Seismic Swarm SVS20211126.1: Analysis of Activity East of West Yellowstone, Montana

A seismic swarm designated SVS20211126.1 was recorded 31 km east of West Yellowstone, Montana, from 16:27 on 25 November 2021 to 04:34 on 28 November 2021. Over 60 hours and 7 minutes, the swarm comprised 61 earthquakes. This event aligns with patterns of clustered seismicity common in the Yellowstone region, where tectonic and hydrothermal processes frequently generate swarms rather than isolated mainshock-aftershock sequences.

The swarm exhibited predominantly low magnitudes, with events ranging from 0.0 to 1.9. Depths were generally shallow, concentrated between 2 km and 10 km, though a few outliers reached 22 km or registered at negative depths likely due to location uncertainties. Activity peaked on 26 November, with multiple events between 14:34 and 15:06, including a magnitude 1.7 quake at 7 km depth. Later phases on 27 November included the swarm's largest event, a magnitude 1.9 at 7 km depth. The sequence tapered by early 28 November. Such temporal clustering and limited energy release are characteristic of fluid-driven swarm behavior in volcanic terrains.

Regional Geological Context

The swarm location lies within the Yellowstone Plateau, part of the eastern Snake River Plain and influenced by the North American tectonic plate's interaction with the underlying Yellowstone hotspot. This supervolcano features a large caldera formed approximately 631,000 years ago during the most recent major eruption. Ongoing crustal deformation, magma movement, and hydrothermal fluid circulation sustain elevated seismicity. The area forms part of the Intermountain Seismic Belt, where extensional faulting contributes to background earthquake rates.

Earthquake swarms here often result from pressurized fluids migrating through fractured rock, triggering slip on small faults without significant stress accumulation. Depths in this swarm, mostly under 10 km, correspond to the brittle upper crust overlying the shallower hydrothermal system and deeper magmatic reservoirs mapped by geophysical surveys.

Historical Swarm Patterns

Seismic swarms have been documented in the region since systematic monitoring began. Since 1 January 2000, 69 swarms have occurred in the broader area. Annual counts include seven in 2000, six each in 2002, 2008, 2014, and 2020, with lower activity in intervening years such as single swarms in 2001, 2011, 2016, and 2017. The 2021 total reached six swarms prior to SVS20211126.1. These recurrent episodes underscore the persistent dynamic state of the Yellowstone volcanic system, where swarms serve as indicators of subsurface fluid or magmatic processes rather than immediate eruption precursors.

Monitoring by the U.S. Geological Survey and the University of Utah Seismograph Stations provides continuous data on such events, supporting hazard assessment for the Yellowstone National Park region. No escalation to higher-magnitude events or surface deformation beyond typical levels was associated with this swarm.

Implications and Monitoring Value

Analysis of SVS20211126.1 reinforces the value of dense seismic networks in distinguishing swarm activity from tectonic mainshocks. Continued observation aids in refining models of hydrothermal and magmatic interactions beneath the plateau. Residents and visitors should remain aware of the region's natural seismic background, though individual swarms like this one pose minimal direct hazard.

References

U.S. Geological Survey, Yellowstone Volcano Observatory reports (2021–2023 updates).
University of Utah Seismograph Stations, Intermountain Seismic Belt catalog.
Yellowstone National Park geologic summaries, National Park Service.