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Scientists: Oso landslide was no anomaly

Image: Landslide site
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.

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