Seismic Swarm S20000329.2: Analysis of Activity Near Hebgen Lake, Montana

Seismic swarm S20000329.2 was recorded 14 km east of Hebgen Lake Estates, Montana. The sequence began at 14:11 on 29 March 2000 and concluded at 17:34 on 7 April 2000. Over 219 hours and 22 minutes, 185 earthquakes were detected.

Analysis of the first 100 events reveals a predominance of microseismicity. Magnitudes ranged from -9.9 to 2.0, with the majority below 1.0 and only two events reaching 2.0. Depths clustered between 5 km and 11 km, consistent with shallow crustal processes. Temporal distribution showed initial clustering on 29 March, followed by sporadic activity through 31 March. No events exceeded magnitude 2.0 within this subset, indicating limited energy release.

The Hebgen Lake region occupies the Intermountain Seismic Belt in southwestern Montana. This zone forms part of the Basin and Range extensional province, where normal faulting accommodates crustal stretching. The Red Canyon and Hebgen faults dominate local structure, both striking northwest-southeast. Historical deformation has produced repeated surface ruptures.

The most significant prior event occurred on 17 August 1959, when a magnitude 7.3 earthquake struck the Hebgen Lake area. That quake generated extensive surface faulting, triggered landslides, and caused subsidence that formed Earthquake Lake. It remains the largest historical earthquake in the Intermountain Seismic Belt. Post-1959 aftershocks and subsequent swarms illustrate ongoing strain release along the same fault system.

Since 1 January 2000, two swarms have been documented in the region, with S20000329.2 representing the first. Such episodic clusters are characteristic of the area and reflect fluid migration or aseismic slip triggering brittle failure at shallow depths.

Seismic monitoring in the Hebgen Lake vicinity continues to track background rates and swarm recurrence. Data from this sequence contribute to refined hazard assessments for the northern Intermountain region, where moderate-to-large earthquakes remain possible given the active tectonic setting.

References

USGS Earthquake Catalog
Montana Bureau of Mines and Geology
Intermountain Seismic Belt Historical Reports