Seismic Swarm S20050604.1 Near Hebgen Lake Estates, Montana

A seismic swarm designated S20050604.1 occurred 6 km south-southeast of Hebgen Lake Estates, Montana, from 10:07 on 3 June 2005 to 18:24 on 5 June 2005. Over 56 hours and 16 minutes, the swarm comprised 49 earthquakes. Event magnitudes ranged from -0.1 to 1.9, with the majority occurring at focal depths of 7–8 km. The sequence began with a magnitude 1.5 event and featured several clusters of activity, including peaks on 3 June and early 4 June, before tapering to a final magnitude 0.2 event. Depths remained consistently shallow, rarely exceeding 10 km, consistent with upper-crustal faulting in the region.

The Hebgen Lake area lies within the Intermountain Seismic Belt, a zone of active extensional tectonics stretching from Montana into Idaho and Utah. This belt accommodates Basin and Range-style normal faulting driven by the westward expansion of the North American plate. The swarm location is situated near the Hebgen Lake fault, a prominent normal fault that produced the magnitude 7.3 Hebgen Lake earthquake on 17 August 1959. That event caused substantial surface rupture, subsidence of Hebgen Lake, and landslides that resulted in 28 fatalities. The 1959 earthquake remains the largest historical event in the area and underscores the potential for significant seismic hazard.

Geologically, the region sits at the northern margin of the Yellowstone volcanic system and within a transition zone between the stable craton to the east and the actively deforming western United States. Quaternary fault scarps and ongoing microseismicity reflect continued tectonic strain accumulation. Historical records document recurrent earthquake swarms in this sector since at least 2000, with 21 swarms recorded through the end of 2004. Annual counts varied as follows: seven swarms in 2000, four in 2001, five in 2002, three in 2003, and two in 2004. These episodes typically involve low-magnitude events at shallow depths and are thought to result from fluid migration or aseismic slip on favorably oriented faults.

The 2005 swarm fits the established pattern of episodic, low-energy activity that releases strain without producing a mainshock–aftershock sequence. No surface rupture or felt reports of damaging intensity were associated with the events. Continued monitoring of the Hebgen Lake segment remains important because the 1959 rupture did not relieve all accumulated stress on adjacent fault sections.

References
SeismoSight internal swarm catalog S20050604.1.
U.S. Geological Survey Earthquake Hazards Program, Hebgen Lake 1959 event summary.
Montana Bureau of Mines and Geology, Quaternary fault database.