Landslides

In 2014, a landslide began above the Stillaguamish River in Washington. In less than 5 minutes, it had buried a rural neighborhood. Forty-three people died, while eight more were carried along and lived to tell the tale. The slide dammed the river, which created a lake 2-and-a-half miles long. As violent and frightening as landslides can be, they’re a natural part of the constant erosion and reformation of Earth. Most often, erosion is a very slow process—except about 400 times a year, when there’s a landslide. They can creep downhill at just a few inches a day. Or they can build up speed, hitting nearly 200 miles an hour. Landslides can be massive, moving many cubic miles of earth in a single event. The wind they push in front of them can strip the leaves off of trees. Landslides require both gravity and water. Natural erosion, or manmade activities like a roadcut, can make a slope overly steep or unstable. Too much water—from rainfall, snow, or even groundwater—can then weaken the internal cohesion of the soil. After that, it’s just waiting for a trigger. That could be a natural tremor, from an earthquake or even thunder. Or it could be manmade, like vibrations from a mining operation or a train. Studies of the Washington slide have helped us better understand all these contributing factors. And as our knowledge of landslides continues to grow, hopefully we’ll get better at getting out of their way.