The research indicates the landslide, the deadliest in U.S. history, happened in two major stages. The first stage remobilized the 2006 slide, including part of an adjacent forested slope from an ancient slide, and was made up largely or entirely of deposits from previous landslides. The first stage ultimately moved more than six-tenths of a mile across the north fork of the Stillaguamish River and caused nearly all the destruction in the Steelhead Haven neighborhood. The second stage started several minutes later and consisted of ancient landslide and glacial deposits. That material moved into the space vacated by the first stage and moved rapidly until it reached the trailing edge of the first stage, the study found.
The report, released Tuesday on the four-month anniversary of the slide, details an investigation by a team from the Geotechnical Extreme Events Reconnaissance Association, or GEER. The scientists and engineers determined that intense rainfall in the three weeks before the slide likely was a major issue, but factors such as altered groundwater migration, weakened soil consistency because of previous landslides and changes in hillside stresses played key roles.
"Perhaps the most striking finding is that, while the Oso landslide was a rare geologic occurrence, it was not extraordinary," said Joseph Wartman, a University of Washington associate professor of civil and environmental engineering and a team leader for the study.
"We observed several other older but very similar long-runout landslides in the surrounding Stillaguamish River Valley. This tells us these may be prevalent in this setting over long time frames. Even the apparent trigger of the event – several weeks of intense rainfall – was not truly exceptional for the region," Wartman said.
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