The Hebgen Lake, Montana, earthquake of August 17, 1959, MST

The main shock at 11:37 p.m. MST was felt by persons in nine western states and three Canadian provinces. Area of perceptibility (600,000 square miles), maximum intensity (X, Modified Mercalli scale), and Richter magnitude (7.1) all were greater for the 1959 shock than for any earlier Montana earthquake on record. The high intensity rating near the epicenter is justified not by the building damage, but rather by the extensive topographic changes which accompanied the earthquake: spectacular vertical fault scarps, severe warping of the ground surface around Hebgen Lake, and landsliding of various types. None of the 28 fatalities resulted from failure of works of man-two were killed by a rolling boulder, the other 26 were buried by a 43 million cubic yard rockslide in the Madison River gorge six miles below Hebgen Dam.

The aftershock record, although limited by the scarcity of seismographic stations in the Rocky Mountain region, indicates that the aftershocks are decreasing in frequency and severity in the irregular fashion usual for an event of this magnitude.