Seismic Swarm S20230207.2: Analysis of Activity Near Hebgen Lake Estates, Montana

A seismic swarm designated S20230207.2 occurred in southwestern Montana, centered 16 km east-northeast of Hebgen Lake Estates. The sequence began at 23:19 on 6 February 2023 and concluded at 08:57 on 9 February 2023, spanning 57 hours and 37 minutes. During this period, 63 earthquakes were recorded, with magnitudes ranging from -0.1 to 2.9 and focal depths predominantly between 5 km and 10 km.

The swarm exhibited typical characteristics of clustered microseismicity, featuring numerous low-magnitude events interspersed with a few events exceeding magnitude 2.0. Notable activity included a magnitude 2.3 event at 11:51 on 7 February, followed by several magnitude 2.4 shocks on 8 February and a peak magnitude 2.9 event at 03:44 that same day. Depths remained shallow throughout, consistent with activity along local fault structures in the upper crust.

This swarm aligns with a long-term pattern of seismic clustering in the region. Since 1 January 2000, 97 swarms have been documented in the area, with annual counts varying from one to ten events per year. Notable years include 2000 with 10 swarms, 2022 with nine, and multiple years recording five or more, indicating persistent episodic activity.

Regional Geological Context

The Hebgen Lake area lies within the Intermountain Seismic Belt, a zone of active crustal extension extending from Arizona to Montana. This belt accommodates Basin and Range-style normal faulting driven by tectonic forces associated with the westward expansion of the North American plate. The local geology features Quaternary sediments overlying Paleozoic and Mesozoic sedimentary rocks, cut by active normal faults such as the Hebgen Lake fault.

Proximity to the Yellowstone volcanic system, approximately 50 km to the east, contributes to elevated heat flow and fluid migration that can influence seismicity. Historical records highlight the region's potential for larger events, most notably the 18 August 1959 Hebgen Lake earthquake of magnitude 7.3, which produced surface rupture, extensive landslides, and significant ground deformation along the Madison Range.

Seismicity Patterns and Implications

Swarm sequences like S20230207.2 typically reflect fluid-driven processes or aseismic slip rather than mainshock-aftershock sequences. The shallow depths and tight spatial clustering observed here suggest activation along minor faults or fracture networks. Such events provide valuable data for monitoring stress accumulation in a tectonically active corridor that has produced multiple magnitude 6+ earthquakes in the past century.

Ongoing surveillance by regional networks remains essential for distinguishing background swarm activity from precursors to larger ruptures. The 2023 sequence underscores the persistent, low-level seismic hazard in this portion of Montana despite the absence of damaging events.

References

• U.S. Geological Survey Earthquake Hazards Program reports on Intermountain Seismic Belt.
• Montana Bureau of Mines and Geology seismic monitoring summaries.
• Historical data from the Advanced National Seismic System.