Oso Mudslide: Area Had Mudslides Every 140 Years Since Glaciers

When a landslide engulfed a neighborhood in Oso, Washigton in March 2014, the massive impact of that earth movement was shocking even to geologists.

A new study from the University of Washington, recently published in the journal Geology, found that over the past 2,000 years that same area on the Stillaguamish River's North Fork has experienced a big slide every 140 years on average.

"The takeaway is that this is a very dynamic landscape," said Sean LaHusen, a UW doctoral student and lead author on the paper, in an article in the Seattle Times. "It's very unlikely that landslides will stop happening in this valley."

Last year's disaster killed 43 people. In response, the researchers used a laser-scanning technique, lidar, to look at maps of the area. The technique clears away vegetation in the view, showing the traces of past landslides along the valley, according to a release.

The team collected deposits from prior landslides by wading the river, then hiking nearby slopes to find trees or wood bits left under slide remnants, which could be radiocarbon dated.

Just downstream from the Oso location, a huge slide known as the Rowan slide had been twice as large as the Oso slide and had sent material skidding across the valley, just as the Oso slide had. Dating the materials, they learned that the Rowan slide occurred about 500 years ago.

In geologic time, that's a short while ago. It adds to the team's knowledge of the dates of previous slides, too. "So Oso can't be dismissed as an outlier," LaHusen said in the article.

It turns out that since the glaciers retreated around 15,000 years ago, that river valley has been a hot spot for landslides. The departure of the glaciers left a rubble of till, clay, sand and lake sediments that were an unstable mix and prone to slides.

Regarding human use of the area, "...Knowing this place is so unstable, we ought to take a careful look at our land use practices," UW geologist and co-author Alison Duvall said in the article. 

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