Seismic Swarm S20170802.1: Analysis of Activity Near Hebgen Lake, Montana
Seismic swarm S20170802.1 was recorded in the Hebgen Lake region of Montana, beginning at 21:44 on 1 August 2017 and concluding at 04:58 on 19 August 2017. Over 415 hours and 13 minutes, the swarm produced 784 earthquakes. The events were centered approximately 10 km east-northeast of Hebgen Lake Estates. Analysis of the first 100 events reveals a concentration of low-magnitude activity. Magnitudes ranged from -0.5 to 2.8, with the majority below 1.5. Depths were predominantly between 5 km and 10 km, consistent with shallow crustal seismicity. Early events on 1–2 August showed sparse occurrence, followed by increased frequency and several magnitude-2 events on 2 August. Peak activity included a magnitude-2.8 event on 3 August at 00:18, accompanied by additional events above magnitude 2.0. The sequence demonstrated typical swarm characteristics, with events clustered in time and space without a dominant mainshock. The Hebgen Lake area lies within the Intermountain Seismic Belt, a zone of active extensional tectonics extending from Montana into Idaho and Utah. This belt accommodates Basin and Range-style deformation and intersects the northeastern margin of the Yellowstone volcanic system. Historical records document frequent earthquake swarms driven by fluid migration and magmatic processes associated with the Yellowstone hotspot. The 1959 Hebgen Lake earthquake (magnitude 7.3) remains the largest instrumentally recorded event in the region and triggered extensive aftershock sequences. Since 2000, 61 swarms have occurred in the area, with notable years including 2000 (9 swarms), 2008 (6), 2013 (6), and 2014 (6). This pattern underscores persistent low-level seismic unrest linked to regional tectonics and hydrothermal circulation. The swarm data align with established patterns of Yellowstone-area seismicity, where swarms often exhibit maximum magnitudes below 3.0 and focal depths under 15 km. No surface rupture or significant ground deformation was associated with this sequence.
References
USGS Earthquake Hazards Program – Hebgen Lake region seismicity reports
Montana Bureau of Mines and Geology – Regional seismic monitoring data
Yellowstone Volcano Observatory – Swarm classification and tectonic summaries