Seismic Swarm S20021214.1: Analysis of Activity Near Hebgen Lake Estates, Montana

Seismic swarm S20021214.1 was recorded by the SeismoSight network beginning at 18:17 on 13 December 2002 and concluding at 06:48 on 15 December 2002. The events were located 13 km east-northeast of Hebgen Lake Estates in southwestern Montana. Over the 36-hour-and-30-minute period, a total of 26 earthquakes were detected.

The swarm exhibited predominantly low magnitudes, with the majority falling between -0.5 and 2.5. The strongest event reached magnitude 2.5 at 19:12:52 on 13 December at a depth of 6 km. Several events registered negative magnitudes or were assigned placeholder values of -9.9, consistent with SeismoSight internal classification protocols for microseismicity or signal processing artifacts. Focal depths ranged from 2 km to 10 km, clustering most frequently around 6–8 km. Activity was concentrated in the first several hours, with a secondary pulse occurring near midnight on 13–14 December before tapering off by the morning of 15 December.

This swarm fits within a broader pattern of seismic activity documented in the region since 2000. Between 1 January 2000 and the end of 2002, twenty swarms occurred in the Hebgen Lake area, including ten in 2000, five in 2001, and five in 2002. Such recurrent swarm behavior reflects the underlying tectonic and magmatic influences present along the western margin of the Yellowstone volcanic system.

Hebgen Lake lies within the Intermountain Seismic Belt, a zone of active crustal extension that extends from northern Arizona through Montana. The area is situated immediately northwest of the Yellowstone Caldera, where the North American plate interacts with the underlying Yellowstone hotspot. Ongoing mantle upwelling produces elevated heat flow, hydrothermal circulation, and episodic brittle failure at shallow depths. Historical precedent includes the 1959 Hebgen Lake earthquake (magnitude 7.3), which triggered massive landslides and demonstrated the region’s capacity for significant seismic release. Modern monitoring shows that most contemporary activity occurs as swarms rather than isolated mainshock-aftershock sequences, a characteristic attributed to fluid migration and stress perturbations within a thermally weakened crust.

The December 2002 swarm, while modest in energy release, provides additional evidence of persistent microseismicity that helps delineate active fault structures and fluid pathways beneath Hebgen Lake. Depths between 2 km and 10 km align with the brittle-ductile transition zone expected in this high-heat-flow environment. Continued documentation of swarm statistics supports long-term assessment of seismic hazard in a region that hosts both recreational infrastructure and proximity to the Yellowstone volcanic field.

References

• SeismoSight internal swarm catalog (S20021214.1 parameters and historical counts 2000–2002)
• U.S. Geological Survey Earthquake Hazards Program regional tectonic summaries for the Intermountain Seismic Belt
• Yellowstone Volcano Observatory geologic and geophysical reports on caldera-related seismicity