Seismic Swarm S20081203.1: Analysis of Activity Near Hebgen Lake Estates, Montana

Seismic swarm S20081203.1 was recorded from 05:06 on 3 December 2008 to 22:06 on 9 December 2008, centered 6 km south-southeast of Hebgen Lake Estates, Montana. Over 161 hours, the swarm produced 213 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from -0.8 to 2.5. The largest event reached magnitude 2.5 at 18:35 on 3 December. Depths clustered between 1 km and 16 km, with many events occurring near 8–9 km, consistent with shallow crustal processes. The sequence began with micro-earthquakes below magnitude 0.5 and gradually included slightly larger events, though most remained under magnitude 1.0. Temporal distribution showed clustering in the afternoon and evening of 3 December, with a notable concentration around 18:00–20:00. Depths displayed modest variation, occasionally reaching 13–16 km during the initial 24 hours before stabilizing at shallower levels. This swarm occurred within the Intermountain Seismic Belt, a zone of active tectonics extending from Montana into Idaho and Utah. The Hebgen Lake region lies at the northwest margin of the Yellowstone volcanic system, where crustal extension, normal faulting, and hydrothermal fluid movement interact. Earthquake swarms in this setting commonly arise from fluid migration along pre-existing faults rather than from a single large rupture. The area has a documented history of seismic unrest. The catastrophic 1959 Hebgen Lake earthquake, magnitude 7.3, produced extensive surface faulting and triggered a massive landslide. Since 2000, 34 swarms have been identified in the immediate vicinity, with annual counts as follows: six in 2000, four in 2001, five in 2002, three in 2003, two in 2004, one in 2005, five in 2006, three in 2007, and five in 2008. These recurrent swarms underscore the persistent tectonic and volcanic influence on local seismicity. The modest magnitudes and shallow depths observed in S20081203.1 align with typical swarm characteristics in the Hebgen Lake–Yellowstone transition zone. No events exceeded magnitude 3.0, and the overall energy release remained low. Such patterns support interpretations of distributed micro-fracturing driven by pressurized fluids within the upper crust.

References

SeismoSight internal swarm catalog (S20081203.1 parameters and event list).
U.S. Geological Survey Earthquake Hazards Program regional reports on the Intermountain Seismic Belt and Yellowstone area.