SEATTLE - At the University of Washington's seismology lab, where even the tiniest earthquakes are tracked and measured, the Pacific Northwest Seismic Network recorded not one, but two slides Saturday morning less than four miles east of the town of Oso.

The first slide lasted about two and one half minutes.

And five minutes later there was a secondary failure that was a little bit smaller, said Kate Allstadt, a seismologist and researcher at the University of Washington. The whole event, along with more chunks moving downhill lasted about an hour she said.

It illustrates how we need to think about how those hazards actually move across the landscape, sometimes in surmising ways, said U.W. geologist and landslide expert Dave Montgomery.

The area has seen large slides before. One in 2006 blocked the Stilliguamish River, forcing it through the very neighborhood wiped out by the current slide.

Another slide hit in 1969. The Washington State Department of Transportation has found itself carrying out various projects to try and stabilize State Highway 530, now buried by the slide.

The debris area runs roughly 4,400 feet by 4,400 feet according to Dave Norman, the state geologist with the Department of Natural Resources. The so-called scarp - the whitish gouge in the face of the mountain now a cliff - is about 1,500 feet wide by 600 feet tall. There is concern for more tension cracks that could allow more debris to fall.

Professor Montgomery says the slide carries many of the hallmarks of a so-called rotational slide, meaning that the land literally turns on itself. Often times as the support part of a hill slides down, the toe or the foot of the slide area goes up. Only this time the toe went out, driving the soggy mess forward into the populated area.

If that's how it worked, what was the hill made up of? Pretty much the leftovers from the last ice age, things like sand and silt, clay and gravel - not much rock.

It's an amalgam of different kinds of glacial units piled on each other, but none of which are terribly strong, said Montgomery.

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