VANCOUVER, Wash. -- The Cascades Volcano Observatory (CVO) is well positioned to study landslides, as landslides and fast moving slides known as lahars frequently accompany eruptions.
Therefore, it is no surprise that a unit of the U.S. Geological Survey is being relied on to contribute expertise on why and how the Oso landslide happened.
At the headquarters of the CVO in Vancouver, Wash., near Portland, one can see how the Oso landslide unfolded in animation from a sophisticated computer model that's taken 20 years to develop. The model can take information like the suspected volume of material (estimated at 10 million cubic yards in this case), take into account how mud flows - particularly when that mud is made of certain types of dirt and rock - and come up with an analysis of how the event happened.
Here are some highlights: In 20 seconds, the slide moved down the scarp of Hazel Hill and across the North Fork of the Stilliguamish River. In another 20 seconds the slide had bulldozed through the neighborhood along Steelhead Drive. At exactly 60 seconds, it had reached the other side of the valley just over a mile from the upper limits of the scarp.
"In this case, it would be like a wall of semi-trucks, shoulder to shoulder, a kilometer wide-- all moving at 60 miles per hour. That's how I picture it," said Dr. Richard Iverson, a landslide specialist at CVO.
He says that was an average - the specific speed at times could have been well above that.
The slide wiped out everything in its path: homes, cars, farm equipment, state route 530 and lives. At this writing 35 are now confirmed dead by the Snohomish County medical examiner, another 11 are still listed as missing.