Seismic Insights from the June 2021 Earthquake Swarm Near West Yellowstone
The earthquake swarm designated SVS20210621.1 began at 14:33 on 21 June 2021 and concluded at 03:53 on 23 June 2021. Centered 27 km east-southeast of West Yellowstone, Montana, the sequence produced 117 earthquakes over 37 hours and 19 minutes. This event fits the established pattern of seismic swarms that characterize the Yellowstone volcanic region.
The area overlies the Yellowstone Caldera, a large volcanic system shaped by multiple super-eruptions over the past 2.1 million years. Ongoing magmatic and hydrothermal processes drive frequent earthquake activity. Swarms arise when fluids or magma movement triggers numerous small quakes rather than a single large event. Depths in this swarm ranged mainly from 2 to 10 km, consistent with shallow crustal processes typical of the caldera’s eastern margin.
Analysis of the first 100 recorded events reveals predominantly low-magnitude activity. Magnitudes clustered between 0.0 and 2.5, with the strongest shock reaching 2.5. Most events fell below magnitude 2.0, and depths showed a concentration around 4–7 km. Temporal distribution indicated peak activity within the first several hours, followed by a gradual decline. No events exceeded magnitude 3.0, aligning with the region’s history of modest swarm intensities.
Historical records document 72 swarms in the vicinity since 2000. Yearly counts include nine in 2000, seven each in 2002 and 2006, six in both 2013 and 2014, and six again in 2020. These recurrent episodes underscore the persistent dynamic state of the Yellowstone hydrothermal and magmatic system.
The June 2021 swarm reinforces the value of continuous seismic monitoring for understanding volcanic unrest. Such sequences provide data on fluid migration and stress changes within the crust, supporting hazard assessment for nearby communities and infrastructure.
SeismoSight internal classification SVS20210621.1
USGS Earthquake Hazards Program – Yellowstone region reports
Smithsonian Institution Global Volcanism Program – Yellowstone caldera overview