Seismic Swarm S20000409.1: Analysis of Activity Near Hebgen Lake Estates, Montana

Seismic swarm S20000409.1 occurred 14 km east of Hebgen Lake Estates in Montana. The sequence began at 21:17 on 8 April 2000 and concluded at 20:30 on 10 April 2000, spanning 47 hours and 12 minutes. During this period, 41 earthquakes were recorded, with magnitudes ranging from -0.5 to 2.1 and focal depths primarily between 5 km and 7 km.

The swarm exhibited typical characteristics of clustered microseismicity, featuring numerous low-magnitude events interspersed with a few events of slightly higher magnitude. Notable peaks included two events of magnitude 2.0 and 2.1 on 9 April, both at depths of 6–7 km. Activity was concentrated in the evening hours of 8 and 9 April, with a gradual decline toward the end of the sequence on 10 April. Such patterns often reflect fluid migration or stress adjustments along pre-existing faults rather than a single mainshock-aftershock progression.

This swarm represents the first of three documented swarms in the region since 1 January 2000. The Hebgen Lake area lies within the Intermountain Seismic Belt, a zone of elevated seismicity extending from Arizona through Montana. The local geology is dominated by Quaternary normal faults associated with Basin and Range extension. The 1959 Hebgen Lake earthquake, magnitude 7.3, ruptured segments of the Hebgen Lake fault and Red Canyon fault immediately west of the swarm location, producing significant surface offsets and triggering landslides. Post-1959 aftershocks and subsequent swarms indicate ongoing tectonic adjustment in the fault zone.

The swarm's shallow depths align with the brittle upper crust in this extensional setting, where faulting accommodates regional strain. Proximity to the Yellowstone volcanic system, approximately 50 km to the east, may contribute to elevated background seismicity through hydrothermal influences, although the 2000 events show no direct volcanic signature.

Further monitoring by regional networks continues to track microseismicity in the Hebgen Lake fault zone, contributing to improved understanding of swarm recurrence and seismic hazard in southwestern Montana.

References

USGS Earthquake Hazards Program – Hebgen Lake region seismicity records
Montana Bureau of Mines and Geology – Quaternary fault database
USGS Professional Paper 435 – The Hebgen Lake, Montana, earthquake of August 17, 1959