Seismic Swarm S20151014.1: Analysis of Activity Near Hebgen Lake, Montana

Seismic swarm S20151014.1 was recorded 3 km east of Hebgen Lake Estates, Montana, beginning at 07:42 on 13 October 2015 and concluding at 04:31 on 15 October 2015. Over 44 hours and 49 minutes, the swarm comprised 31 earthquakes, with magnitudes ranging from -0.3 to 2.0 and focal depths primarily between 1 km and 13 km. The largest event reached magnitude 2.0 at 03:10 on 14 October at 13 km depth. Most events clustered between 7 km and 12 km depth, consistent with shallow crustal seismicity in the region.

This swarm exemplifies typical swarm behavior, featuring numerous small-magnitude events without a dominant mainshock-aftershock sequence. Such patterns often reflect localized stress perturbations or fluid migration along faults rather than large-scale tectonic rupture. The temporal distribution shows peak activity during the early hours of 14 October, followed by gradual decay, with events tapering by mid-morning on 15 October.

Hebgen Lake lies within the Intermountain Seismic Belt of southwestern Montana, a zone of active extension influenced by the Yellowstone hotspot and Basin and Range tectonics. The area experiences normal faulting along structures such as the Hebgen Lake fault and Red Canyon fault. Historical precedent includes the 1959 Hebgen Lake earthquake (magnitude 7.3), which produced significant surface rupture, subsidence, and a major landslide that formed Earthquake Lake. The region remains one of the most seismically active in the western United States outside of California.

Since 2000, 46 swarms have occurred in the vicinity, with annual counts varying from one to six. Notable years include 2000 (6 swarms), 2002 (5), 2008 (5), 2013 (5), and 2014 (3). This recurrence underscores persistent low-level seismic unrest driven by regional tectonics and possible magmatic or hydrothermal influences from the nearby Yellowstone volcanic system.

The 2015 swarm aligns with this established pattern of episodic, low-magnitude activity. Depths and magnitudes remained well below thresholds for felt shaking or structural impact, posing no significant hazard. Ongoing monitoring by regional networks continues to track such sequences to refine understanding of fault interactions and volcanic-tectonic linkages.

References

USGS Earthquake Hazards Program catalog
Montana Bureau of Mines and Geology seismic reports
Yellowstone Volcano Observatory geologic summaries