Seismic Swarm S20170821.1 Near Hebgen Lake Estates, Montana

Seismic swarm S20170821.1 occurred in a tectonically active region of southwestern Montana, approximately 1 km east-northeast of Hebgen Lake Estates. The sequence began at 11:27 on 20 August 2017 and concluded at 05:30 on 25 August 2017, spanning 114 hours and registering 43 earthquakes. Magnitudes ranged from -0.1 to 3.2, with the largest event recorded at 03:00 on 21 August at a depth of 9 km. Depths throughout the swarm were predominantly between 3 km and 10 km, consistent with shallow crustal seismicity in the area.

Hebgen Lake lies within the Intermountain Seismic Belt, a zone of elevated earthquake activity extending from Arizona to Montana. This belt results from extensional tectonics associated with the Basin and Range province, where normal faulting accommodates regional stretching. The local geology features Precambrian metamorphic rocks overlain by Paleozoic sedimentary sequences, influenced by Laramide orogeny and subsequent Cenozoic extension. Proximity to the Yellowstone hotspot adds a component of volcanic and hydrothermal influence, though the swarm itself showed no direct surface volcanic manifestations.

Historical records indicate frequent swarm activity in this locale. Since 2000, 48 swarms have been documented, with annual counts varying from one to five events per year in most periods. Notable prior years include 2000 and 2002, each with five swarms, and 2013 with five. These episodes typically involve low-to-moderate magnitude events clustered over days to weeks, often linked to fluid migration along faults or stress adjustments in the crust.

The 1959 Hebgen Lake earthquake (magnitude 7.3) remains the most significant historical event in the immediate vicinity, producing extensive surface faulting along the Hebgen Lake fault and triggering a massive landslide that dammed the Madison River. Modern monitoring by regional networks continues to track microseismicity, revealing that swarms like S20170821.1 represent a common mode of strain release rather than precursors to larger mainshock sequences.

Analysis of event timing shows clustering on 20–21 August, followed by a gradual decline, with isolated events persisting until 25 August. Depths remained stable in the mid-crust, suggesting a consistent source volume. Such patterns align with known swarm behavior in the Intermountain Seismic Belt, where episodic activity occurs without clear foreshock-mainshock-aftershock signatures.

References

USGS Earthquake Hazards Program reports on Montana seismicity.
Montana Bureau of Mines and Geology regional geological summaries.
Intermountain Seismic Belt historical earthquake catalogs.