Seismic Swarm S20170827.1: Analysis of Activity East of Hebgen Lake Estates, Montana

Seismic swarm S20170827.1 occurred 14 km east of Hebgen Lake Estates, Montana, beginning at 07:08 on 27 August 2017 and concluding at 15:35 on 30 August 2017. Over approximately 80 hours and 26 minutes, the swarm produced 66 earthquakes. Magnitudes ranged from -0.6 to 2.8, with the majority falling below 1.0. Depths clustered between 4 km and 12 km, indicating shallow crustal activity typical of the region.

The sequence initiated with low-magnitude events near 0.5 and progressed through clusters of slightly larger shocks. Notable events included two magnitude-2.8 earthquakes on 27 August at depths of 11 km and 12 km, alongside several magnitude-1.8 to 2.4 shocks distributed across the afternoon and evening. Later stages on 28–30 August featured predominantly smaller events under magnitude 1.0, with activity tapering to isolated micro-earthquakes by the swarm’s end. This pattern reflects the characteristic migration and decay of swarm sequences, where energy release occurs through numerous small ruptures rather than a single dominant mainshock.

The Hebgen Lake region lies within the Intermountain Seismic Belt, a zone of active crustal extension in the western United States. Tectonic forces arise from a combination of Basin and Range extension and the influence of the Yellowstone hotspot, which drives elevated heat flow and fluid migration that can trigger swarms. The 1959 Hebgen Lake earthquake of magnitude 7.3 remains the largest historical event in the immediate area, produced by slip on the Hebgen Lake fault and generating substantial surface rupture and landslides. Since 2000, the region has recorded 68 swarms, with annual counts varying from one to ten; notable years include 2000 (10 swarms), 2008 (7), and 2013 (7). These recurrent swarms underscore persistent seismic unrest linked to both tectonic stressing and hydrothermal processes.

Swarm S20170827.1 fits within this established pattern of episodic, low-magnitude activity. Depths predominantly between 7 km and 11 km align with the brittle–ductile transition zone influenced by local geothermal gradients. The absence of a clear magnitude progression or aftershock sequence following a dominant event further distinguishes the swarm from typical mainshock–aftershock sequences.

Ongoing monitoring by regional seismic networks continues to document such activity, providing data essential for assessing long-term hazard in this tectonically dynamic setting near the Yellowstone volcanic system.

References

USGS Earthquake Hazards Program – Yellowstone Region Seismicity
Montana Bureau of Mines and Geology – Hebgen Lake Fault Studies
International Seismological Centre – Regional Earthquake Catalog