Wednesday, June 7, 2017

Earthquake at Hebgen Lake, West Yellowstone, Montana

It was around midnight on August 17, 1959, when campers and locals were sound asleep. They were awoken by a loud roar and the ground shaking when one of the most powerful earthquakes ever recorded in the Rocky Mountains struck the Madison River Canyon. The quake registered 7.5 on the Richter scale and triggered a massive landslide, sending over 80 million tons of rock crashing down into the canyon, blocking the Madison River.  The water backed up behind the slide and formed Earthquake Lake. High velocity winds and a gigantic wall of water swept through the area.

Earthquake Lake

The Landslide

When the quake hit, the earth's crust dropped 19 feet. The land under Hebgen Lake tilted upward; cabins on the north shore were immersed in water and portions of the south shore lay high and dry. Huge waves washed over Hebgen Dam and although it cracked, it held. Several sections of Highway 287 fell into the lake. As a result of the disaster, hundreds of people vacationing in the area were trapped, and 28 people lost their lives.

The massive landslide instantly buried Highway 287 under tons of boulders, rocks and debris. The Madison River, now blocked by the landslide, sweeps through and floods Rock Creek Campground. Muddy water laden with broken trees starts backing up the canyon, beginning to form Earthquake Lake.

As the earthquake tips Hebgen Basin down to the north, Highway 287 is severed by minor landslides, collapsing entire sections into Hebgen Lake. Campers who flee eastward in hopes of reaching Highway 191 are trapped.

Grace Miller, who ran the Hilgard Lodge and rented out cabins, awakes that night to the strange feeling that she has to get out of her house immediately. She and her dog leap across a rapidly widening fissure in the earth just as her house drops into Hebgen Lake.

The earthquake's impact shocked the world. No one expected something so tragic to happen to them while they lay sleeping. Today, even though the area seems tranquil, geologic tensions are still active underground. 

The earth's crust is comprised of 7 major slabs called tectonic plates. As these plates move against each other, energy is stored then rapidly released in the form of earthquakes that most often occur where the plates meet. Here in western Montana, which is part of the Intermountain Seismic Belt, the seismic activity is not occurring where two plates meet. Instead, it is a result of land deformation that is extending eastward from the western edge of the North American Plate. The earth's crust is slowly deforming, stretching and thinning, causing a high level of seismic activity. As massive land movement occurs, the forces produced by a major earthquake can uplift mountains, drop adjacent valleys and form fault scarps - the exposed face of a fault.

There are two huge boulders that were left in place from the earthquake, which have been dedicated to the people who lost their lives in the disaster.

Memorial Boulder
This boulder commemorates the lives lost in the earthquake. This plaque lists the names of the people who lost their lives. This 3,000 ton boulder rode the crest of the slide across the canyon. Undisturbed lichens on its side indicate it did not roll or tumble while crossing.

Although there were over 250 people in the Canyon that night, nobody saw the mountain come down. Only the light from the next day did the nature and magnitude of the slide become clear. Over the weeks and months after the disaster, geologists built a story of what had happened. One of the key clues was the scatter of dolomite boulders (such as this one, known as "Sister Boulder," a companion to the Memorial Boulder) that came to rest at the top edge of the slide.

Massive Memorial Boulder and Sister Boulder today rest at approximately the same elevation as the dolomite ledge that they probably came from across the river. They were blasted across the canyon on the leading edge of the sliding mountain, "floating" on debris that was, for those moments, behaving more like a liquid than like rock. In just 20 seconds these six-million-pound boulders traveled half a mile.

Into the valley
There were 250 campers in the canyon that quiet moonlit night when the earthquake set off the slide. Choking dust clouds filled the air. Waves surged against the Madison River. Boulders bounded and crashed from the cliffs above. Families were separated, some injured, some lost forever. Escape was blocked until help arrived after daylight. When the rescue efforts, and the days of accounting for who had been there, came to a close, 28 lives had been lost. 

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