“Active” faults are those that have had earthquakes occur along their trace in recent geologic time, and have the potential to produce future earthquakes. Hazards associated with active faults include earthquake damage due to ground shaking, surface displacement due to fault rupture, triggering of landslides, and liquifaction. Most active faults are located in western Montana, with the Brockton-Froid fault zone in Roosevelt County of northeastern Montana being a prominent exception.
Seismic hazards, seismic activity, and Quaternary faults in western Montana. Faults are from the USGS Quaternary Faults and Folds
Database. Green to red colors and contours show the USGS regional seismic hazard assessment represented as a 2% chance in 50 years
of peak ground acceleration exceeding a set percentage of gravity. Blue lines show faults used in construction of the National
Seismic Hazard Maps. Black lines are faults not used in the seismic hazard maps because they lack the basic information (average
slip rate and time elapsed since last major earthquake) to be included.