JUDY WOODRUFF: 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: 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.
I don’t know whether that contributed to actually this slide. People are obviously going to be looking into things like that in the future. But you had these three factors that conspired to make that particular hillside the one that went.
JEFFREY BROWN: Well, so it has happened before. So this is not an atypical experience, or is it in terms of scale?
DAVID MONTGOMERY: Well, this one is big.
The failure that just happened was bigger, traveled farther, did more damage than the ones that had happened at that same site. But pieces of that slope had failed in the past, and this is an enlargement of that, a much bigger, more violent, more rapid one that did far more damage, tragically.
But that slide had failed, you know, seems like about every decade or so going back to the 1960s, as far as I can tell in sort of a quick look into things. It was a fairly unstable place.
JEFFREY BROWN: And what about other possible contributing factors? Some people have asked about logging, for example.
DAVID MONTGOMERY: Well, I understand that the site had been logged back I believe in the 1980s, and that was before the last slide that happened in 2006.
And whether that actually played a contributing role is probably an open question. There’s questions about whether or not logging may have influenced the recharge of the groundwater to the site. Those kind of things, I’m sure, will be — those questions will be asked and looked into. It’s a little early to have a firm opinion on that now.
But the site had been unstable. There’s certainly no chance that logging the site would increase its stability, in my view, but whether that actually contributed to this second failure since that had occurred is an open question.
JEFFREY BROWN: What about the difficulties of the rescue mission now? I see that geologists are continually checking to make sure that it’s safe to go in. What exactly are they looking for?
DAVID MONTGOMERY: Well, when a hillside like that that’s made of a lot of silt and sand essentially liquefies and flows out into — and you take a hillside and turn it into a thin sheet of very loose, shaky material, what you have essentially is create quicksand.
And that’s not the kind of material that is easy to move through. Having sunk into quicksand myself in the Philippines once, you basically can’t go anywhere. You need somebody else to pull you out if that happens.
It’s a nightmare scenario for a rescue team trying to actually get to people who are then trapped within it. It keeps moving. It’s essentially like working in sand Jell-O or sandy slurry. But that will set up over time and become harder as the water drains out of it.
So what you don’t want is to put the rescuers at risk as well. It’s a fine line to walk obviously between trying to get people out and have people get in safely to try to recognize. You couldn’t ask for worse conditions than what they had there.
JEFFREY BROWN: So, just in thinking about potential other places where this could happen — or actually just how much people even know, are aware of where they’re building — you’re talking about knowing this has happened several times in the last. How much do the general public? How much do local governments know? How much awareness is there of where people should and shouldn’t build?
DAVID MONTGOMERY: That question really goes to the heart of opportunities that this tragedy presents for education about geological hazards.
The odd thing about this particular case was that the people who were impacted by the landslide lived on flat ground, on a floodplain on the far side of the river from the hill that actually failed. If you had looked at sort of that geologic map for their properties, it would have listed flooding as the dominant hazard.
And that’s obviously not, in hindsight, what the major hazard at that place was. So looking to see what the hazard was across the river on a slope that is on somebody else’s property, that actually creates — it’s a problem for the information flow in terms of actually getting to homeowners, so that they might even be aware that they might be at risk.
Most people probably don’t have a terribly sophisticated understanding of the geology of the area that they live in. That’s why geologists can make a living doing what we do. But consulting a geologist in terms of understanding the hazards that a piece of property that you might be thinking of buying or living on is a really good idea to actually get the benefit of that.
But there’s a real challenge as to how we get the knowledge that’s in the geological community out and available to homeowners in the general public in ways that they could actually use it.
JEFFREY BROWN: All right, David Montgomery is a geologist from the University of Washington. Thanks so much.
DAVID MONTGOMERY: Great. Thank you.