New research: Devastating Oso landslide wasn't an outlier

SEATTLE — A new study from the University of Washington finds that the devastating Oso landslide in 2014 wasn't an outlier.

The landslide wiped out neighborhood north of Seattle and left 43 people dead when it roared down a hillside above the north fork of the Stillaguamish River.

It was the deadliest landslide in U.S. history.

University geologists used radiocarbon dating of woody debris buried in earlier slides as well as a review of erosion characteristics to help map the history of landslides in the area.

They found that slopes in the area have collapsed every 500 years, and with even greater frequency in the past millennium: every 140 years or so.

The authors say the study disproves the notion that the previous slides in the area all occurred thousands of years ago after the ice sheets retreated.

[A lidar image of the Stillaguamish River with newly calculated ages for the landslides. Radiocarbon dating of woody debris shows that the huge Rowan Landslide, on the left, happened only about 500 years ago.Alison Duvall/University of Washington]

"The soil in this area is all glacial material, so one hypothesis is the material could have fallen apart in a series of large landslides soon after the ice retreated, thousands of years ago," said corresponding author Sean LaHusen, a UW doctoral student in Earth and space sciences. "We found that that's not the case — in fact, landslides have been continuing in recent history."

It's not known whether the findings for the Oso site's history would apply to other parts of the Stillaguamish River, according to UW, or to other places in Washington state. The researchers are still studying debris from other locations.

The results do have implications for the immediate area.

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The Associated Press contributed to this report

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