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Oso Mudslide: Links to the Geology Behind the Tragedy

The views expressed are those of the author and are not necessarily those of Scientific American.

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On Saturday, the Seattle region experienced one of the worst landslide disasters in its history. A lot of the hills around here are unconsolidated glacial deposits, and they’re ready to fall at the slightest provocation. Heavy rains and a river determinedly eroding the toe of a previous slide, possibly combined with some unwise land use practices, caused an appreciable part of the steep slope opposite the tiny town of Oso, Washington to come down. The debris flow blocked the Stillaguamish River and buried at least 25 houses, killing a confirmed 16 people. Over a hundred more are missing. Chances of finding survivors are slim.

Aerial photo showing the extent of the slide. Courtesy Gov. Jay Inslee via Flickr. (CC BY-ND 2.0)

Aerial photo showing the extent of the slide. Courtesy Gov. Jay Inslee via Flickr. (CC BY-ND 2.0)

I’m working on an in-depth post of my own, though it’s heavy going when I have to stop every few moments to calm the rage. It infuriates me when officials know an area is unsafe, and allow people to build there anyway. It enrages me when hapless county emergency management directors claim in one breath that “[The area] was considered very safe. This was a completely unforeseen slide. This came out of nowhere,” and in the next pompously proclaim, “This entire year we have pushed message after message that there’s a high risk of landslides.” Snohomish County: please fire John Pennington for breathtaking inanity. Or at least keep him from babbling at the media.

While I count to ten every twenty seconds and try to contain my fury at incompetent officials, I would like to direct your attention to several places where you can find good information on the geology behind this utterly foreseeable tragedy.

Geology Blogs

Our very own Dan McShane at Reading the Washington Landscape has a fantastic series of posts on the slide and various geologic aspects of it. He knows the area well, and in fact had taken a look at the site of the slide last year and came away very Not Happy At All. He included this LiDAR image that even a landslide amateur such as myself didn’t need a word of interpretation for:


LiDAR image of slide area, courtesy Dan McShane. The smaller red outline is the location of Saturday's deadly slide. The enormous outlined area is the blazingly obvious trace of a gigantic slide that hasn't been dated, but is recent enough to stand right out. Anyone looking at this LiDAR image should have no doubt that this hill is tremendously unstable, and people should not be allowed to build near it.

Dave Petley at The Landslide Blog has several posts up with good information and resources.

This slide appears may be a reactivation of the 2006 Hazel Landslide, which also blocked the river. You can find more information about it at The Sliding Thought blog.

The USGS Blog has information on the USGS response to the situation.

News Reports

The Seattle Times has been doing an excellent reporting job. A pair of posts, “Site has long history of slide problems” and “Risk of slide ‘unforeseen’? Warnings go back decades,” give a lot of information on who knew what and when, plus plenty of quotes from local geologists.

King 5 has an interview with Washington State Geologist Dave Norman, plus seismograph readings from the University of Washington showing the slide came down in two distinct movements.

And PBS Newshour has a talk with a geologist about “what happens in a mudslide”.

Thank you to my readers, who called my attention to this. It’s a fantastic object lesson in the fact that we need geologists involved in decisions on where to build our communities. And we need the officials who make these decisions to be scientifically literate enough to understand the hazards they’re supposed to plan for. This slide demonstrates what happens when no one pays enough attention to the repeated warnings the terrain was kind enough to give before burying a village.

We have to do better.

Dana Hunter About the Author: Dana Hunter is a science blogger, SF writer, and geology addict whose home away from SciAm is En Tequila Es Verdad. Follow her on Twitter: @dhunterauthor. Follow on Twitter @dhunterauthor.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. M Tucker 1:45 pm 03/26/2014

    “It infuriates me when officials know an area is unsafe, and allow people to build there anyway.”

    You ain’t alone! There is a whole branch of geology devoted to ensure the safe location of structures. The county should have maps indicating geologic hazards. As I understand it a home, now buried, was under construction when the landslide hit. It is a perfect example of people insisting on living in places of high risk and no authority to stop them. I believe the body count is now 24.

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  2. 2. psanity 9:48 pm 03/26/2014

    Great resource, Dana. One thing — the link for Dan McShane’s posts is to a news story instead; I think this is right?

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  3. 3. PeterChopelas 11:24 pm 03/27/2014

    Please do not start the finger pointing until more is known about the reasons for this tragedy. I live only a few miles from the Oso landslide, many of the victims are our friends and neighbors. I am also a Licensed Engineer and have done slope stability analysis up and down this valley to assist landowners in getting building permits. Before you criticize those involved you should know something about the history of the area. The lots on Steelhead drive in Oso were created before 1970, and many of the homes were built around that time, when there was not much consideration for such hazards. The oldest geologic assessment of the slope opposite the residential area was 1976, with another one done in 1988 when a logging permit for the hillside was declined because of the hazard. At the time it was very rare that a logging permit was turned down because of geologic hazard. Most of the hillsides in the area are composed of very dense till over bedrock and are usually only prone to shallow surface slumps. As far as I can tell from the county records, no new building permits were approved on the north side of the highway until about 2005, and these were on the opposite side of the valley close to the hwy, away from the river and over a 1/4 of a mile away from the toe of the slope. A subsequent landslide occurred at this location in 2006, changing the course of the river. Another investigation was performed and made available in 2008 (approx). No permits were issued for new homes since that time. All of the reports indicated that the hillside was unstable and there has been no logging permits approved since before 1988, nor building permits for new construction issued since the last landslide in 2006. None of the assessments estimated the extent of hazard, just that it was at extreme risk of slope failure. I understand from media reports that the focus of these reports was about the protection of fish habitat, not public safety. But also consider the building department is often reluctant to deny permits to property owners of existing legal lots where they have been paying property taxes for decades, unless there is a clear imminent hazard to the building site. State regulations consider a building site to be safe if it is half the height of the slope away from the the toe, in this case it would have been only 300 ft. I do not think anyone would have estimated that being 2000-3000 away from an unstable slope only 600 ft high would have been at extreme risk. The state Geologist has a limited budget to do detailed investigations of such sites, similar sites are all over the state and there was no way the department could afford such investigations of all of them. Had they had the budget they may have done a more detailed investigation. But even with better information it would have been difficult to force people to abandon their homes that have stood for over 40 years. Some of the homes destroyed were over a half mile away, uphill and across the highway from the slope, it would have been difficult to foresee that kind of energy in the landslide. And it is way beyond the budget of either the land owner or the county to do a detailed geological investigation of an off site hazard so far away. It is extremely rare for landslides in this area to extend so far from the toe of a hill side even in some of the most severe failures. Usually the building department will require the applicant to record on the property title known hazards such as flood zones or landslide hazard zones, so future buyers of the property will know of the hazard. Whether that was done here or not is unknown, but I have personal knowledge that this has been the case for some time. It is always easy in hindsight to criticize those on the ground from a distance, but unless you expect everyone to be clairvoyant, or have unlimited budgets to investigate such hazards, these kinds of tragedies will happen. I have already started working with my state law makers to increase the budget of the state Geologist’s office so we can get better information available to the public, and to change the focus to public safety. All planning from public works to individual home construction depends on this kind of information to guide future land use decisions. You do your readers and yourself a disservice to criticizes those involved in hindsight before more is known about what lead up to this tragedy.

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  4. 4. BHoover 12:58 pm 03/29/2014

    Before you find yourself “infuriated” “…when officials know an area is unsafe, and allow people to build there anyway,” please read some basic land use and zoning law; spend a day at a land use permit counter; and attend a plan commission meeting or two where zoning changes are proposed. Or ask one of those “infuriating” local officials about the time, money, and lawsuits involved if even the simplest project is actually denied. Read about the Agenda 21 activists who have derailed half-million-dollar, multi-year collaborative efforts to limit building in hazard areas across the west. And then please contain your fury at those awful local officials. In a nation with rabid, lavishly funded right-wing activism and zero common concept of the public good or basic property law, those local officials are probably doing the very best they can keeping any basic order.

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