Deadly consequences of the Hebgen Lake earthquake

Editor's Note: Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week's contribution is from Mike Stickney, director of the Office of Seismic Studies at the Montana Bureau of Mines and Geology.

The Greater Yellowstone landscape has evolved gradually over thousands of years, punctuated by rare catastrophic events such as volcanic eruptions, major earthquakes, and glacial floods. The only such historical event occurred 65 years ago. The M7.3 Hebgen Lake Earthquake occurred at 11:37 pm local time on Monday, August 17, 1959, unleashing the largest earthquake-triggered landslide in North America in recorded history.

Centered near the mouth of Madison Canyon west of Hebgen Lake, Madison Slide extends a mile (1.6 km) from head to toe and is three-quarters of a mile (1.2 km) wide. A 1,300-foot (400 m) section of the south valley wall collapsed north into the Madison River Valley, washing out 430 feet (131 m) up the north valley wall in places and creating a 220-foot (67 m) high dam across the Madison River, which Behind him, the Earthquake Lake immediately began to fill. The slide's mass volume is estimated at 37 to 43 million cubic yards (28 to 33 million cubic metres) and covers 130 acres (0.5 km2).

View of the Madison slide on August 21, 1959, as the earthquake lake quickly filled. Rock Creek Campground was near flooded trees. Camping equipment was left behind by survivors who searched for higher ground after the slide / Professor William P. Hall, Department of Mine Geology at Montana School.

A thick layer of Precambrian dolomite dips steeply northward along the base of the southern valley wall. This solid layer appears to have formed a buttress that reinforced the weakly weathered schist and gneiss deep in the slopes above. The strong shaking of the earthquake caused the dolomite buttress to collapse, and the entire valley wall slid. The slide block has maintained its stratigraphic arrangement with dolomite forming a prominent tan-colored band along the leading edge of the slide, followed by dark gray schist and gneiss. Fallen trees and topsoil of the north canyon wall settled above the slide mass, indicating no churning and mixing as the slide mass moved.

According to witnesses, Madison's slide occurred from just a few seconds to perhaps a half-minute after the strongest tremor of the Hebgen Lake earthquake subsided. Tragically, the eastern (upstream) edge of Madison Slide buried the overflow camping area below the US Forest Service's Rock Creek Campground. Twenty-six people died there, including 19 whose bodies were never recovered and are presumed to be buried in the slide block.

A wave of muddy water accompanied the slide down the upstream edge, carrying trees, driftwood and small rocks up to 100 feet (30 m) above the river on the north canyon wall. The violent wave and accompanying hurricane-force airburst dislodged by the slide mass caused numerous casualties and destroyed vehicles and trailers at the Rock Creek Campground.

By 6:30 a.m. the next morning, the water in the newly formed Earthquake Lake covered all the cars and trailers at Rock Creek Campground and was at least 20 feet (6 m) deep. About 250 survivors gathered at Refuge Point, the highest area between Madison Slide and Hebgen Dam. They were effectively trapped in the area because the earthquake also damaged the road upstream, so there was no way out of the Madison River Valley. The most seriously injured were evacuated by Air Force helicopters around midday, and by late afternoon, a bumpy road around landslides that cut off roads along Hebgen Lake, upstream of the Madison landslide, had been bulldozed, allowing the remaining survivors to escape. 25 people who were seriously injured were treated in hospitals in Bozeman, Ennis, Butte and Sheridan, Montreal.

The slope of the Hebgen Lake Fault in 1959/USGS, J.R. Stacy.

The U.S. Forest Service maintains a number of geological sites in the Hebgen Lake area, including a visitor center built on Madison Slide, interpretive signs marking where survivors gathered on the night of the earthquake, and the cliff that formed during the earthquake itself.

Next week, we'll continue the Madison Slide story with a look at the hazards the slide posed when it blocked the Madison River, and how engineers worked to stabilize the slide and mitigate those hazards.

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