Historic Earthquakes

Hebgen Lake, Montana
1959 08 18 06:37:15 UTC (local 08/17)
Magnitude 7.3
Intensity X

Largest Earthquake in Montana

• Isoseismal Map
• Damage Photos: 1   2   3
• Damage Photos from the USGS Photographic Library

This earthquake caused 28 fatalities and about $11 million in damage to highways and timber. It is characterized by extensive fault scarps, subsidence and uplift, a massive landslide, and a seiche in Hebgen Lake. A maximum MM intensity X was assigned to the fault scarps in the epicentral area. The instrumental epicenter lies within the region of surface faulting. Area of perceptibility, maximum intensity, and Richter magnitude all were larger for this earthquake than for any earlier earthquake on record in Montana (from May 1869).

The most spectacular and disastrous effect of the earthquake was the huge avalanche of rock, soil and trees that cascaded from the steep south wall of the Madison River Canyon. This slide formed a barrier that blocked the gorge and stopped the flow of the Madison River and, within a few weeks, created a lake almost 53 meters deep. The volume of material that blocked the Madison River below Hebgen Dam has been estimated at 28 - 33 million cubic meters. Most of the 28 deaths were caused by rockslides that covered the Rock Creek public campground on the Madison River, about 9.5 kilometers below Hebgen Dam.

New fault scarps as high as 6 meters formed near Hebgen Lake. The major fault scarps formed along pre-existing normal faults northeast of Hebgen Lake. Subsidence occurred over much of an area that was about 24 kilometers north-south and about twice as long east-west. As a result of the faulting near Hebgen Lake, the bedrock beneath the lake was permanently warped, causing the lake floor to drop and generate a seiche. Maximum subsidence was 6.7 meters in Hebgen Lake Basin. About 130 square kilometers subsided more than 3 meters, and about 500 square kilometers subsided more than 0.3 meters. The earth-fill dam sustained significant cracks in its concrete core and spillway, but it continued to be an effective structure.

Damaged and collapsed buildings at the Blarneystone Ranch, near Hebgen Lake, Montana, caused by the August 18, 1959, earthquake. (Photograph by I.J. Witkind.)
 

Landslide and slumping damage to State Highway 287, along the shore of Hedgen Lake. Montana, caused by the August 18, 1959, earthquake. (Photograph by J.R. Stacy.)

Many summer houses in the Hebgen Lake area were damaged: houses and cabins shifted off their foundations, chimneys fell, and pipelines broke. Most small-unit masonry structures and wooden buildings along the major fault scarps survived with little damage when subjected only to vibratory forces. Roadways were cracked and shifted extensively, and much timber was destroyed. Highway damage near Hebgen Lake was due to landslides slumping vertically and flowing laterally beneath pavements and bridges, which caused severe cracks and destruction. Three of the five reinforced bridges in the epicentral area also sustained significant damage.

High intensities were observed in the northwest section of Yellowstone National Park. Here, new geysers erupted, and massive slumping caused large cracks in the ground from which steam emitted. Many hot springs became muddy.

On the basis of vibration damage (and excluding geologic effects), damage to buildings along the fault zone was singularly unspectacular (MM intensity VIII at places, intensity VII generally). Minor damage occurred throughout southern Montana, northeast Idaho, and northwest Wyoming. Felt as far as Seattle, Washington, to the west; Banff, Canada, to the north; Dickinson, North Dakota, to the east; and Provo, Utah, to the south. This area includes nine Western States and three Canadian Provinces. Aftershocks continued for several months.

Abridged from Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.