Seismic Swarm S20240423.1 Near Hebgen Lake Estates, Montana
A seismic swarm designated S20240423.1 was recorded in southwestern Montana, centered approximately 11 km east-northeast of Hebgen Lake Estates. The sequence began at 03:32 UTC on 23 April 2024 and concluded at 11:21 UTC on 27 April 2024, spanning 103 hours and 49 minutes. During this period, 79 earthquakes were detected, with magnitudes ranging from -0.5 to 3.0 and focal depths predominantly between 5 and 9 km.
The swarm exhibited typical characteristics of clustered seismicity, featuring an initial moderate event of magnitude 3.0 followed by numerous smaller aftershocks. Peak activity occurred within the first 24 hours, including several events above magnitude 2.0, after which the rate gradually declined. All events remained below the threshold for widespread felt shaking, consistent with the shallow but low-magnitude nature of the sequence.
The Hebgen Lake region lies within the Intermountain Seismic Belt, a zone of elevated crustal extension and normal faulting that extends from northern Arizona through western Montana. This belt accommodates ongoing tectonic deformation associated with the western margin of the North American plate. The local geology is dominated by Precambrian basement rocks overlain by Paleozoic sedimentary sequences and Quaternary glacial and alluvial deposits. Proximity to the Yellowstone volcanic system further influences regional stress fields through magmatic and hydrothermal processes.
Seismicity in the area has a well-documented history. The most significant event was the magnitude 7.3 Hebgen Lake earthquake of 17 August 1959, which ruptured the Hebgen Lake fault and triggered a massive landslide that dammed the Madison River, forming Quake Lake. That earthquake caused substantial surface deformation and remains one of the largest historic events in the contiguous United States outside California. Since 2000, the broader Hebgen Lake–Yellowstone region has experienced 84 documented swarms, with annual counts varying from one to eight. Notable swarm years include 2000 (8 swarms), 2008 (6), 2013 (6), 2014 (6), and 2022 (7). These recurrent swarms reflect the interplay between tectonic extension and fluid migration along pre-existing faults.
The April 2024 swarm fits within this established pattern of episodic, low-magnitude activity. No surface rupture or significant ground deformation has been reported, and the events are interpreted as resulting from minor stress adjustments on faults within the Hebgen Lake fault zone. Continued monitoring by regional seismic networks remains essential for distinguishing background swarm behavior from potential precursors to larger events.
References:
U.S. Geological Survey Earthquake Catalog
Montana Bureau of Mines and Geology
Yellowstone Volcano Observatory reports
Historical records of the 1959 Hebgen Lake earthquake