Geological Circumstances Behind Washington Mudslide

Mar 26, 2014

As search efforts intensify around the site of Washington state’s devastating mudslide, geologists are looking into causes of the rapid collapse of the 1,500-foot-wide segment of hillside in Snohomish County that suddenly cut away and crushed the homes and roads below.

The chief culprit appears to have been the glacial composition of the hillside, which is made of silt, clay and soil, and very little rock, which tends to be very loose.

When these collapse they create something called a “rotational slide,” meaning that the land turns on itself, with the base of the hills moving upward as the top collapses.

David Montgomery, a geologist at University of Washington explains the geological circumstances behind the mudslide.


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So what causes a devastating landslide like the one in Washington state? And can they be predicted? We turn now to Professor David Montgomery, a geologist at the University of Washington. He's with us from KUOW. Professor, welcome.


HOBSON: Well, we've read in a 1999 report filed with the U.S. Army Corps of Engineers that there was a warning of a potential for a large catastrophic failure on the hill where this slide occurred. So, based on that, should experts have been able to predict this?

MONTGOMERY: Well, that slide had happened before. There had been - that slide had moved before. This is one - the most recent reactivation of an active landslide. I don't think that people imagined that it could be as large as it went. The warning of a catastrophic landslide in the 1999 report at that site now seems prophetic. Looking at the geologic evidence in that valley, you can see evidence for that kind of an event having happened in the past across the valley.

But the recent slides on that site had not been as big, or gone as far. This is a reminder of just how violent landsliding can be, and how dangerous sites downslope of active landslides can turn.

HOBSON: So you're saying you could predict that there would be some kind of event, but you just have no idea how big it's going to be. And it could be quite small and not worth moving people out of the area for.

MONTGOMERY: Well, to deal with the first part, you could - it was inevitable that site would fail again in the future. How large it would be and when it would happen were open questions that nobody could address earlier. In hindsight, obviously, having a community living in that spot was not a terribly good idea. There can be sort of no debate about that at this point.

But how you make - how you translate those lessons to other areas that might be subject to similar hazards in the future is really the challenge of this for geologists and for those involved in land use regulation and planning. The hazard mitigation is not simply a scientific exercise. It involves how society interfaces with hazards. And this tragic event reminds us that landslide hazards - although one of the more underappreciated of natural hazards - really need to be in that mix and thought about and in terms of how we mitigate hazards as a society.

HOBSON: Well, what makes these hillsides in Snohomish County, where this landslide took place, so unstable?

MONTGOMERY: Well, you know, part of it is the glacial - the legacy of glaciation that deposited the sediments that make up that slope. That landscape is out of equilibrium. It's been responding to the post-glacial landscape evolution. You could consider it a glacial hangover, if you will. The river cutting down into these very loose, very weak sediments created the recipe for a very unstable slope that had failed in the past. And then we had a very, very, very wet winter this year. Those things all combining were a recipe for big slides. What's really hard to predict in advance, though, is which slides will actually fail in any given year or any storm.

HOBSON: So if you have a place like this that you know it's prone to a slide, it's had slides in the past, is there anything you can do to go in and reinforce it a little bit, so that it might not be so susceptible?

MONTGOMERY: Well, you know, the only real way to completely eliminate the hazard to people is to not have people living at the bottom of a slope like that. That - the demand of engineering that would - it would have taken to actually stabilize that particular slope is virtually unimaginable. This was a very large, very unstable slope. And, you know, you could conceivably do some work to try and reinforce the toe. There had been work to try and get the river from cutting into the toe of the landslide and taking out the buttressing at the bottom.

What more could've been done is, you know, in hindsight, perhaps not the right question to be asking in this case. Obviously, this was a terrible disaster and tragedy. Looking at the lessons that could be drawn from other sites and how we can prevent this from happening again is the lesson for geologists and planners and politicians and homeowners, people who'd be most affected by these events.

HOBSON: David Montgomery, professor of geology at the University of Washington, who's also the author of "The Rocks Don't Lie: A Geologist Investigates Noah's Flood." Professor Montgomery, thanks so much.

MONTGOMERY: OK. Thank you.

HOBSON: This is HERE AND NOW. Transcript provided by NPR, Copyright NPR.