Montana is one of the most seismically-active states in the United States. Montana’s earthquake activity is concentrated mostly in the mountainous western third of the state, which lies within the Intermountain Seismic Belt. This belt includes at least 45 potentially active faults in Montana and has been the site of two of the largest historic earthquakes in the Rocky Mountains, the August 18, 1959 Hebgen Lake, Montana earthquake (magnitude 7.3) and the October 28, 1983 Borah Peak, Idaho earthquake (magnitude 6.9).

Although it has been over five decades since the last destructive earthquake in Montana, small earthquakes are common in the region, occurring at an average rate of 4-5 earthquakes per day.

A great earthquake (magnitude 8.0 or greater) is not likely in Montana, but major earthquakes with magnitudes as large as 7.5 may occur in the state.

The Earthquake Hazard & Northwestern Montana

Northwestern Montana includes the northern Intermountain Seismic Belt. Although historic earthquakes in northwestern Montana have not exceeded magnitude 5.5, there are several faults that could potentially produce earthquakes as large as magnitude 7.5. Seismic hazard maps indicate that northwestern Montana has the second highest hazard level compared to other areas of Montana.

The Flathead Lake-Swan Lake region experienced a significant earthquake on March 31, 1952. With an estimated magnitude of 5, this trembler caused minor damage along the eastern side of Flathead Lake and was felt through northwestern Montana and adjacent parts of Idaho and Canada. A total of 17 earthquakes with magnitudes ranging from 4.0 to 4.8 have occurred in northwestern Montana during the past four decades. These earthquakes, along with frequent smaller earthquakes and large active faults such as the Mission Fault, demonstrate the potential for a larger destructive earthquake.

Adapted from the 2010 Update to the State of Montana Multi-Hazard Mitigation Plan and Statewide Hazard Assessment and the USGS Earthquake Hazards Program.

Please visit the following links for more information on earthquake hazards and preparedness:

American Red Cross of Montana

Governor’s Office of Community Service: Ready Montana

Lewis & Clark County Earthquake Hazards: 1935 Helena Earthquakes

Montana Bureau of Mines and Geology: Earthquake Studies

Personalizing the Earthquake Threat

State of Montana Disaster and Emergency Services
Montana Mitigation Plan
Let's Mitigate Montana

USGS Montana Earthquake Information
Montana Earthquake Hazards