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Sean LaHusen
UW graduate student Sean LaHusen points to buried debris at a landslide site on the North Fork of the Stillaguamish River. (Credit: Alison Duvall / UW)

A newly published analysis of the geological record for the area around the site of 2014’s Oso landslide shows that the slopes have been collapsing every 140 years or so on average. That’s significantly more frequent than previously estimated.

Based on laser elevation measurements and radiocarbon dating of woody debris around the North Fork of the Stillaguamish River, researchers from the University of Washington found that a collapse five times as big as the Oso event, known as the Rowan landslide, took place sometime between 300 and 694 years ago.

The researchers’ study, published online by the journal Geology on Tuesday, came up with an average collapse rate of once per 500 years for the area around Oso, Wash., over the course of thousands of years. Over the past 2,000 years, the average rate has been about 140 years, the scientists said.

“This was well known as an area of hill slope instability, but the question was: ‘Were the larger slides thousands of years old, or hundreds of years old?’ Now we can say that many of them are hundreds of years old,” UW professor Alison Duvall, a co-author of the study, said in a news release.

Alison Duvall
UW geologist Alison Duvall inspects a sample of wood from the Rowan Landslide. (Credit: Dennis Wise / UW)

The findings could have implications for how the area is developed in the wake of the fatal slide. Forty-one people died when the rain-soaked hillsides over Oso collapsed, sending down a wall of mud that buried houses and covered the highway along the Stillaguamish on March 22, 2014. A quick assessment of the landslide threat, published just months after the tragedy, suggested that the area experienced large landslides every 1,000 to 3,750 years.

The new assessment is likely to change the risk-vs.-benefit equation for the North Fork Stillaguamish River Valley. “It suggests that the Oso landslide was not so much of an anomaly,” Duvall said.

UW graduate student Sean LaHusen, the study’s principal author, said the methods used in the study “can provide some information about how often these events recur, which is the fist step toward a regional risk analysis.”

However, he and other researchers emphasized that the analysis applied only to the region immediately surrounding Oso rather than to the entire Stillaguamish River Valley or other places in Washington state. To get a wider assessment of the landslide threat, samples from other locations would have to be gathered and tested.

LaHusen and Duvall are part of the team for the UW’s M9 Project, which is studying potential hazards from magnitude-9 earthquakes along the Cascadia subduction zone. Their objective is to determine whether there are linkages between large earthquakes and the big landslides in the region’s past. Such findings could help officials and the general public prepare for the next “Really Big One” in the Pacific Northwest.

In addition to LaHusen and Duvall, the authors of “Surface Roughness Dating of Long-Runout Landslides Near Oso, Washington (USA), Reveals Persistent Postglacial Hillslope Instability” include Adam Booth and David Montgomery.

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