Roughly 6.5 million people in the U.S. live in areas that are highly susceptible to landslides. West Coast residents — including those on the Oregon Coast and Washington’s Puget Sound — face the second-greatest exposure to landslides, after Appalachian communities in and around W. Virginia.
University of Washington researchers made these findings using the Landslide Exposure Database, an online tool they spent three years creating and recently made publicly available. The scientists say the database provides the first-ever building-level assessment of landslide susceptibility across the U.S. for nearly 130 million homes and buildings.
The database also accounts for socioeconomic data, revealing an urban-rural divide in how natural disasters can impact communities. For example, residents in rural areas who are vulnerable to landslides may have fewer financial resources to move or recover from landslides compared to urban residents living in homes perched on hillsides in affluent neighborhoods.
Joe Wartman is a professor of civil and environmental engineering at the University of Washington and the director of the Natural Hazard and Disaster Reconnaissance Facility. He joins us for more details.
Note: The following transcript was transcribed using AI and validated for accuracy, readability and formatting by an OPB volunteer.
Geoff Norcross: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Geoff Norcross. Folks who live here on the West Coast face the second highest risk of landslides anywhere in the U.S., after parts of Appalachia. That’s the finding of researchers at the University of Washington. They’ve spent the last three years compiling a landslide exposure database, and it is now online. Joe Wartman is part of that team. He’s a professor of civil and environmental engineering at the University of Washington, and he joins us now. Joe Wartman, welcome to Think Out Loud. It’s good to have you.
Joe Wartman: Thank you. I’m delighted to be here and join you.
Norcross: So Oregon and Washington are big places, and they have lots of different geographies. What parts of our states are especially susceptible to landslides?
Wartman: Well, perhaps unsurprisingly, it’s the mountainous portions of the state along that kind of Cascadia corridor, along the Oregon coast, along the I-5 corridor. But we also see distribution of landslide hazards across the eastern portion of the state as well, so that is the physical geography of landslides. Our work here is really looking at the human geography of landslides and how human populations intersect with these susceptible areas.
Norcross: Yeah, and we’ll talk about how the humans are impacted by these in a minute, but can you tell me a little bit more about what is going on with the land in those areas that make it more likely to slide?
Wartman: There’s a very active landscape across the Pacific Northwest, and we have highly landslide prone terrain. There’s a lot of geologic factors that have led to this, but it’s also the high rates of rainfall, and also we’re a seismically active region. So those are the two kinds of triggering factors that often are enough to push this susceptible ground over the edge and to trigger a landslide. So I think the combination of somewhat unstable terrain, high slopes, lots of relief. It’s many of the reasons why we live in this part of the world, in combination with the rainfall and with the seismic hazards.
Norcross: I just want to take a little sidebar conversation here with you because I noticed that the number one most susceptible area to landslides in the country was Appalachia and specifically West Virginia, and that made my ears perk up because I grew up there, and it sounds like there are some similarities, but also maybe some key differences between where we are now and there.
Wartman: That’s right. I think when you look at the Appalachian chain, there is really very little land that is buildable on, at least on flat ground. And so a lot of people have positioned themselves in the landscape based on, in a sense, what is left and what’s possible to build on, and that’s often on sloping ground. So from a landslide density perspective, and it’s also from a population density perspective, those two really overlap in that mountain chain.
It’s a different story really across all of the west. We have a lot of landslide susceptible ground, but people have actually, in a relative sense, been quite good at avoiding that terrain. Nationally, about 20% of the ground has a high susceptibility to landslides, and that represents just about 2% of where people have settled. So it’s a much different story. Those are the national averages, but it’s a really different picture on the East Coast versus the West Coast.
Norcross: Can you briefly explain to me what a landslide is and what typically causes them?
Wartman: Yeah, at the most basic level, a landslide is simply the movement under gravity of rock and soil, down a slope. And so that is the story of what makes ground susceptible. The second part of that you’ve asked about, is what are the triggering factors that push the susceptible ground over the edge and actually trigger a landslide, and those are rainfall and as I mentioned earlier, in somewhat more infrequent seismic events as well.
But basically, it’s this downslope movement. Now, that’s a very broad classification, and I should also say that there are many different types of landslides. Some creep along over the course of months and even years, particularly along the coast of Oregon. Others move very quickly. The ones that move quickly are highly mobile. Those are the ones that pose the greatest risk to individuals in terms of their life safety.
Norcross: Is climate change making landslides more frequent?
Wartman: Climate change is having a few different effects. One part of climate change is drought, and there’s scientific research that shows this has actually served to slow some of the large landslides across the Western United States. I don’t think that those are some of these large landslides, for example, that exist along the coast. I think what I am more concerned about is the increase of atmospheric rivers, the changing precipitation, and short, intense precipitation events.
And then I think something that we’re really now beginning to see much more frequently, which is the cascading chain of wildfire followed by extreme precipitation events. Wildfire denuding the landscape, making the ground temporarily much more susceptible to landslides and much more vulnerable to slides, then followed by intense rain in the fall. That’s a somewhat new cascading phenomenon that I think is related to climate change and something we’re all keeping our eye on.
Norcross: An urban affluent community can be just as geologically vulnerable to a landslide as a rural poor community. The land really doesn’t care who’s living on it or how much money they make. Can you explain to me how the ability to prepare for and recover from the landslide might be very different among these communities?
Wartman: Yeah, one of the things that was most interesting to me in this study that we found is that there is a really pronounced divide between metropolitan America and rural America. Generally, metropolitan America is better off for those who are situated in terrain that has a high susceptibility of landslides, they are better off than the background urban population. And that holds across Oregon, that holds across the entire United States to differing degrees, but it is especially pronounced across all of really the West Coast.
When we move to the rural areas, we see that those who are situated in terrain that has a high susceptibility, have less resources, are more vulnerable than their background rural population. And the reason that that matters is that those in urban settings often have resources such as, for example, a county geologist who is on staff. They have mitigation actions that can often be publicly funded or community funded. And that doesn’t exist to the same degree in some of the rural areas. They often don’t have a full-time geologist on staff. And often because of a lesser tax base may not have the same ability to undertake mitigation options and actions to reduce those risks.
Norcross: A recent example that jumps out at me as maybe pointing up what you’re talking about is the 2014 landslide near Oso, Washington. 43 people were killed, about 50 homes destroyed. Is that an example of what you’re talking about, that it can be harder to jump back from a landslide because of where it happened?
Wartman: Yeah, exactly. Another factor with rural slides, such as the Oso slide, is that it’s geographically isolated, so you can’t reach those areas as quickly. We find in some of this research and some earlier research that we’ve done, that actually a significant number of the rescues that are done in rural areas are from neighbors, not from first responders, simply because of the time required for first responders to access those areas, often in situations where landslides have blocked roads. And so that is another complicating factor for rural versus urban populations and it played out tragically in Oso.
Norcross: Yeah, it sure did. Joe, you’ve been working on this landslide exposure database for the past three years now. It is online. How would you describe it?
Wartman: The landslide exposure database is a national database of 128 million buildings, the building population essentially of the United States, and we have looked on a building-by-building level of what the individual susceptibility is. 98% of the buildings across the U.S. do not have a high susceptibility, and that’s the good news. The other part of that is that the remaining 2% of the population is 6.5 million people. And I think really the story of landslides in America is about those 6.5 million people. Those people are often clustered in these very high susceptible areas, high risk areas.
And so our database identity, it really is a picture of the human geography of landslides and their distribution, and it’s a much different way of thinking about landslide hazards in comparison with the way we’ve traditionally mapped susceptible terrain. Instead of looking at what ground is most dangerous, we’re specifically identifying which parcels are most susceptible.
Norcross: OK, so the data is a little more granular than what we had, and you looked at like 128 million buildings, I think you said. How do you even begin to crunch that data?
Wartman: Yeah, that’s been the work of the past three years and our scientific paper reports on the findings, but it really doesn’t reflect all of the labor that went into this. Much of our time was in developing and synthesizing a number of databases of human occupancy of building footprints. These have come from a number of different sources trying to synthesize these, then merge those with socioeconomic data measures from the Census Bureau, as well as a landslide susceptibility map that was developed by the United States Geological Survey. So matching these up, removing duplicates and so forth has been really the big part of the effort here. It is computationally expensive and again, this has been a 3-year effort to bring this to fruition.
Norcross: And is it the socioeconomic data that you crunched along with all of the science that filled a gap that existed in our terms of knowing about landslide risk? Was it that layer, that social layer that makes your project unique?
Wartman: I think it’s one of the things that makes the project unique. When we think about landslide exposure in the U.S., I think we often have this view from mass media about these spectacular landslide events that are often impacting urban communities. Los Angeles has had a number of these. The Montecito landslide that occurred about five years ago, just in the Santa Barbara area, has made the headlines. And I think that that’s been a lot of the picture. That’s actually just about one-third of the picture.
About two-thirds of the landslide exposed population in the U.S. are rural. And again, those individuals who are in those settings tend to be less resourced than their other nearby rural neighbors. And so I think when we start to look at the socioeconomic data, we begin to unravel some interesting trends. And in fact, I think if anything, what we see is landslides are a mechanism that don’t just reflect that urban rural divide, they actually widen it.
Norcross: I can imagine a lot of stakeholders having interest in the data that you’ve put together and that would be, yeah, homeowners, but also policymakers, lawmakers, insurance companies, FEMA. Who do you think this database is for?
Wartman: This database is for everyone. It’s a really general response to that, which is to say that we have really done our best to try to tailor this to a wide range of audiences. We have made a simplified version of the data available for homeowners who want to check their parcels. Some of this reflects the many requests that people email to me about, I’m in this location and atmospheric river is forecasted, do I need to be concerned? Am I in landslide susceptible terrain? This can be a first step in trying to answer that question.
But we’ve also had more formal databases that can be used by policymakers for making strategic investments in mitigation and in risk reduction. And I think that because we are looking at the socioeconomics as well, it helps us tailor mitigations specifically to communities and understand specifically what those vulnerabilities are. In some cases, they’re in poverty. In some places across the West, they’re limited English proficiencies, and so that can really help us inform what kind of outreach products we develop.
Norcross: What were the biggest challenges in creating this database?
Wartman: It was really in synthesizing 128 million buildings.
Norcross: Well, that’s a crunch.
Wartman: Yeah, and I think that that has been the greatest challenge. I think once we worked through that, it was really the fun of them beginning to really explore the data in some depth and to try to unravel this story.
Norcross: I don’t need specifics, Joe, but do you happen to live in one of these landslide-prone areas?
Wartman: Of course, I checked my house and I do. I live in a moderate susceptibility landslide class. Everything that we’ve been talking about thus far has really referred to the high susceptibility class. I live in Seattle, and Seattle is full of landslide prone terrain, much like Portland and some other major cities across the West. That didn’t entirely surprise me, but it was interesting to look at my own property, and I understand that risk. I recognized that when I bought the house and so noted.
Norcross: Yeah, well, I’m wondering how having that knowledge about your own home and going through this process, how it made you rethink about your home.
Wartman: Yeah, I think one of the things to keep in mind is that our mapping is indicating susceptibility. So it’s telling people where susceptible buildings are, or even if they are in landslide susceptible terrain. And I’ll tell you, much to the relief of many people have learned that they have been concerned that they’re in the West and they are in landslide susceptible terrain and have learned in fact that they’re not. But that is just susceptibility. It is not a forecast of what is happening and what will happen.
That really is kind of a larger picture of what kind of specific risk is posed. And that’s a function of how much rainfall we have, and if there will be a large earthquake and what would be the impacts of such an event. So, I do want to be clear that we’re identifying susceptibility, we are not forecasting what is going to happen at a particular parcel. And so, I think that we need to look at this database with that in mind, and certainly that’s the way I look at it for my own home.
Norcross: Joe Wartman, this was a great conversation. Thank you so much.
Wartman: Thank you, Geoff. It’s great to speak with you.
Norcross: Joe Wartman is a professor of civil and environmental engineering at the University of Washington. He also is the director of the school’s Natural Hazard and Disaster Reconnaissance Facility. You can find a link to all of this data at our website, OPB.org/thinkoutloud.
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