Monday, March 24th, I was perusing blog posts about the Oso, Washington mudslide, some of which were written by people knowledgeable about local geology. I was disturbed by what I found. By Tuesday, the major news stations and newspapers around Seattle had started to publish stories asking the same question—were there red flags indicating that the mudslide would happen? Could a landslide happen again soon?
I’m not an expert, but the more I learn about the geology in this area, the more concerned I become about the possibility of another slide and the more I question whether rebuilding in the same spot—if that’s on the table– is a wise idea. As a society we see rebuilding as a sign of resilience and so might not question the wisdom of doing so.
No disrespect is meant to the victims of this disaster—this is not about assigning blame, this is about bringing up important questions so that this doesn’t happen again. Besides, how many of us could describe the geology we live on? When does that come up in a conversation with a real estate agent? When do we think about getting extra insurance for something like this?
Part of downtown Seattle is built on a liquefaction zone, and in the Northwest, we seem to build all over the place despite being at risk of huge earthquakes. We have five volcanoes— Baker, Glacier Peak, Rainier, St. Helens, and Adams– and live in the Cascadia Subduction Zone. The earth is alive and changing in Washington State. Some will say it’s too soon to be bringing up the tough issues like this, but given the very real danger all of those brave rescuers face as they continue with recovery efforts on a rainy week, I disagree.
Last night I interviewed David Tucker of the Mount Baker Volcano Research Center, a 501 (c) (3) non-profit research and educational center informally affiliated with the Geology Department at Western Washington University. Dave blogs at Northwest Geology Field Trips and the MBVRC site.
Please note that the views and opinions expressed in this interview are those of the interviewee and myself. All data and information provided in this article is for informational purposes only and not intended to replace expert advice. Media reports are presented as is and no guarantee is given as to the accuracy of the information.
DT: I have a master’s degree in geology from Western Washington University. I grew up in western Washington and have lived here my whole life. I loved climbing our local volcanoes in the Boy Scouts and that got me hooked. When I was a teenager, someone told me that Puget Sound was sculpted by ancient glaciers and that only added to my interest.
Back in the 1980s I served as a professional mountain guide. I’ve published research papers, principally about Northwest volcanism, but also coauthored a paleontology paper. Additionally, I love to do adult education. I am not a professional geologist, but I do it all out of love for the science and to educate the general public.
WN: Is it safe to rebuild? Or should Oso residents be relocating elsewhere (meaning not near slide areas)? The Oso area is their home and relocating seems so unrealistic; I understand that. But the more we learn about geology in this area, the more I’m questioning if rebuilding or relocating is the answer. This is the same question many had about New Orleans.
DT: It’s not safe to rebuild in that location. People should not have been allowed to build there in the first place, or to rebuild after the 2006 landslide.
WN: Someone said on the radio yesterday that there were yellow flags indicating that a mudslide could happen in this area, but others argue that there were red flags. Were there clear signs that something like this mudslide was going to happen?
DT: There were absolutely clear signs. Did you see the article in this morning’s Seattle Times? It was all about the geologic reports written on this place back in the 1990s. One report said it had “potential for a large catastrophic failure”. The various blog posts by Dan McShane I linked to yesterday make it clear that this place had slid many times before.
WN: Looking at the DNR’s geologic map, I’m still trying to decipher the landslide layer. What’s your interpretation? Does this show a history of recent mudslides in the Oso area?
DT: The DNR map shows several landslides in the Oso area, including a deposit of a really big one right where last Saturday’s occurred. I’ve attached a screen shot I made (below). If you open the DNR website, in the upper left are four little squares. Point at it and select the “topo” option. This adds the topographic overlay.
Click the “map contents” icon near the upper right. Move the slider beside “surface geology” to make the geo overlay more transparent. Now zoom in to the Oso area. Every pale yellow geologic unit labeled “Qls” is a quaternary landslide deposit. Move up and down the valley. There are lots of those Qls units. They don’t give the actual age of these slides, but you can bet they are all relatively recent (occurring in the past few thousand years).
Just south of Steelhead Drive is another large yellow landslide marked Qls . It runs down off the hill and the mapped deposit nearly reaches the Steelhead Drive are with a north-pointing lobe. It crosses 530 at the same place that the fault does (the dotted line).
I wouldn’t doubt if the point that the Steelhead Drive homes were built on, projecting north into a curve in the river, is actually part of the landslide deposit. There’s sort of a double whammy facing that location plus the fault.
WN: Some have speculated that another slide is possible which could complicate rescue and recovery efforts. Rain is in the forecast all week. What are the chances of another event like Saturday’s?
DT: You can’t forecast these things. There could be another soon; it could be decades away. Maybe not in exactly the same place, but possibly immediately adjacent, since the slope that slid has now created unstable scarps to either side.
WN: Lastly, what advice do you have for Oso area residents and their loved ones as they fight their way back from this disaster?
DT: In general I’d say that all people should learn a little geology, which is the ultimate power in the Northwest’s landscape. Learn how to read a geologic map and study an area carefully before buying land or building. Ask a geologist if you don’t understand, then ask another one for another interpretation.
The houses at Steelhead Drive were built on top of a big landslide deposit (of unknown age to me, but that is not really important). And don’t live in floodplains, which are at risk for floods and in some places, lahars. Learn from past mistakes. Pay attention to past history.
WN: Thanks Dave.
Here are some other links that have mentioned geological issues and raised questions. Please pray that as the recovery and rebuilding efforts advance our leaders will make wise choices that give area residents the best chance of never having to go through such a horror again. This can be accomplished by all agencies and property owners working together and sharing data.
As rescue workers slogged through the muck and rain in search of victims Tuesday, word of the 1999 report raised questions about why residents were allowed to build homes on the hill and whether officials had taken proper precautions.
“I knew it would fail catastrophically in a large magnitude event,” though not when it would happen, said Daniel Miller, a geomorphologist who was hired by the U.S. Army Corps of Engineers to do the study. “I was not surprised.”
The Herald: A relentless river erodes the hills
“It’s pretty clear to me that one thing’s that made that area so unstable over the years is that the river is taking the toe out of that slope,” he said. “Whether that contributed to this recent reaction of this old slide is an open question.”
At least two smaller slides have occurred in recent decades along the same hill — one in 2006, another in the late 1960s…
The top layers of the surrounding hills consist of porous glacial soil that becomes easily saturated with heavy rainfall.
Geologists agree that Saturday’s slide was a deep-seated one, meaning several layers of soil gave way.
A state geologic map shows that in many areas just north of the current river route — including along the base of the hill that collapsed — the soil is made up in part of river silt.
Dan McShane, Reading the Washington Landscape: Arm Waving Notes on the Stillaguamish Blocking Landslide
This slide was a preexisting problem. There was concern that it would block the river a few years ago. Landslide wonks knew exactly where this slide was as soon as it made the news. I looked at this slide area last summer. It was clearly bad news then and with very heavy multiple rain events the past six weeks landslides – particularly big ones are not a surprise.
Dave Petley, Durham University, The Landslide Blog: Oso landslip: useful resources and the rising human cost
Whilst I am referring to this as the Oso landslip, in fact it is a reactivation of an existing landslide, known as the Hazel Landslide. This landslide is known to have moved 1988, and went through a second phase of movement in 2006. It is well described in a blog post from 2009 that can be found at:https://slidingthought.wordpress.com/tag/north-fork-of-stillaguamish/.
Dave Petley, Durham University, summarizing information from the Yakima Herald: ‘Unforeseen’ risk of slide? Warnings go back decades. and The Steelhead landslide in Oso, Washington State
- 1949: A large landslide (1000 feet long and 2600 feet wide) affected the river bank
- 1951: Another large failure of the slope; the river was partially blocked
- 1967: Seattle Times published an article that referred to this site as “Slide Hill”
- 1997 report, by Daniel Miller, for the Washington Department of Ecology and the Tulalip Tribes
- 1999: US Army Corps of Engineers report by Daniel and Lynne Rodgers Miller that warned of “the potential for a large catastrophic failure”
- 25 January 2006: large movement of the Steelhead landslide blocked the river
A 2010 report commissioned by Snohomish County to comply with a federal law warned that neighborhoods along the Stillaguamish River were among the highest-risk areas, The Seattle Times reported.
The hillside that collapsed Saturday outside of the community of Oso was one highlighted as particularly dangerous, said the report by California-based engineering and architecture firm Tetra Tech.
“For someone to say that this plan did not warn that this was a risk is a falsity,” said report author and Tetra Tech program manager Rob Flaner.
KING 5 video: UW geologist on Oso landslide
Seattle Times: State allowed logging on plateau above slope
Others echoed his concerns. Noel Wolff, a hydrologist who worked for the state, wrote that “Timber harvesting could possibly cause what is likely an inevitable event to occur sooner.” And Pat Stevenson, an environmental biologist for the Stillaguamish Tribe, cited “the potential for massive failure,” similar to a slide that occurred in 1967.
Seattle Times: County’s own 2010 report called slide area dangerous
Snohomish County officials were warned in a federally funded report in 2010 that neighborhoods along Steelhead Drive, nestled along the Stillaguamish River, were ranked as one of the highest risk areas for deadly and destructive landslides.
Geological reports warning the hill is in imminent danger of collapse date back to the 1950s.
But the alerts, issued by experts every ten years, went unnoticed.
Seismometers showed no earthquake triggered Saturday’s landslide at Oso, but seismic signals show there were two major slides during the event, about four minutes apart.
The image shows a bend in the north fork of the Stillaguamish and what appear to be broken-off logs cluttering the river at the northeast end of the wall. The wall was swept way Saturday in a mile-long mudslide that flattened homes on the other side of the river, killing at least 17 people.
It’s too early to say whether the wall’s failure played a part in the slide, but it could be studied in the event the river had eaten away at the hillside’s stability, said Dave Montgomery, a geologist at the University of Washington.
“You can clearly see the river cutting into the toe of the slope,” Montgomery told KING 5.
There are some very interesting photos and graphics here.
This is what Dan McShane has been talking about on his blog– lidar. Dan’s blog is one of the best places to go for images and analysis.
Several of the slides along the Stillaguamish that pop out in lidar images aren’t included in the Washington Department of Natural Resources’ (DNR) statewide landslide inventory, said Ralph Haugerud, a lidar expert with the U.S. Geological Survey. And while even some experts were shocked that Saturday’s slide plowed across the river, the lidar images show that some of the old slides in the vicinity were clearly powerful enough to have run out even further.
This article contains an interesting Q&A with wildly popular UW Professor Cliff Mass. Cliff has a very informative article titled The Landslide State on his weather blog.
Washington State Department of Natural Resources: Their Ear to the Ground blog has a question and answer feature about landslides and geology.
Seattle Times: Expert baffled by ferocity, distance of ‘freakish’ slide
Using a new computer model, Iverson estimated that the mass of mud, rocks and trees was traveling about 60 mph when it slammed into the river.
ABC News (Australia): Washington mudslide: before and after photos show scale of destruction
USGS: Oso Landslide Preliminary Simulation