Earthquake Swarm S20021104.5: Seismic Activity Near Hebgen Lake, Montana

The earthquake swarm designated S20021104.5 occurred in a tectonically active region of southwestern Montana, centered 6 km east-southeast of Hebgen Lake Estates. This sequence began at 23:20 on 3 November 2002 and concluded at 12:06 on 7 November 2002, encompassing 80 events over 84 hours and 45 minutes. The area lies within the Intermountain Seismic Belt, where extensional faulting associated with the Basin and Range province interacts with volcanic influences from the nearby Yellowstone hotspot. The 1959 Hebgen Lake earthquake of magnitude 7.3 remains the region's most significant historical event, producing surface rupture along the Hebgen Lake fault and triggering a massive landslide that dammed the Madison River.

Seismic activity in this zone reflects ongoing crustal extension and fluid migration, common precursors to swarm behavior. Depths of recorded events predominantly ranged between 2 km and 13 km, with occasional deeper occurrences up to 23 km, consistent with the brittle-ductile transition in the upper crust of this volcanic-tectonic setting. Magnitudes varied from microseismic levels near zero to a peak of 2.8, illustrating the swarm's characteristic lack of a dominant mainshock. Multiple events clustered on 4 November, including several above magnitude 1.0, followed by a gradual decline through 6 November.

Historical records indicate 15 swarms in the vicinity since 1 January 2000, with nine occurring in 2000, four in 2001, and two in 2002. Such recurrent swarms underscore the region's persistent low-to-moderate seismicity driven by regional stress fields and hydrothermal processes linked to the Yellowstone volcanic system.

Insights from the temporal distribution reveal an initial burst of activity within the first 24 hours, followed by episodic bursts on subsequent days. Most events exhibited depths around 10–11 km, suggesting a primary source volume at mid-crustal levels where fluid pressure variations may facilitate slip on pre-existing faults. The swarm's compact duration and moderate event count align with patterns observed in other Yellowstone-adjacent sequences, where magmatic or hydrothermal fluids contribute to triggering without leading to larger tectonic release.

This episode highlights the value of dense seismic monitoring in distinguishing swarm activity from foreshock sequences, aiding in hazard assessment for nearby communities and infrastructure.

References

USGS Earthquake Hazards Program – Hebgen Lake region seismicity Montana Bureau of Mines and Geology – Regional tectonic summaries Yellowstone Volcano Observatory – Swarm characteristics and volcanic-tectonic interactions