Damkeeper George Hungerford points out how much the tidal waves, or seiches, overtopped Hebgen Dam at the time of the quake. Hungerford is standing on earthfill washed down from dam-top level by the gigantic waves. The quake also cracked the dam’s concrete core as shown.(Montana Power Co.)
It ranked right along with San Francisco’s 1906 shakedown as among the severest earthquakes on the North American continent.
In seismic measurements, it rated 7.8 on the Richter Scale, as compared with San Francisco’s 8.2.
It set up so called tidal waves, or seiches, on Hebgen Lake. There were at least three of these huge waves—20-ft. high—which overtopped the entire 721-ft. length of the dam by four feet. Eyewitness statements relate that the velocity of the tidal wave was so great that it caused the water literally to leap over the top of the dam. It filled the small generating plant with a 2 to 4 ft. deep layer of rocks.
Although the dam stood, the quake caused several fractures in the core wall, one of which showed a 3- to 4-inch separation, and shattered the dam’s concrete spillway.
The earthquake created three major faults, with displacement on the Red Canyon Fault running as much as 20 ft., which stacks up impressively alongside the 26-ft. maximum displacement resulting from San Francisco’s quake. (These two earthquakes differed, however, in that the Montana displacement was vertical, while San Francisco’s was horizontal.)
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