Excerpt
JUDY WOODRUFF (NewsHour): Rescue teams are continuing a difficult, hazardous and grim hunt this evening for survivors in a large mudslide in Washington State. More than 100 people are unaccounted for, although the precise number of people missing is uncertain. At least eight are confirmed dead.
The huge slide has prompted many questions about the cause — the cause of this.
Jeffrey Brown explores that part of the story.
JEFFREY BROWN (NewsHour): The mudslide destroyed some 30 homes and occurred about 55 miles northeast of Seattle, near the town of Oso right off state Route 530. Just before 11:00 a.m. Saturday, a wall of mud and debris slammed into the former fishing village, covering an area about one-square-mile wide and 15-feet deep in some places.
David Montgomery is a geologist at the University of Washington. And he joins us now with more.
Well, thanks for being with us. So what can we say so far? What appears to have happened to cause this mudslide?
DAVID MONTGOMERY, Geologist, University of Washington: Well, the proximal cause is that we had one of the wettest months of March on record. That region had something north of seven inches of rain, as I understand it, in the last month.
It’s been very wet out here lately. But that slide — that slide had actually slid before. This is a reactivation of a prior slide that was actually a reactivation of a much older prior slide. So the hill had failed before, so it was prone to weak — it had been weakened by the act of sliding in the past.
And the material that forms that hill is glacial sediments. It’s fairly weak material for such a tall cliff. And so you had a naturally unstable exposure that received an awful lot of rainfall lately. And there’s this other factor of a river that’s been cutting into the toe of the slide for the past few decades.
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