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| EARTHQUAKE!!! |
At 11:37 P.M. on August 17, 1959, a Richter Scale 7.8 earthquake struck near Lake Hebgen in Montana, about 25 miles northwest of the west entrance to Yellowstone Park. The area is an intersection of the Hebgen fault and the Red Canyon fault. The lake lies at 6550 elevation, in a narrow valley between 7600 foot mountains; and is held back by a concrete core, earthen dam at it's west end. The quake caused three 20-foot tidal waves, called seiches, to slosh back and forth in the lake over a period of an estimated 40 minutes, sloshing over the dam and destroying its spillway, but not breeching the dam.
The Madison river feeds Lake Hebgen, and flows west beyond the dam through Madison Canyon for about five miles before flowing onto a broad plain. The canyon was home to a number of vacation/fishing cabins, and the Rock Creek campground. On the evening of August 17, the campground was full, and many other campers had parked trailers and tents at numerous ad hoc camping spots along the river.
Somewhere between five and 20 minutes after the quake, a half-mile wide section of the south side of the canyon wall came loose, releasing an estimated 43 million cubic yards, or 80 million tons, of rock, trees and mountainside debris.
It took eight seconds for the mass at the top of the mountain to fall to the valley floor 1200 feet below at an estimated 174 miles per hour, and another 30 seconds to sweep 1/2 mile across the river and flow 300 feet up the opposite shore; then spread three-quarters of a mile each way east and west. Twenty-one campers at the formal and ad hoc campsites were buried, and remain in the debris.
Another seven died in hospitals, or were killed driving into eight-to-twenty-foot scarps, or vertical displacements of earth which developed on the plain west of the valley.
It took eight seconds for the mass at the top of the mountain to fall to the valley floor 1200 feet below at an estimated 174 miles per hour, and another 30 seconds to sweep 1/2 mile across the river and flow 300 feet up the opposite shore; then spread three-quarters of a mile each way east and west. Twenty-one campers at the formal and ad hoc campsites were buried, and remain in the debris.
Another seven died in hospitals, or were killed driving into eight-to-twenty-foot scarps, or vertical displacements of earth which developed on the plain west of the valley.
With the river blocked, water began rising upstream at a rate of nine inches an hour. Many campers not caught in the slide itself, lost their vehicles and supplies as the water rapidly inundated the area, and escape roads were impassible. Most scrambled for high ground, shivering together in the darkness and listening to falling rocks as aftershocks continued.
The fact that timber from the face of the mountain spread in a relatively even pattern over the entire surface of the slide is interpreted to mean that there was little tumbling action – that the slide moved as a single, if shattered, mass. Two 30-ton boulders stand erect at the top of the debris pile on the north side of the canyon. Lichen growth and lack of scuffing indicate that they rode the top of the mass of moving earth from the south side to their new resting place without rolling or tumbling. One of them now is home to a plaque listing the 28 victims of the slide.
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| These two big rocks are named: Memorial and Sister |
The two fault plates slid vertically against each other and created numerous vertical scarps for several miles.
Hebgen Lake tipped: The north side dropped 8 feet, while the south shore rose 8 feet. Cabins on the north shore were floated away, or were flooded. Cabins on the south shore were no longer on the shoreline; their docks were out of the water. Causeways were later cut to be able to get boats to the cabins.
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| See the scarps (faults) today in the valley below. |
Allowing the water to build to the 300 foot depth of the slide would mean
flooding the valley, burying the existing dam and causing huge economic losses.
Moreover, if the water was allowed to reach that depth and then flow down the
side of the slide unhindered, it could erode the slide rapidly and endanger
downstream homes, towns, and businesses. The Corps of Engineers therefore cut
two channels through the slide, lined with huge boulders, at a level that made
the new lake formed upstream of the slide (“Slide Lake”) about 220 feet deep at
its deepest. The new lake runs nearly to the Hebgen dam. It’s shoreline is
filled with snags of drowned trees; it’s bottom an underwater forest of standing
and fallen drowned trees. A fish’s heaven; a fisherman’s nightmare.
Thirty miles away in Yellowstone park, the quake did 3 1/2 million dollars damage to roads and structures. (The huge stone fireplaces in Old Faithful Inn, and the park headquarters at Mammoth Park collapsed.) There was considerable juggling of intervals and playing time of some of the better-known geysers. Great Fountain, Riverside, Daisy, Castle and Oblong shortened their eruption intervals. Clepsydra Geyser went frantically wild, and has erupted continuously since the quake. Steady Geyser just up and quit. Old Faithful erupted for a continuous 100 hours.
(Credit: 5 of the above photos/drawings are from The Night the Mountain Fell by Edmund Christopherson 1962)
