In late March, a major landslide occurred a few miles east of Oso, Washington, killing at least 35 people and engulfing an area approximately one square mile (2.6 km2). Geologists are now studying the event and they're baffled by its ferocity and speed — a slide that rushed down at an astounding 60 mph (97 km/hr).
Using a new computer model, Richard Iverson of the U.S. Geological Survey estimated that the mass of mud, rocks, and trees was traveling about 60 mph when it reached the river below.
Here's the animation from start to finish (sped up considerably — but you can see the clock for reference):
More from the Seattle Times:
He blames the disaster on a combination of unusually wet weather, erosion at the toe of the slide and local geology.
The slope that failed is largely made up of loose, sandy soil deposited by retreating glaciers. But that porous material is underlain by a compacted layer of silt and clay, which blocks the flow of water and allows it to accumulate deep within the hillside.
The slide may have started at that clay layer, Iverson said, but more field work is needed to be sure.
He believes the first block of soil to start moving was the jumbled pile of debris at the foot of the slope from a previous slide in 2006. The movement of that chunk destabilized the upper slope, which then collapsed.
The witness, who was in his yard across the valley, said the first thing he noticed was his dogs running away. "They probably either heard it or felt it first," Iverson said. Then the man heard a loud roar and saw the river being flung into the air as the debris raced across the landscape.
"In my view, the thing liquefied very quickly after beginning to move," Iverson said.
As the sandy slope collapsed, the weight probably compressed the sodden soil, which would have increased water pressure between soil grains and turned the mass to soup.
GIF via USGS.