Oso Disaster Rooted in 'Remobilization' of Earlier Landslide

The Oso landslide disaster in March 2014 involved the "remobilization" of a 2006 landslide on the same hillside, a new geological study released Tuesday concluded.

Oso, which killed 43 people in the rural Washington state, was the most deadly landslide in US history. It occurred in two stages: first, it remobilized the 2006 slide, moving more than sixth-tenths of a mile of land across the north fork of the Stillaguamish River and causing nearly all the destruction in the Steelhead Haven neighborhood.

The second stage started several minutes later and consisted of ancient landslide and glacial deposits. That material moved into the space vacated by the first stage and moved rapidly until it reached the trailing edge of the first stage, the study found.

The report - released exactly on the four-month anniversary of the disaster - revealed that intense rainfall in the three weeks prior to the slide was likely responsible for the incident. Other factors such as altered groundwater migration, weakened soil consistency because of previous landslides and changes in hillside stresses also played key roles.

"Perhaps the most striking finding is that, while the Oso landslide was a rare geologic occurrence, it was not extraordinary," lead author Joseph Wartman, a University of Washington associate professor, said in a statement.

"We observed several other older but very similar long-runout landslides in the surrounding Stillaguamish River Valley. This tells us these may be prevalent in this setting over long time frames. Even the apparent trigger of the event - several weeks of intense rainfall - was not truly exceptional for the region," Wartman added.

The study's team began their research eight weeks after the slide, once search and recovery activities were completed. The basis of their findings comes from direct observations of the area, but also includes information such as local geologic and climate conditions and eyewitness accounts.

Interestingly, the team estimated that large landslides such as the Oso event have happened in the same area as often as every 400 years to every 1,500 years during the last six millennia, based on mapped landslides and carbon dating.

But the March landslide takes the cake for the biggest one yet, and researchers hope this study can help warn people in the future before such a disaster happens again.

"For me, the most important finding is that we must think about landslides in the context of 'risk' rather than 'hazard,'" Wartman said.

"Landslide hazard," he explained, "which was well known in the region, tells us the likelihood that a landslide will occur, whereas landslide risk tells us something far more important - the likelihood that human losses will occur as a result of a landslide."

The report, funded by the National Science Foundation, can be found here.