Location:

Period:

29 Aug 2025 01:11:17 - 1 Sep 2025 10:42:32 (3 days 9 hours 31 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

3 swarms found nearby.

2004

S20040717.1(29.0km)

16 Jul

1 day 16 hours

38 earthquakes

2014

S20140819.4(27.1km)

18 Aug

1 day 15 hours

42 earthquakes

2017

S20170821.1(28.8km)

20 Aug

4 days 18 hours

43 earthquakes

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Seismic Swarm S20250830.1: Analysis of Recent Activity West of Hebgen Lake Estates, Montana

A seismic swarm designated S20250830.1 occurred 26 km west of Hebgen Lake Estates, Montana, between 01:11 on 29 August 2025 and 10:42 on 1 September 2025. In 81 hours and 31 minutes, the sequence produced 38 earthquakes, with magnitudes ranging from -0.1 to 1.6 and focal depths between 8 and 15 km. The events clustered tightly in both space and time, characteristic of swarm behavior rather than a mainshock-aftershock sequence.

The largest event reached magnitude 1.6 at 19:36 on 29 August at 11 km depth. Most activity remained below magnitude 1.0, with 22 events between 0.0 and 0.5. Depths averaged near 11–12 km, consistent with brittle failure in the upper crust of this extensional regime. Activity peaked during the evening of 29 August, when 11 events occurred within four hours, before declining steadily through 31 August and tapering off on 1 September.

This region lies within the Intermountain Seismic Belt, where active normal faulting accommodates east-west extension linked to the Yellowstone hotspot and the broader Basin and Range province. The Hebgen Lake area experienced the magnitude 7.3 earthquake of 17 August 1959, which ruptured the Hebgen Lake and Red Canyon faults, triggered a massive landslide, and caused significant surface deformation. Ongoing microseismicity reflects continued strain accumulation along these and related structures.

Historical records maintained by SeismoSight indicate three prior swarms in the immediate vicinity since 2000: one each in 2004, 2014, and 2017. Each episode involved low-magnitude events at similar depths, suggesting recurrent fluid-driven or aseismic-slip-triggered processes rather than large tectonic stress release. The current swarm aligns with this pattern, showing no events above magnitude 2.0 and a rapid decay after the initial burst.

Such swarms are common in the Yellowstone volcanic field and its margins, where hydrothermal fluids and magmatic processes can modulate fault slip at shallow to mid-crustal levels. Depths of 8–15 km place the activity below the typical hydrothermal regime yet above the brittle-ductile transition, consistent with regional focal-mechanism studies that document normal and strike-slip faulting.

Continued monitoring remains essential. Although the present sequence released negligible seismic energy compared with the 1959 event, swarms can sometimes precede larger earthquakes or signal changes in subsurface fluid pressure. No damage or felt reports have been associated with S20250830.1.

References

SeismoSight internal swarm catalog (S20250830.1 parameters and historical statistics)
U.S. Geological Survey Earthquake Hazards Program – Hebgen Lake region tectonics and 1959 event documentation
Montana Bureau of Mines and Geology – Intermountain Seismic Belt seismicity summaries