The landslide that caused a major mudslide in Oso, Washington in March 2014 has been termed as the deadliest landslide in the history of United States. However, a new research has indicated that the landslides in the region could be a recurring phenomenon. The study published in the journal Geology has suggested that the region might have witnessed a massive landslide nearly 500 years ago.
Using radiocarbon dating and analysis of woody debris buried in the region, the research team from University of Washington has found that the soil in the region is all glacial material. And it could have faced major landslides in the past as well. The team of geologists suggests that landslides in this region could have been continuing in recent history. The team estimated that slopes around Oso, Washington may have faced a landslide every 140 years on an average and a major collapse every 500 years.
The study was co-authored by Alison Duvall, a UW assistant professor of Earth and space sciences, Sean LaHusen, a UW doctoral student in Earth and space sciences, David Montgomery at the UW and Adam Booth at Portland State University.
The study was funded by the National Science Foundation, the Geological Society of America, and the UW’s Quaternary Research Center.
The new study uses the radiocarbon dates for two slides to establish a roughness curve to date other events along a 3.7-mile (6-kilometer) stretch of the north fork of the Stillaguamish River. A roughness curve uses the amount of surface erosion to establish each slide’s age. The two dates put firm limits on the curve, so that other nearby slides can be dated from their roughness characteristics without having to find material buried inside each mass of soil.
“This is the first time this calibrated surface dating method has been used for landslide chronologies, and it seems to work really well,” LaHusen said. “It can provide some information about how often these events recur, which is the first step toward a regional risk analysis.”
Applying the new method for other locations would require gathering samples for each area, they cautioned, because each site has its own soil composition and erosion characteristics.
The method shows that the slopes in the area around Oso have collapsed on average once every 500 years, and at a higher rate of about once every 140 years over the past 2,000 years.
“This was well known as an area of hillslope 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,” said Alison Duvall.
The team managed to unearth samples of wood buried in the Rowan landslide, just downstream of the Oso site, and the Headache Creek landslide, just upriver of the 2014 slide. Results from several debris samples show that the Rowan landslide, approximately five times the size of the Oso slide, took place just 300 to 694 years ago. The Headache Creek landslide is within a couple hundred years of 6,000 years old.
Previous UW research had shown a history of geologic activity at the Oso site, including previous major landslides and a recent small slide at the same slope that collapsed in 2014.