Seismic Swarm S20200529.1: Analysis of Activity Near West Yellowstone, Montana

Seismic swarm S20200529.1 occurred 24 km east-northeast of West Yellowstone, Montana, within the active Yellowstone volcanic region. This area lies at the edge of the Yellowstone Plateau, shaped by the Yellowstone hotspot that has driven massive caldera-forming eruptions approximately 2.1 million, 1.3 million, and 0.63 million years ago. Ongoing hydrothermal and magmatic processes continue to generate frequent seismicity along the Intermountain Seismic Belt.

The swarm began at 09:15 on 29 May 2020 and concluded at 20:07 on 30 May 2020, spanning 34 hours and 51 minutes. During this interval, 88 earthquakes were recorded. Magnitudes ranged from -0.1 to 3.0, with the largest event (magnitude 3.0) occurring at 10:39 on 29 May at a depth of 9 km. Depths throughout the swarm clustered between 2 km and 9 km, consistent with shallow crustal activity typical of the region.

Event timing showed peak activity in the first several hours after initiation, followed by a gradual decline. Multiple events exceeded magnitude 2.0, including magnitudes 2.5, 2.3, 2.2, and 2.0, all occurring within the initial 24 hours. Negative and near-zero magnitudes indicate the inclusion of microseismicity detectable only by sensitive instrumentation. Depths remained shallow, suggesting involvement of fluid migration or stress adjustments in the upper crust rather than deeper magmatic movement.

This swarm aligns with the broader seismic character of the Yellowstone area, where swarms arise from hydrothermal fluid circulation, tectonic stress release, and minor magmatic influences. Historical records document 79 swarms in the region since 2000, with notable yearly totals including 10 in 2000, 7 each in 2008, 2013, and 2014. The 2020 count reached three swarms, underscoring the persistent nature of clustered seismicity here.

Such swarms rarely precede larger tectonic events but serve as indicators of dynamic subsurface conditions. Depths under 10 km and moderate magnitudes reflect the brittle failure regime above the deeper magmatic system. Continued monitoring by regional networks remains essential for distinguishing routine hydrothermal activity from any escalation tied to volcanic unrest.

References

• U.S. Geological Survey Earthquake Hazards Program data archives
• Yellowstone Volcano Observatory periodic reports on regional seismicity
• SeismoSight internal swarm classification records