Seismic Swarm SVS20201225.1: Geological Context and Event Analysis Near West Yellowstone

The seismic swarm designated SVS20201225.1 occurred approximately 30 km east-southeast of West Yellowstone, Montana, in a tectonically active zone influenced by the Yellowstone volcanic system. The sequence began at 23:36 on 24 December 2020 and concluded at 12:36 on 25 December 2020, encompassing 47 earthquakes within a span of 12 hours and 59 minutes. Magnitudes ranged from -0.4 to 2.0, with focal depths primarily between 2 km and 8 km, consistent with shallow crustal processes in this region.

This portion of southwestern Montana lies on the periphery of the Yellowstone Plateau, underlain by a partially molten magma reservoir that drives hydrothermal circulation and recurrent microseismicity. The broader geological setting features the Yellowstone caldera, formed by massive rhyolitic eruptions approximately 2.1 million, 1.3 million, and 631,000 years ago. Ongoing deformation arises from magma recharge, fluid migration along faults, and regional extension associated with the Basin and Range province. Earthquake swarms here typically reflect brittle failure triggered by pressurized fluids rather than primary tectonic slip, producing clustered events of low to moderate magnitude over short durations.

Historical records indicate persistent swarm activity in the vicinity. Since 1 January 2000, a total of 65 swarms have been documented in the area, distributed across multiple years as follows: 2000 (7), 2001 (3), 2002 (6), 2003 (3), 2004 (1), 2006 (7), 2007 (2), 2008 (7), 2009 (2), 2010 (2), 2013 (5), 2014 (6), 2016 (1), 2017 (1), 2018 (4), 2019 (3), and 2020 (5). These episodes underscore the episodic nature of seismicity linked to the volcanic plumbing system, where swarms often occur without escalation to larger mainshock-aftershock sequences.

Within SVS20201225.1, event timing shows an initial cluster of smaller shocks followed by a modest increase in magnitude around 02:24–02:57 on 25 December, peaking at 1.7. Depths remained shallow throughout, with many events between 4 km and 7 km, aligning with the depth range of hydrothermal and magmatic interactions beneath the plateau. Negative and near-zero magnitudes reflect the high sensitivity of local seismic networks in detecting microfracturing. Such patterns are characteristic of fluid-driven swarms in Yellowstone, where pore-pressure changes facilitate slip on pre-existing fractures without significant stress accumulation for a major rupture.

Monitoring of these events contributes to understanding volcanic unrest indicators. While swarms can coincide with ground deformation or gas emissions, the limited energy release and rapid decay observed here suggest routine background behavior rather than precursory activity. Continued surveillance by regional networks supports hazard assessment for the Yellowstone area, where annual earthquake counts routinely exceed several thousand.

References

SeismoSight internal swarm classification data (SVS20201225.1 parameters and historical counts).
USGS Earthquake Hazards Program – Yellowstone region seismic reports and caldera geology summaries.
Smith RB, et al., "Yellowstone Supervolcano: Geophysical and Geological Context," updated through 2020 monitoring data.