Seismic Swarm S20151107.1: Characteristics and Context in the Hebgen Lake Region

The earthquake swarm designated S20151107.1 began at 02:04 on 7 November 2015 and concluded at 01:09 on 9 November 2015, approximately 7 km east of Hebgen Lake Estates, Montana. Over 47 hours and 5 minutes, the sequence produced 85 events. Magnitudes ranged from -0.2 to 2.2, with the majority below 1.0 and depths clustered near 10–11 km. The largest event reached magnitude 2.2 at 18:59 on 7 November.

This swarm exemplifies typical low-magnitude clustering observed in the region. Event rates peaked during the first 12 hours, followed by a gradual decline. Depths remained consistent between 5 and 13 km, indicating activity within the shallow brittle crust. No events exceeded magnitude 3.0, consistent with the swarm’s non-destructive character.

The Hebgen Lake area occupies the northern margin of the Intermountain Seismic Belt in southwestern Montana. This zone experiences extensional tectonics driven by Basin and Range province dynamics and proximity to the Yellowstone hotspot. The underlying geology consists of Precambrian metamorphic rocks overlain by Paleozoic sedimentary sequences and Quaternary volcanic deposits from the Snake River Plain–Yellowstone system. Faults such as the Hebgen Lake fault and segments of the Red Canyon fault accommodate normal and strike-slip motion.

Historical records document repeated seismic swarms in this setting. Since 2000, 58 swarms have occurred in the immediate vicinity, with annual counts varying from one to nine. Notable prior years include 2000 (9 swarms), 2008 (6), 2013 (6), and 2014 (6). These episodes reflect episodic fluid migration and stress transfer along pre-existing fractures rather than mainshock-aftershock sequences.

The 1959 Hebgen Lake earthquake (magnitude 7.3) remains the largest instrumental event in the area, producing surface rupture, a major landslide, and permanent changes to Hebgen Lake’s shoreline. Subsequent monitoring has revealed persistent microseismicity linked to hydrothermal and magmatic processes associated with the Yellowstone caldera, located roughly 30 km to the southeast.

Swarm S20151107.1 fits within this established pattern of diffuse, low-energy activity. Its spatial and temporal distribution aligns with historical clusters, reinforcing the region’s classification as a zone of recurrent swarm-type seismicity rather than isolated large events.

References

• USGS Earthquake Catalog and Quaternary Fault Database
• Montana Bureau of Mines and Geology seismic reports
• Intermountain Seismic Belt historical summaries (updated through 2023)