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A deadly 2014 landslide’s power came from soils weakened by past slides

The Washington mudflow moved almost like an earthworm, extending and contracting

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3:33pm, October 27, 2017
Oso landslide

LETHAL LANDSLIDE  Researchers reconstructed the anatomy of the Oso landslide, one of the deadliest in U.S. history, to understand why it was so widespread. Liquefied sediments, weakened by previous earthquakes, rafted the debris far down the hill. 

SEATTLE Earth weakened by previous landslides and soils behaving like water were responsible for the unusual size of a deadly 2014 landslide, two scientists reported October 24 at the Geological Society of America’s annual meeting. Understanding why this landslide was so mobile could help geologists better map the hazards that could lead to others like it and prevent future loss of life.

In March 2014, following more than a month of heavy rainfall, a wall of mud suddenly rushed down a hillside near Oso, Wash., engulfing houses and trees before spilling into the Stillaguamish River valley (SN: 4/19/14, p. 32). The debris flow killed 43 people and destroyed dozens of homes. The valley had seen landslides before, most recently in 2006. But the “run-out” — the size of the debris flow — of the Oso landslide was

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