hours. Magnitudes remained low, with no events exceeding 2.0, and focal depths predominantly under 6 km, indicating activity within the upper hydrothermal regime. The absence of a dominant mainshock and the rapid decay of event rate after 8 December further characterize the episode as a classic swarm.

This activity underscores the continuous seismic monitoring required in the Yellowstone region to distinguish background swarm behavior from potential precursors to larger volcanic or tectonic events. Ongoing surveillance by regional networks provides essential data for assessing hazard levels in this geologically active landscape.

References

USGS Earthquake Hazards Program, Yellowstone Volcano Observatory reports (2020–2023).
Smith, R. B., et al., "Yellowstone hotspot volcanism and tectonics," Geological Society of America Bulletin (updated syntheses through 2022).