A team from the U.S. Geological Survey is leading the effort to understand what happened. Richard Iverson, a hydrologist at the USGS’s office in Vancouver and one of the world’s leading landslide experts, told the Seattle Times that the slide was caused by “a combination of unusually wet weather, erosion at the toe of the slide and local geology.”
The slope that failed is largely made up of loose, sandy soil deposited by retreating glaciers. But that porous material is underlain by a compacted layer of silt and clay, which blocks the flow of water and allows it to accumulate deep within the hillside.
According to the Times, Iverson believes the sandy soil collapsed and “probably compressed the sodden soil, which would have increased water pressure between soil grains and turned the mass to soup.”
The “mass of mud, rocks and trees was traveling about 60 mph when it slammed into the (Stillaguamish River).”
“It was a freakish thing,” said Iverson, of the U.S. Geological Survey’s Cascades Volcano Observatory. “I’m not sure anybody would have anticipated a slope like that would liquefy the way it did.”
Another official from the USGS told the Wall Street Journal that heavy rainfall was the primary cause.
Randall Jibson, a USGS research geologist, said in an interview that while USGS scientists are certain heavy rains were the cause, they are still investigating why the landslide was so large and traveled so far.
“The far-above-average rainfall in the weeks and months prior to the landslide raised water levels in the hillside, and that is what ultimately caused the slope to fail,” Mr. Jibson said. “There is a consensus among landslide experts in the USGS that this is the cause.”
Nearly a month of torrential storms dumped 8.7 inches of rain on Arlington, a data-collection site near Oso, recording the second-wettest March there on record, said Josh Smith, a meteorologist with the National Weather Service.
Washington State Forester Aaron Everett said the slide was 600 feet below the ground, far from the surface.
“There’s a distinction to be made about the type of landslide we’re talking about here. This was a deep-seated landslide. That originates far, far underground. And that’s in contrast to shallow fast moving landslides, where the root strength of the trees has a lot to do with the landslide hazard,” said Everett.
David Montgomery, a geology professor at University of Washington, agreed with the USGS that rainfall was the primary cause, further destabilizing a hillside that was already weakened by the river and sandy glacial soil.
“So you had weak materials exposed on a high, steep slope, and we had an awful lot of rainfall this winter. The proximal cause of this landslide was the great amount of rain we’ve had over the month of March. We had something like 200, 300 percent of the normal rainfall that we get.”
More details will come as scientists continue to do their work.
If you would like to donate to those affected by the landslide, the Everett Herald has a contact list of local charities that are taking donations.