Seismic Swarm S20110504.1 Near Hebgen Lake, Montana

Seismic swarm S20110504.1 was recorded in southwestern Montana, centered 6 km southeast of Hebgen Lake Estates. The sequence began at 03:31 on 3 May 2011 and concluded at 02:07 on 5 May 2011, encompassing 33 earthquakes over 46 hours and 35 minutes. Magnitudes ranged from -0.9 to 1.5, with the majority falling below 1.0. Focal depths varied between 1 km and 18 km, indicating activity across shallow crustal levels.

Events clustered primarily on 3 and 4 May, with peak activity in the evening of 3 May when several events of magnitude 1.0–1.5 occurred at depths of 7–10 km. Later events on 5 May included a magnitude 1.1 earthquake at 11 km depth. Such patterns reflect typical swarm behavior, where numerous small events occur without a dominant mainshock.

Since 1 January 2000, the region has experienced 43 swarms. Yearly 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, and 1 in 2010. These recurrent swarms underscore persistent low-level seismicity.

The Hebgen Lake area lies in the Intermountain Seismic Belt, a north-south trending zone of active extensional tectonics within the Basin and Range Province. The belt accommodates crustal stretching through normal faulting, particularly along structures such as the Madison Range fault. This setting produced the magnitude 7.3 Hebgen Lake earthquake of 17 August 1959, which caused significant surface rupture and ground deformation. Proximity to the Yellowstone volcanic system adds complexity, as regional stress fields can be modulated by magmatic or hydrothermal processes, although most swarms remain tectonic in origin.

Seismic monitoring by regional networks has documented ongoing microseismicity, consistent with the area's position in a tectonically active transition zone between stable cratonic blocks and extending lithosphere. Depths observed in the 2011 swarm align with brittle failure in the upper 15–20 km of crust typical for this province.

References

USGS Earthquake Hazards Program: Hebgen Lake region tectonics and historical events.
Montana Bureau of Mines and Geology: Seismicity of southwestern Montana.
Incorporated Research Institutions for Seismology (IRIS): Intermountain Seismic Belt overview.