When the Oso landslide engulfed a neighborhood along the North Fork of the Stillaguamish River last year, its size and ferocity were stunning even to geologists. But a new analysis shows that the deadliest landslide in U.S. history was not a fluke.

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When the Oso landslide engulfed a neighborhood along the North Fork of the Stillaguamish River last year, its size and ferocity were stunning even to geologists.

But a new analysis from the University of Washington makes it clear that the deadliest landslide in U.S. history was not a fluke.

In fact, the researchers estimate that over the past 2,000 years, the same stretch of valley has been slammed by a major slide every 140 years, on average.

“The take-away is that this is a very dynamic landscape,” said Sean LaHusen, a UW doctoral student and lead author on the paper published online Tuesday by the journal Geology. “It’s very unlikely that landslides will stop happening in this valley.”

After the March 22, 2014, disaster, which killed 43 people, geologists reviewed maps of the area based on a laser scanning technique called lidar. By stripping away vegetation, the lidar revealed tracks left by large, past landslides up and down the valley. But it was impossible to tell how long ago those slides occurred.

 

Oso landslide coverage

The Seattle Times’ complete coverage of the Oso landslide, including investigative stories, profiles of the victims, interactive maps and a photo gallery.

If they were truly ancient — dating back to soon after the last ice age — then the Oso slide might have been just an outlier, and not representative of the present-day hazard, explained UW geologist and co-author Alison Duvall. “A really big question on everybody’s mind was just how rare an event like Oso is,” she said.

The researchers set out to answer that question by determining the dates of more than two dozen major slides along a 3.7-mile stretch of river centered on the Oso site.

To get to the landslide deposits, they had to wade the river in several places. Then they hiked up and down the slopes, looking for trees or bits of wood buried in the slide that could be used for radiocarbon dating.

The scientists were particularly interested in a huge slide just downstream from the Oso site. Called the Rowan slide, the collapse was more than twice as big as Oso and sent massive amounts of mud, trees and rocks racing across the valley bottom — just as the Oso slide did.

“This was a catastrophic event,” LaHusen said. “It would have been absolutely terrifying to be there.”

The radiocarbon results put the date at about 500 years ago.

While that sounds like a long time to most people, it’s a blink of an eye on the geologic scale. And it proves that slides even bigger than Oso have occurred along the North Fork in historic times.

“So Oso can’t be dismissed as an outlier,” LaHusen said.

Dating past landslides is key to improving landslide hazard assessments and guiding land use, said Jonathan Godt, program coordinator for the U.S. Geological Survey’s Landslide Hazards Program.

The UW project focused only on a small part of one valley, but the general approach could be applied elsewhere, said Godt, who was not involved in the study.

“It gives us a way to look at past landslide activity and use it as a guide to what we can expect in the future,” he said.

The UW team was able to find datable bits of wood on only a handful of landslides. To estimate the timing of the other events, they relied on the fact that slide deposits get smoother as they age.

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Using lidar images, the scientists zoomed in on the old slides and quantified how rough their surfaces are. Then they compared slides of known ages to slides of unknown ages to come up with an approximate scale.

The results show that the valley has been a landslide hot spot since the glaciers retreated more than 15,000 years ago, leaving behind an unstable mix of till, sand, clay and lake sediments.

That means the slopes were failing long before humans were logging or otherwise altering the environment in a major way, Duvall pointed out. But it doesn’t mean human activity couldn’t raise the danger.

“We know that the ingredients are there for unstable hill slopes, regardless of what we do,” she said. “But knowing this place is so unstable, we ought to take a careful look at our land-use practices.”

The lidar images used for the study can only differentiate major slides, so the 140-year recurrence rate doesn’t account for smaller events like the multiple slides that sloughed off the Oso hillside throughout the past century.

The question of whether those smaller slides should have served as a red flags for logging and homebuilding in the area is being argued in lawsuits filed by slide survivors and the families of those who were killed.