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MICHAEL DUNNE: I'm Michael Dunne. We all love the Oregon coast. With all due respect to our neighbors to the north and south, in Washington and California, our part of the coast is the most beautiful. But of course, as we've all come to know over the past decade, this beautiful coast contains a truly terrifying threat, the Cascadia Subduction zone and the potential for a massive earthquake and tsunami. Well, I've got some more scary news for you. A new report shows that, in addition to these calamities, the subduction zone holds the potential for a sinking of the entire tidal plane that could usher in massive flooding. Today on the show, you'll hear from the lead author of that study and learn about this threat and what it could mean for our beautiful coast. The Cascadia Region Earthquake Science Center, or CRESCENT studies the Cascadia Subduction Zone, and a new report is out from the organization, which adds another threat to our coastal region. Tina Dura, Assistant Professor of Geosciences at Virginia Tech University. Thanks so much for coming on and talking to us.
TINA DURA: Thank you for having me.
MICHAEL DUNNE: Folks might be wondering, what is an Oregon talk show host talking to a Virginia Tech professor? But you’re part of the Cascadia Region Earthquake Science Center, or CRESCENT, which is a center housed at the University of Oregon. Can you talk about what CRESCENT is?
TINA DURA: Crescent is funded by the National Science Foundation, and it is an Earthquake Center, not so much a physical center, even though it is kind of run out of Oregon, but it involves researchers from all around the west coast and across the US. That's why I am Virginia Tech. I'm also involved in it. So, it's bringing together researchers from a lot of different types of subduction zone science, with the goal of kind of increasing our understanding of what kinds of earthquakes and tsunamis can occur at Cascadia, what kinds of effects that they are going to have on populations. And a big part of CRESCENT is also to kind of improve our relationships with partners such as local and regional and federal agencies that have a part in preparing coastal residents for the next earthquake.
MICHAEL DUNNE: Okay, before I get into your study, I know a lot of people here in Western Oregon have heard the term subduction zone much because of Cascadia, but could you kind of help us understand exactly what a subduction zone is?
TINA DURA: A subduction zone is a very large fault, so it's where two tectonic plates are colliding. And in a subduction zone, you have one plate that is subducting or kind of going beneath the other tectonic plate. And so, in this case, we have the Juan de Fuca plate subducting beneath the North American plate. And this covers from Northern California all the way up into southern Canada and Vancouver Island area. So, it's a very long fault. And subduction zones are the types of faults that produce the largest earthquakes on Earth, basically because they're they are very, very long, and they accumulate a lot of strain, as one plate is trying to subduct beneath the other one, and kind of gets locked and accumulates that energy, and so when, when that fault slips, there's a lot of area for it to slip, because it's such a long fault, and this can create the biggest earthquakes that we have on Earth.
MICHAEL DUNNE: Let's talk about your study that you are an author of, and certainly for those of us living in this subduction zone, we've heard obviously about the potential for an earthquake. And we've heard about a tsunami, but your study talks about this idea that another big risk we should be cognizant of is flooding. Talk about the findings of your study.
TINA DURA: Like you mentioned, we wanted to look at this kind of lesser known, lesser talked about consequence of the next earthquake, which is that the land is going to subside or drop during the earthquake from between one to six feet – six feet being in the realm of possibility. And we're basing this on the evidence that we can see in the coastal sediments along Cascadia. So, I'm a coastal paleo seismologist or coastal geologist, and I use coastal sediment records to kind of reconstruct what has occurred during past earthquakes. And so, we can go to these estuaries in Northern California, Oregon, Washington, and we can dig pits and take cores, and we can see really clear evidence that these estuaries have in the past dropped down suddenly during past earthquakes. So, we'll see a very nice organic soil that is suddenly sharply overlain by a tidal mud, indicating that that estuary used to be vegetated and happily growing marsh plants, and then suddenly dropped down and was basically converted to a tidal flat. And so, in conducting this work, I find myself standing in these marsh areas, looking at the stratigraphy and thinking about the next earthquake and how much is going to change the coastal landscape. And sometimes we do see evidence of tsunami inundation as well. So, we might see a nice little sand bed right on top of the soil and then the mud, but sometimes we don't. Sometimes we just see the transition from the soil to the mud, indicating that the land dropped down. And so, a lot of the focus does end up on the tsunami, because it is such a catastrophic event that is definitely going to have large impacts along the coast. But the part that we looked at where the land is going to drop down up to six feet, that is going to persist over decades to centuries, whereas the tsunami will impact the coast and then recede and be over within hours. But the subsidence is going to persist. So those areas that were close to sea level, maybe not experiencing a lot of inundation, will suddenly be in the floodplain, and that's going to cause more frequent tidal inundation, more sensitivity to king tides and these things and storm surges, basically everything is going to be affected now that the land has dropped down so much.
MICHAEL DUNNE: We're talking with Professor Tina dura. She's a professor of Geosciences at Virginia Tech and also part of the Cascadia Region Earthquake Science Center. Talk about the computer modeling that you did.
TINA DURA: Yes. So, as I described, we have this coastal geology that we can look at to get an estimate of how much the land dropped down. So, we can use techniques like looking at how much the land needed to drop down to transition from that soil to mud. We can also use these little critters called micro fossils that tell us kind of how much the tidal range has changed. And so, we have these really important estimates of subsidence from the geology all along the subduction zone, but that's just one number. And so, we have an estimate, let's say, of a meter at a certain estuary. But if we can compile all those and then start thinking about as a whole, if we can see all this deformation, all this land level change along the subduction zone, how can we then kind of feed that information into earthquake models that can give us some upper and lower bounds on what might occur during the next earthquake. And so instead of just having one estimate from the geology, we can use these models to say, well, this amount of slip offshore can match the geology in all these different ways, because there's a lot of different places you can put the earthquake and the amount of slip and still match the geology estimates of subsidence onshore. And so, we use the modeling to get a better picture of the upper and middle and lower bounds of what to expect during the next earthquake. And so, we were considering not just the geology and not just the worst-case estimate, but kind of that range of possibilities.
MICHAEL DUNNE: Okay. By doing your research, and I apologize if this is an overly simplistic question, but I'm just wondering, were you able to sort of see a historic timeline of these floods that were brought on by earthquakes?
TINA DURA: So, with our geologic studies, we can go to some of these estuaries, and we can find evidence of up to 10 or 11 instances of the marsh dropping down and going from that soil to the mud. And so definitely, we can see these repeated Cascadia earthquakes over the last six or seven thousand years on the coast.
MICHAEL DUNNE: Okay, and then, if my memory serves, the last big earthquake in the subduction zone was that like 1700 or something?
TINA DURA: Yeah, so in 1700 we had what's commonly accepted in the scientific community as the last great or large earthquake at Cascadia, and we have really compelling evidence of that along the subduction zone in these marshes. And we also have really interesting evidence in Japan that a tsunami hit Japan in 1700 CE, January 26 1700, because they have historical records of a tsunami that impacted the coastline but did not have a parent earthquake. And so, it's known as the orphan tsunami, because they had these tsunami impacts but did not have an earthquake that was associated with it. And so, modeling and kind of linking that to this coastal geologic evidence at Cascadia, it kind of led to the understanding that the parent earthquake for that tsunami in Japan was from Cascadia. And so, it's a really interesting story that kind of lets us understand that the last time we had possibly magnitude nine or larger earthquake at Cascadia was in 1700 so about more than 300 years has passed since we've had an earthquake, which kind of leads to some within the earthquake culture to say that Cascadia isn't as developed as somewhere like Japan or Chile, where they have more frequent earthquakes on their subduction zone, it's very, very quiet, and so it's, it becomes a challenge to kind of communicate to the public the, you know, the possibility of a really large earthquake there.
MICHAEL DUNNE: Yeah, help me understand something. In this study that you did, finding that the land drops down. A non-expert understanding of earthquakes, as I do, I think of those faults sort of rubbing against each other. How does that cause the ground to sink up to six and a half feet?
TINA DURA: So, I might have to use my hands here just to show that during, you know, when the subduction zone is locked in between earthquakes, the shoreline is diving down. And kind of where my knuckles are, that strain is accumulating, the plate is trying to dive down, and the upper plate is kind of being compressed and rising up. And so right now at Cascadia, we actually have millimeters a year of uplift going on along the coast, and that has kind of helped temper any sea level rise that the Pacific Northwest has felt up to now, because the land is actually rising a little bit right now. And so, it's either kind of keeping up with sea level rise, or in some places, sea level is a little bit faster. And so, that's kind of your inner seismic or in between earthquakes, saying the land is rising. When the earthquake occurs, you have the plates suddenly unstick offshore. So, my fingers right here pop up, and that's what displaces the ocean water and creates a tsunami. But onshore, my knuckles kind of go down. And so, with that slip of the fault, you have the land dropping down and deforming during the earthquake. And that's going to happen basically over minutes during the earthquake. So, while you're feeling the shaking, that's when the land is dropping down. So, this is going to occur very quickly, and the effects will be felt basically immediately afterwards. But if you were in a place that was very close to sea level, near mean tidal level, or below, and this whole earthquake occurred over minutes, and you might find yourself suddenly inundated. But there are places that are where you might not necessarily perceive it, but it's kind of the thing where everything will be lower, and then it might not be immediately clear, because, let's say it's low tide or something, but then the next, subsequent high tide, it would be very clear that everything has moved down, and the tide is able to reach higher than it did before.
MICHAEL DUNNE: I see. We obviously have heard so much about climate change and sea level rise. How does that sort of factor into the potential for drastic flooding on its own, but then also in conjunction with the potential for a Cascadia Subduction Zone earthquake?
TINA DURA: So, we wanted to explore that aspect of the sea level rise also. It’s very hard to predict when the next earthquake is going to occur, so the first thing we looked at was what are the effects? But then we also wanted to think about, well, it might not occur until 2100 let's say, and during that time, sea level is projected to rise along the Pacific Northwest. Like I said today, the effects have been a bit tempered, because the subduction zone is kind of gradually rising, but eventually the climate driven sea level rise rates are projected to increase to the point where it is going to overtake that uplift of the subduction zone, and so by 2100 at these estuaries, we use the NASA sea level projection tool to look at what the sea level rise could be and we found between about 0.5 to one meter of sea level rise at these estuaries. Every estuary had its own kind of value, because the rates kind of depend on that inner seismic uplift of the coast. So, between half meter and a meter of sea level rise by 2100 so if this occurs before the earthquake, you're basically raising the baseline sea level. You're already expanding the floodplain in these estuaries just from the climate driven sea level. And then if you have an earthquake, on top of that, lower the land up to six feet, and you now have an even more expanded floodplain than you would have from just the earthquake.
MICHAEL DUNNE: Wow. I can only imagine the damage there. Talk a little bit how unique the Cascadia Subduction Zone is?
TINA DURA: Yeah, it's very, very seismically quiet. And so, I alluded to that earlier, where in places like Japan and Chile, you might see in the news, you know, earthquakes from time to time. And we had the big earthquake in Japan in 2011, we had a big magnitude earthquake in Chile in 2010, and one in the Indian Ocean, you have a subduction zone that also has frequent small earthquakes and a very, very large earthquake in 2004. Alaska is, you know, constantly having sixes, sevens. And 1964 had a 9.2, so Cascadia is kind of the outlier there, in that it hasn't had a great earthquake in the historical period since the 1700s - that’s the last big event. And on top of that, it doesn't really even have these magnitude six sevens that we expect to see at this kind of subduction zone. So, there are other types of seismicity going on, like these slow slip earthquakes that are much deeper. And, you know, for a while it was thought that Cascadia just didn't make these large earthquakes. But then we had kind of the geologic detective story of discovering this evidence in the coastal marshes, the sediment, sedimentary evidence, and the Japan tsunami, that kind of needed an earthquake to for a parent and kind of putting together that whole detective story. And we also have that really compelling kind of offshore turbidite record, which is another type of sedimentary record that we see, kind of offshore in the canyons, where you can see evidence of mass wasting, or kind of sediment flow events that end up on the ocean bottom there. And so, by piecing that together, Cascadia, everyone's pretty much agreed that it can make very large earthquakes, but it is eerily quiet, and it would be, for preparedness’s sake, a little bit better if we had a few small earthquakes from time to time, just to keep people you know prepared and kind of understanding that there is a threat offshore, it might influence the way things are built and such. There's a lot of buildings in Portland, Seattle and such that have been built without being stressed by a very large earthquake yet, and so it's definitely a challenging subduction zone to sort of communicate the risks to the public about, because it is so quiet.
MICHAEL DUNNE: You know, I know this science is important in and of itself, but obviously it does inform what we as a populist need to do to prepare, right? I mean, you're talking about something, and I don't know that I could even imagine what a 9.0 earthquake would feel like, but this flooding is, is kind of a whole new layer, or at least a whole new understood layer, of the damage that that this event could cause, isn't it?
TINA DURA: Yeah, unfortunately, yes, it's, you know, it's, there's already going to be a very strong shaking. We have the tsunami impacts, then we need to consider this subsidence, and it is difficult to kind of take in the magnitude of what could occur, but I think we're a lot better off if we are aware of it and can kind of prepare for it.
MICHAEL DUNNE: Yeah, my last question for you is this: and I know that as the science stands right now, we cannot predict when an earthquake will occur. But in the work, in the study that you do, do you feel like the science is getting closer to a day where we might be able to accurately predict an earthquake?
TINA DURA: I don't think we're near predicting an earthquake, but we do have some key advancements that are going to help us be prepared for an earthquake, such as the Shake Alert system, where people can get minutes or seconds of warning before an earthquake to at least be able to shut down important infrastructure and take cover or do whatever you need to do in the environment you're in to keep yourself safe. So that's a really important advancement. And I think the geology that we're doing, the modeling that we're doing, gives us more information about where to expect strong shaking or tsunami inundation, and that's really important for preparedness. The geology we do can also help us understand better the recurrence intervals of earthquakes. So, we know from the geology and from the offshore turbidite records that these very large earthquakes occur 450 to 500 years apart. But we do also see evidence, especially in southern Cascadia, of more frequent, maybe every two- or three-hundred-year earthquakes. So that's important to kind of put everything in context. There are some faults that you can do a paleo seismic study on, and they rupture or have earthquakes 1000s of years apart. So, we know Cascadia happens more frequently than that. So that's already a really important piece of information to be prepared, but predicting exactly when it's going to occur, we're not there yet.
MICHAEL DUNNE: Fascinating stuff. Tina Dura, she is a professor of Geosciences at Virginia Tech and part of the Cascadia Region Earthquake Science Center. Really appreciate you coming on and talking to us about this fascinating topic.
TINA DURA: Thank you so much for having me.
MICHAEL DUNNE: That's the show for today. All episodes of Oregon on the record are available as a podcast at klcc.org. The refugee program in America has been upended in unprecedented ways by the Trump administration. Monday on the show, you'll hear from a national and local organization who work with refugees throughout America and here in Oregon, and you'll hear how difficult and heartbreaking the current system has become. I'm Michael Dunne, and this has been Oregon on the record from KLCC. Thanks for listening.
