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 uncommonly large, spreading a fan of mud and debris across 1.4 kilometers.
To unravel the sequence of events leading to the landslide, Brian Collins and Mark Reid, both with the U.S. Geological Survey in Menlo Park, Calif., first mapped the debris that made up the landslide, including large still-intact blocks of hillside called hummocks, glacial sediments and fallen trees. The researchers then used those maps to track where the different parts of the debris had originated and where they ended up. From that, the duo determined that sediments weakened and previously mobilized by the 2006 landslide failed first, followed by sediments that had failed in a prehistoric landslide and finally by intact sediments.
Once it began to flow, the landslide didn’t sweep smoothly down the hill, the researchers determined. Instead, segments collided into one another and then stretched apart — extending and contracting earthwormlike — as more and more of the slope fell and transferred its momentum to the landslide. The sudden piling-on of mass also caused the soils beneath the hummocks and larger debris to weaken and become “liquefied,” or behave like water. And those liquefied soils then helped raft the hummocks and trees much farther out into the valley.