Seismic Swarm S20200509.3 Near Hebgen Lake Estates, Montana

Seismic swarm S20200509.3 was recorded 15 km east-northeast of Hebgen Lake Estates, Montana, beginning at 08:37 on 9 May 2020 and concluding at 14:18 on 11 May 2020. Over 53 hours and 40 minutes, 80 earthquakes were detected, with events clustered at shallow to moderate depths primarily between 5 km and 10 km. Magnitudes ranged from negative values to a peak of 1.9, indicating a typical low-to-moderate energy swarm sequence without a dominant mainshock.

The temporal distribution showed the highest activity on the first day, with frequent events between 16:00 and 22:00 on 9 May, including multiple events above magnitude 1.0. Activity decreased on 10 May and tapered further on 11 May, ending with a single event of magnitude 0.4. Depths remained consistent, averaging around 7 km, consistent with regional faulting in the upper crust.

This swarm occurred within the Intermountain Seismic Belt, a zone of active tectonics extending from Montana through Idaho and into Utah. The Hebgen Lake region lies at the northern margin of the Yellowstone volcanic system, where crustal extension interacts with magmatic processes. Historical seismicity includes the magnitude 7.3 Hebgen Lake earthquake of 1959, which triggered widespread faulting and demonstrated the area's capacity for larger events. Seismic swarms here often arise from fluid migration along faults or subtle volcanic influences, producing sequences of small events rather than isolated large shocks.

Since 2000, the broader area has experienced 71 documented swarms, with notable annual counts including 10 in 2000, 7 in 2013, and 6 each in 2008 and 2014. These recurrent swarms underscore persistent crustal stress and fluid involvement in the Hebgen Lake–Yellowstone transition zone. The 2020 swarm fits this established pattern, contributing to ongoing monitoring of strain accumulation near the Yellowstone hotspot.

Data from the sequence reveal a high proportion of events below magnitude 1.0, reflecting sensitive detection capabilities and the swarm's microseismic character. Such activity provides valuable insight into fault network behavior and helps refine models of seismic hazard in this tectonically dynamic region.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog for Hebgen Lake, Montana region.
Montana Bureau of Mines and Geology seismic reports.