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Oso Mudslide: Links to the Geology Behind the Tragedy

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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.

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

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