Seismic Swarm SVS20210917.1 East of West Yellowstone, Montana

Seismic swarm SVS20210917.1 occurred 29 km east of West Yellowstone, Montana, from 08:20 on 16 September 2021 to 09:09 on 20 September 2021. In 96 hours and 48 minutes, the sequence produced 75 earthquakes, with magnitudes ranging from -0.2 to 1.7 and focal depths between 2 km and 12 km. The majority of events clustered between 5 km and 9 km depth, consistent with shallow crustal processes in the region.

The swarm initiated with a magnitude 0.2 event at 4 km depth, followed by a magnitude 1.7 shock at 7 km within the first hour. Activity remained low-magnitude throughout, with only a handful of events exceeding magnitude 1.0. The largest event, magnitude 1.7, occurred on 19 September at 7 km depth. Event rates peaked on 16–17 September and again briefly on 19 September before tapering to background levels by 20 September.

This location lies on the western margin of the Yellowstone Plateau, within the active Yellowstone volcanic field. The area overlies the Yellowstone hotspot, a mantle plume responsible for bimodal rhyolite-basalt volcanism and the formation of the 0.63-million-year-old Yellowstone Caldera. Ongoing crustal deformation arises from magmatic recharge, hydrothermal fluid circulation, and regional tectonic extension associated with the Basin and Range province. Earthquake swarms here commonly reflect fluid migration or minor magmatic movement rather than tectonic fault rupture.

Historical records since 2000 document 69 swarms in the same general area. Annual counts include nine swarms in 2000, seven in 2002, six each in 2006, 2008, 2014, and 2020, and two in 2021 prior to SVS20210917.1. These episodes underscore the persistent seismic unrest characteristic of the Yellowstone volcanic system.

Swarm SVS20210917.1 fits established patterns of short-duration, low-magnitude sequences that do not indicate imminent volcanic activity. Depths and magnitudes align with the shallow brittle-ductile transition zone influenced by elevated geothermal gradients beneath the plateau. Continued monitoring by the Yellowstone Volcano Observatory remains essential for distinguishing routine hydrothermal or magmatic unrest from any escalation.

References

• U.S. Geological Survey Earthquake Catalog
• Yellowstone Volcano Observatory annual seismicity reports
• SeismoSight internal swarm classification database