Seismic Swarm S20170612.3: Analysis of Activity Near Hebgen Lake, Montana

An earthquake swarm designated S20170612.3 was recorded 13 km east of Hebgen Lake Estates in Montana. The sequence began at 16:46 on 12 June 2017 and concluded at 04:55 on 28 July 2017, spanning 1092 hours and 8 minutes. During this period, 1542 earthquakes were detected, characteristic of swarm behavior where events cluster in time and space without a dominant mainshock.

The Hebgen Lake region lies within the Intermountain Seismic Belt, a zone of active tectonics extending from Arizona to Montana. This area experiences frequent seismic activity due to its position at the intersection of Basin and Range extension and the influence of the Yellowstone hotspot. The 1959 Hebgen Lake earthquake, magnitude 7.3, remains one of the largest historical events in the contiguous United States and triggered significant landslides and ground deformation nearby. Ongoing monitoring reveals that swarms here often reflect fluid migration or magmatic processes associated with the broader Yellowstone volcanic system.

Historical data indicate 61 swarms have occurred in the region since 1 January 2000. Annual counts include 9 in 2000, 4 in 2001, 5 in 2002, 3 in 2003, 2 in 2004, 1 in 2005, 4 in 2006, 3 in 2007, 6 in 2008, 5 in 2009, 1 in 2010, 1 in 2011, 7 in 2013, 6 in 2014, 3 in 2015, and 1 in 2016. These episodes underscore the persistent seismic restlessness of the area, with events typically occurring at shallow to moderate depths.

Examination of the first 100 events from the swarm provides insight into its initiation and evolution. The sequence opened with two events above magnitude 2.8 within the first minute, followed by a rapid succession of smaller shocks. Magnitudes ranged from negative values to a peak of 3.2, with the majority falling between 0.0 and 2.0. Depths clustered between 1 km and 11 km, averaging around 7 km, consistent with brittle failure in the upper crust. Early activity showed bursts of low-magnitude events interspersed with occasional larger shocks, such as the 3.2 magnitude event at 22:54 on 12 June at 10 km depth. This pattern aligns with fluid-driven swarm dynamics commonly observed in tectonically active volcanic margins.

The data reveal no single dominant event exceeding magnitude 3.2 in the initial phase, supporting classification as a true swarm rather than an aftershock sequence. Depths remained stable, suggesting a consistent source volume. Such characteristics aid in distinguishing swarm behavior from mainshock-aftershock patterns and contribute to refined seismic hazard assessments for nearby communities.

Seismic swarms in this setting carry implications for regional monitoring. They may signal changes in subsurface stress or fluid pressure, warranting continued observation by networks such as those operated by the U.S. Geological Survey. While most events remain imperceptible, the cumulative energy release and proximity to infrastructure highlight the value of real-time tracking.

References

• SeismoSight internal swarm classification records for S20170612.3
• U.S. Geological Survey earthquake catalog and regional tectonic summaries
• Historical seismic data compilations for the Intermountain Seismic Belt since 2000