The roots of the devastating March 2014 landslide in Oso, Washington were laid in a 2006 slide, which itself had its origins in legions of ancient slides that had come before, over thousands of years, a new report has concluded.
The main event leading up to the two-stage landslide that claimed 43 lives in March was a previous one in 2006 on the same hillside, the study found. No single factor, including logging, was the culprit, the researchers said, although National Geographic noted that the report implied that logging could have been one of many factors included in the “chain reaction” that led to the tragedy.
The study was conducted by the Geotechnical Extreme Events Reconnaissance Association (GEER), a program funded by the National Science Foundation that studies extreme events. It was released on July 22, four months to the day after the deadliest landslide in U.S. history.
Researchers looked at large landslides in the Stillaguamish Valley in the Oso area dating back 6,000 years, according to a statement from the University of Washington, which led the study. They examined maps that showed that the valley bottom was composed of debris from old mudslides and estimated that landslides on the scale of the Oso tragedy had taken place as often as every 400 years to every 1,500 years in the region. Moreover, the rainfall for the area was not abnormally high.
“Perhaps the most striking finding is that, while the Oso landslide was a rare geologic occurrence, it was not extraordinary,” said Joseph Wartman, an associate professor of civil and environmental engineering at the University of Washington who lead the study team, in the statement. “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.”
Such findings dovetail with what had long been observed by seekers and keepers of traditional knowledge. The Sauk-Suiattle to the east of the landslide area, and Stillaguamish and Tulalip to the west had long known that 10,000 to 15,000 years ago, the slide area had been a lakebed, and the river blocked by ice. The receding glacier left behind an outwash of fine-texture sand and silt common to lakebeds, said Derek Marks, manager of Timber, Fish and Wildlife for the Tulalip Tribes, to ICTMN after the March slide.
A study of laser imaging of the valley clearly showed traces of multiple landslides underneath the vegetation, National Geographic said, as well as revealing that the scope of the slide was not unusual for the area, even though its speed and the distance it traveled shocked scientists and other observers at the time.
“The new report raises questions about how local governments take into account risks of this sort, and how the newest, best science finds its way to the local level,” National Geographic reported on its website. “The hillside that collapsed had been the scene of multiple smaller slides since the first housing development was built there in the 1960s. Government agencies repeatedly attempted to buttress the hillside, though engineers warned it might do little good.”
Such findings could well be considered when evaluating construction and zoning decisions, Wartman suggested.
“For me, the most important finding is that we must think about landslides in the context of ‘risk’ rather than ‘hazard,’ ” said Wartman in the University of Washington statement. “While these terms are often used interchangeably, there is a subtle but important difference. Landslide hazard, which was well known in the region, tells us the likelihood that a landslide will occur, whereas landslide risk tells us something far more important—the likelihood that human losses will occur as a result of a landslide.”
As Wartman told National Geographic, "That landslide mass prior to March 22 was really in some sense poised to fail. It was really unstable.”