Seismic Swarm S20150121.1: Analysis of Activity Near Hebgen Lake Estates, Montana

Seismic swarm S20150121.1 was recorded beginning at 02:26 on 21 January 2015 and concluding at 11:13 on 22 January 2015. The events were centered 7 km east-southeast of Hebgen Lake Estates in Montana. Over 32 hours and 47 minutes, the swarm comprised 135 earthquakes, providing a concentrated window into local crustal dynamics.

The first 100 events exhibited predominantly low magnitudes, ranging from -0.5 to 1.9, with the majority falling between 0.0 and 1.5. Depths clustered between 2 km and 14 km, most frequently 8–12 km, consistent with shallow crustal faulting in the region. Early activity featured a rapid succession of events within the first hour, including multiple magnitude 1.5 shocks at depths of 11 km. Subsequent events showed fluctuating intensities, with occasional deeper occurrences near 13–14 km and shallower ones at 2–4 km. This pattern reflects typical swarm behavior, where energy release occurs through numerous small ruptures rather than a single dominant shock.

Hebgen Lake lies within the Intermountain Seismic Belt, a zone of active tectonics extending from Arizona to Montana. The area experiences ongoing deformation linked to Basin and Range extension and proximity to the Yellowstone volcanic system. Historical records document frequent microseismicity, often manifesting as swarms along reactivated normal faults.

Since 2000, 57 swarms have been identified in the vicinity. Annual counts include eight in 2000, four in 2001, five in 2002, three in 2003, two in 2004, one in 2005, five in 2006, three in 2007, six in 2008, five in 2009, one in 2010, one in 2011, seven in 2013, and six in 2014. These episodes underscore persistent seismic unrest without progression to larger mainshock-aftershock sequences.

The 1959 Hebgen Lake earthquake, magnitude 7.3, remains the benchmark event for the district, producing surface rupture and significant ground deformation. Contemporary monitoring indicates that swarms like S20150121.1 represent routine stress adjustment within this framework.

References

United States Geological Survey Earthquake Catalog Montana Bureau of Mines and Geology Seismic Data SeismoSight Internal Swarm Classification Records