Seismic Swarm S20130928.1 Near Hebgen Lake Estates, Montana
Seismic swarm S20130928.1 was recorded in the Hebgen Lake region of Montana, beginning at 13:42 on 27 September 2013 and concluding at 02:25 on 29 September 2013. Over this 36-hour and 43-minute period, a total of 28 earthquakes were detected, centered approximately 8 km east-southeast of Hebgen Lake Estates. The events exhibited magnitudes ranging from negative values to a maximum of 2.2, with focal depths primarily between 9 and 13 km.
The sequence initiated with low-magnitude tremors around 0.6 to 0.8, followed by a cluster of events in the afternoon of 27 September that included two magnitude 2.2 earthquakes at depths of 13 km and 12 km. Subsequent activity remained subdued, with occasional spikes to magnitude 2.0 and 1.3, interspersed with numerous micro-earthquakes below magnitude 1.0. Depths stayed consistently shallow throughout, reflecting typical patterns for the area.
This swarm fits into a broader pattern of seismic activity in the region. Historical records since 1 January 2000 document 42 swarms in total. Annual counts include 6 in 2000, 4 in 2001, 5 in 2002, 3 in 2003, 2 in 2004, 1 in 2005, 4 in 2006, 3 in 2007, 5 in 2008, 4 in 2009, 1 in 2010, 1 in 2011, and 3 in 2013. Such recurrent swarms underscore the persistent seismicity driven by regional tectonics.
The Hebgen Lake area lies within the Intermountain Seismic Belt, where extensional faulting interacts with influences from the Yellowstone volcanic system. This geological setting promotes episodic earthquake swarms through fluid migration and stress adjustments along faults. Depths observed in the 2013 swarm align with the brittle upper crust characteristic of this zone.
References
SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program regional tectonic summaries.