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Environment

USGS Study: Prolonged Rain Preceding Oso Landslide Made Mild Slope Deadly

USGS_MR_Oso_Aerial_clipped_adjusted.jpg
USGS
This photo shows the entire extent of the landslide source area and path.

The incredible size and speed of the Oso landslide that killed 43 people last March has been a source of wonder, even for the most seasoned geologists investigating it.

Now the U.S. Geological Survey has published its first peer-reviewed study of the event. It focuses on the landslide’s high mobility as a major cause of the destruction.

USGS models show the Oso landslide reaching average speeds of 40 mph and peak speeds of 60 mph. The resulting force destroyed everything in its path, which ran out for a full kilometer before it hit the other side of the river valley.

“Chances are that it would have ran even further except that it ran into the opposite side of the flood plain and essentially stopped when it encountered that topography,” said Richard Iverson, the lead author of the new USGS study.

In 30 years of studying landslides, Iverson says he’s never seen anything like Oso, not even in historical record. He says if the soil had been only slightly denser or drier, it might not have been so destructive. And this underlines the importance of the record rains that preceded it.

“The prolonged wet period prior to the landslide was a very big contributing factor, not only in making the slope fail, but also in influencing how the landslide behaved after it failed,” Iverson said.

According to measurements taken near the slide, the four-year period leading up to it was wetter than any since record-keeping began in 1928, the new paper states. The study also says that the slope that failed in Oso had a long history of prior historical landslides at the site, but they had not exhibited exceptional mobility.

And Oso's slope was not particularly steep by landslide standards. It was less than 20 degrees on average — 10 degrees less than what would be expected — and still it traveled at extremely high speeds over the nearly flat surface of the flood plain in the north fork Stillaguamish River valley.  

“Generally speaking, landslides that mobilize into high-speed flows begin on slopes steeper than 30 degrees,” Iverson said. “So this is unusual in that sense, and it was also unusual in that it spread out so dramatically across an essentially flat surface.”

Knowing that soil density and moisture were key factors in the sequence of events that caused the Oso slide will help direct future research on landslide hazards in similar conditions, Iverson said.

“Our paper also uses some relatively novel, state-of-the-art methods of seismology to better interpret the sequence of events during the landslide onset,” he said.

The paper details how the soil mobilized in a two-step process during the first minute of the slide.  

“We feel that we’ve reached new conclusions about that,” Iverson said.   

The study is a collaboration of 14 scientists at USGS and the University of Washington. It appears online in the journal “Earth and Planetary Science Letters."

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