Seismic Swarm S20160925.1 Near Hebgen Lake Estates, Montana
Seismic swarm S20160925.1 occurred 5 km southeast of Hebgen Lake Estates in Montana. The sequence began at 21:01 on 24 September 2016 and concluded at 13:40 on 26 September 2016, spanning 40 hours and 39 minutes. During this period, 46 earthquakes were recorded.
The events exhibited low magnitudes, ranging from -0.9 to 2.7, with the largest reaching 2.7 at 16:46 on 25 September. Depths varied between 2 km and 14 km, indicating shallow crustal activity typical of the region. Early events on 24 September included a magnitude -0.4 quake at 6 km depth. Activity intensified on 25 September, featuring multiple events above magnitude 1.0 clustered between 13:00 and 23:00, including a magnitude 2.5 event at 17:50. The final recorded event on 26 September was a magnitude 0.8 earthquake at 11 km depth.
Analysis of timing shows peak occurrence during afternoon and evening hours on 25 September, with 22 events registered that day. Depths remained predominantly between 7 km and 11 km for most events, suggesting a consistent focal zone. Negative magnitudes reflect the network's sensitivity to microseismicity, common in detailed swarm monitoring.
This swarm fits into a broader pattern of seismic activity in the Hebgen Lake area. Historical records since 2000 document 52 swarms, with notable yearly counts including seven in 2000, six in 2008, and five each in 2002 and 2013. Such recurrent swarms highlight ongoing tectonic stress release without a single dominant mainshock.
The Hebgen Lake region lies within the Intermountain Seismic Belt, a zone of distributed crustal extension. It is situated near the Yellowstone volcanic system, where magma movement and hydrothermal processes contribute to elevated seismicity. The area experienced the magnitude 7.3 Hebgen Lake earthquake in 1959, which triggered significant ground deformation and landslides. Modern monitoring by regional networks confirms persistent low-level activity driven by both tectonic forces and volcanic influences from the nearby hotspot.
References
USGS Earthquake Hazards Program reports on Intermountain Seismic Belt
Montana Bureau of Mines and Geology seismic bulletins
Yellowstone Volcano Observatory geological summaries