On Aug 17, 1959, Hebgen Lake was hit by one of the largest and most destructive earthquakes in US history. This magnitude 7.5 quake caused 28 fatalities and millions of dollars in property damage; most of which was the result of a massive landslide triggered by the sudden movement of the Earth’s crust.
The Northern Rockies are famous for their superb fishing destinations and beautiful scenery. So much so that when I traveled there earlier this year I found myself to be surrounded by fisherman on the plane, hearing stories of giant trout and stunning mountain vistas before I even touched ground in Montana. The person sitting next to me, a middle aged businessman from New Jersey, explained to me that August and September are the BEST months to go fly fishing in Montana as the weather is cooling down and the fish are returning to shallower waters. I suddenly thought this may be one of the reasons the 1959 quake was so deadly, because on the night of the quake dozens of people were camped along the Madison river just west of the lake, many of whom were fisherman just like my friend here on the plane.
Hebgen Lake sits directly northwest of the Yellowstone caldera close the border of Wyoming and Montana. It is fed by the Madison River, a tributary of the Missouri, which flows towards the east.
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Using seismograph data from the University of Utah I placed a marker showing the approximate location of the epicenter of the fatal earthquake.
Soon after the earthquake, a section of rock 3 quarter miles wide slid from the south wall of the Madison Canyon into the river immediately displacing all the water and air in its path. The incredible momentum of the landslide sent 30 foot tall waves and gale force winds roaring up and down the canyon. Below is a picture we took of landslide scar looking towards the south standing on top of the natural dam.
Immediately after the landslide completely dammed the Madison River water began to rise upstream forming another lake called Earthquake Lake.
You may be wondering by now, what caused a massive earthquake in the middle of the Rocky Mountains? Hebgen Lake is surrounded by a system of active normal faults that are part of the Basin and Range Extension, the literal stretching of the Earth’s lithosphere in the western United States driven by plate tectonics. The fault movement in this area is also thought to be connected to a larger seismic chain originating from the Yellowstone Hotspot (Yellowstone geology will have to be covered in a different post).
A “normal fault” is a fault where one side moves in a downward motion relative to the other. This type of motion is responsible for much of the landscapes you see out west, often creating vertical cliffs on the Earth’s surface called a fault scarp. Here is a picture of the scarp that appeared after the Hebgen Lake Earthquake:
Prior to the 1960’s there were no significant attempts in measuring seismic activity in the Hebgen Lake area, however; this changed after the 1959 quake. Since then there has been seismographs setup to record data, along with surveys and mapping done to further understand the structural geology of the region. Events like this should be a stark reminder to us that the Earth’s surface is a constantly changing environment and fault movement in North America is nothing new but a process that has been ongoing for millions of years.