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The Oso landslide that killed 43 people originated from a relatively mild slope but still managed to gain deadly speed and power, a phenomenon that a new study attributes to a two-step process that unfolded in about a minute’s time.

An initial failure March 22 last year near the base of the hill unleashed loose-packed sediments. Then, a second collapse of the upper bluff compressed, pressurized and liquefied this material, enabling it to cross the North Fork of the Stillaguamish River at speeds that averaged 40 mph and strike the Steelhead Haven neighborhood, according to the study.

This entire process was primed by water, and models indicate that about 5 percent less moisture saturating the unstable slope would have likely resulted in a far less mobile and less lethal slide.

“The calculations show how incredibly sensitive this kind of behavior can be,” said Richard Iverson a, U.S. Geological Survey hydrologist who was the lead author in the study published online Friday in the journal, Earth and Planetary Science Letters. “It’s kind of remarkable and sobering.”

Iverson worked with 13 other scientists from the Geological Survey, the University of Washington and the NOAA National Weather Service to put together the study that investigates why the landslide moved so rapidly over a large area.

Iverson said that scientists should strive to do more than simply identify areas where a slide may occur. The challenge is to improve forecasting about the potential scope of a slide’s run-out to better understand the risks to the public.

The Oso landslide in Snohomish County involved about 18 million tons of sediments — or almost three times the mass of the Great Pyramid of Giza — and sent debris across the 1-kilometer-wide flood plain of the Stillaguamish’s North Fork. It came down from a bluff that has failed numerous times in the past and was identified as an unstable zone for deep-seated landslides.

Estimates indicate that about half of that debris still remains on the slope that failed, and more analysis is required to determine how great a hazard it represents, according to Iverson.

The study follows the July release of a National Science Foundation-sponsored (NSF) team report on the slide, which found that it was not an outlier when compared with other landslides and debris flows around the world. That report noted a 2004 geological map showed landslide deposits under the Steelhead Haven neighborhood.

“Perhaps the most striking finding is that, while the Oso landslide was a rare geological occurrence, it was not extraordinary,” said University of Washington engineer Joseph Wartman, a co-leader of the NSF-sponsored team, in an earlier interview with The Seattle Times.

But the study published this month by Iverson and his colleagues found the mobility of the Oso slide exceeded that of comparable landslides elsewhere. The study noted it began on a slope inclined — on average — less than 20 degrees. Elsewhere, landslides that transform into mobile, high-speed flows “almost invariably” begin on steeper slopes of more than 30 degrees, according to the study.

“I am personally not aware of any modern analogues where something has behaved this way,” Iverson said.

The landslide was primed by rainfall.

The new study cites an analysis of data from a nearby weather station that found the four-year period that ended March 31 of last year was wetter than any other four-year period in the 86 years of record keeping at that station.

In the scenario sketched in the new study developed with the aid of field data and computer modeling, a slow-moving landslide lasting tens of seconds first unfolded on the lower slope of the bluff and undercut support for the mass above it.

That slide did not reach the river, and it started to lose momentum. Then, the upper bluff collapsed, liquefying the earlier slide and giving it new energy.

Iverson said this scenario is buttressed by accounts of some of the eyewitnesses he interviewed as he conducted research in the early days after the disaster.

This scenario has some differences with the one developed by the NSF-sponsored report, which tracked seismic records and noted two major collapses that were a few minutes apart rather than a minute apart.

The future will bring more studies of the Oso landslide, which ranks as the second deadliest in U.S. history.

“I think one of the things that geologists excel at is challenging ourselves to figure out what really happened,” said David Montgomery, a University of Washington geomorphologist, who participated in the NSF-sponsored team.

Hal Bernton: 206-464-2581 or hbernton@seattletimes.com