Subduction Zones, Earthquakes, and Tsunamis with Tina Dura

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Travis

Of all the scary things to pop up in movies. Probably some of the most terrifying things to me are earthquakes and tsunamis. They just seem more realistic to me than a chainsaw-wielding clown in a cornfield, I do hope that I'm correct about that.

So I'm curious what we know about these naturally occurring tears, what we can learn from history, how do we go about doing that, and how does that help us better prepare for them to possibly happen again? Thankfully, Virginia Tech's Tina Dura is not only an expert in this area, she was also kind enough to talk with me about it. Tina is an assistant professor in the Department of Geosciences at Virginia Tech. She's also an affiliate faculty of the Global Change Center, where she leads the Coastal Hazards Lab at Virginia Tech. Her research involves using a variety of different methods to better understand subduction zones, the natural disasters they have produced in the past, and then using that information to help us better predict what they might do in the future. So Tina and I talked a little about what subduction zones actually are, how she goes about investigating them almost like a detective, and what critical information would a person like me living in Southwest Virginia need to know about earthquakes, tsunamis, and subduction zones. We also talked a little about the tsunami movie that scared me a few years ago and she was extremely kind in helping me remember the title.

I'm Travis Williams and this is Virginia Tech's Curious Conversations.

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Travis

I’ve heard a lot about your work and I'm just curious, what is a subduction zone?

Tina

So a subduction zone is where two tectonic plates collide. where you have a oceanic plate that's meeting a continental plate and the two are sliding against each other, that's where you get a subduction zone. They don't always slide freely so it can get locked and where it gets locked it's accumulating energy.

And when that slips again is when you get a big earthquake. So subduction zones are where you can get the largest earthquakes on earth because they are the longest faults that are on earth. And because they're very long, of big area of them can kind of slide and make an earthquake. And so you get, you know, the big magnitude eight or nine earthquakes that you hear about like in Japan and Chile or in the Indian Ocean and places like that.

Travis

Yeah, so how many subduction zones are there around the world? Do we know? I honestly off the top of my head do not know the answer to that. There are the part of the earth that has the most subduction zones is known as the Ring of Fire. So around the Pacific Rim. And the reason I say I don't know the exact numbers because they're the very large ones. And then there's little small plates that are colliding around the world.

 

But the big ones that you might've heard about are, like I said, in Japan, in Alaska, in the Pacific Northwest of the US, down in Chile, there's in the South Pacific, and then in the Indian Ocean. Those are kind of the biggest ones that we hear about and that make the earthquakes, where there are a lot of people concentrated in those areas.

Travis

What's the biggest difference? I know you mentioned the magnitude, are there any other differences between earthquakes that's created from a subduction zone? an earthquake that's created otherwise?

Tina

So, subduction zone earthquakes. mean, all earthquakes are created when basically two sides of a fault slide against each other, so suddenly fail. But in a subduction zone, like I mentioned earlier, they're just the longest faults that we have on Earth and they are thrust faults. So, it's two things colliding kind of against each other. And so, a lot of energy can get stored up in those kinds of faults. If you have like on the San Andreas fault, that's a big fault we've all heard about in California. Those plates are sliding kind of past each other and you can also get quite large earthquakes there, but they don't tend to spread over as large of an area.

Travis

When it comes to subduction zones, what is your favorite subduction zone and why is it the Cascadia subduction zone?

Tina

I do love the Cascadia subduction zone. It is..It's a really interesting subduction zone. It's in the US, for example. So we do have the one in Alaska as well, but kind of in the lower US, we have the Cascadia subduction zone. It's an interesting place because it has not had a large earthquake since 1700 AD. The 1700 AD earthquake, are some kind of native oral accounts of something happening around that time, but there are no written historical accounts of it. And we obviously don't have any kind of instrumental measurements of it. And so for a long time, before we sort of discovered those records, we didn't even know that Cascadia would make big earthquakes because it's very, very quiet right now as far as earthquake activity. The plates are, it looks from GPS measurements and such that the plates are locked against each other. And so it's accumulating that strain, that energy that it can release, but it's not making those small earthquakes that we detect in places like Alaska or Japan or Chile. And so it's kind of a curiosity to scientists and why it's so quiet, but in the geologic record, so over hundreds to thousands of year time scales, we can see evidence that has had big earthquakes in the past, but we're just not seeing them in kind of modern observations. And so because we know it's happened in the past and it's very quiet now, The thinking is, you know, maybe it's kind of ready to go and create a lot of issues for the West Coast, the U.S.

Travis

So it sounds like that you are kind of stuck between trying to figure out if this is like a sleeping giant or just like a completely just completely asleep. I don't know another good analogy for that.

Tina

Yeah, no, that that's a good one. But I just think in the geologic record is so compelling. I mean, that's why it's my favorite subduction zone, not because it's gonna unleash this catastrophe. That's not my favorite part. It's that the geologic record. in the coastal sediments, you can see evidence that the coastline has been impacted by tsunamis and earthquake subsidence in the past. And so I think it's pretty well accepted that it can make big earthquakes. It just hasn't recently.

Travis

How do you go about figuring out or learning this story of what this zone has done, just from looking at the geological record, how do you go about establishing that?

Tina

So I like to think of us as earthquake detectives. trying to go find clues in the landscape that can tell us about what kinds of ecological changes occurred to the environment, what kinds of changes in how the estuaries and the marshes there were configured that can give us clues about what earthquakes might have impacted the coast. So we basically, we're coastal geologists. We go and we investigate these low energy intertidal environments. mostly salt marshes and estuaries and sometimes coastal lakes and things like that. And we look at the sediment record that's contained in those environments. And so we take cores. do these, you know, we can sometimes we do vibro-coring, which is a wider diameter core that we can use kind of motor to power down into the subsurface, or we use push cores and we extract these and we look at the layers of sediment and those can tell us something about how, you know, whether tsunamis came in, washed sand into these environments, or sometimes in a big earthquake the land can subside one or two meters and that can change these big salt marsh intertidal environments from an organic soil that's growing there with all the vegetation that I'm sure everyone has seen, you know, going to the coast, you see these intertidal environments with basically with peat or soils, and then they all get dropped out one or two meters and they all convert to mud flats, basically. And so we get these really distinct changes from a soil to a mud that's in the subsurface. So you can go, you know, four meters down into the subsurface and you see repeatedly soils with mud, soils with mud. And there are just a series of earthquakes that occurred along the coast, drop the land down, and then the land slowly in between earthquakes rises up, and then it drops down in the earthquake. And so it's kind of recording that earthquake cycle. And we can date the sediments using radiocarbon dating to get an age estimate of when those earthquakes occurred in the past.

Travis

Wow, that sounds fascinating. And I love the idea of you being an earthquake detective and looking for these little clues. Like, if you start a podcast, you can name it that. Or you should name it that. That'd be great. Earthquake did.

Tina

Yeah. Yeah, that could be fun.

Travis

Well, I know one of the things you mentioned earlier that is kind of a marker sometimes for looking at earthquakes or maybe the results of the earthquakes being a tsunami. Are these things called, do you call them diatoms? Diatoms. Diatoms, sorry about that.

Tina

Yeah, diatoms.

Travis

What are diatoms and why are they important to your work?

Tina

So diatoms are critical to the work that we do in our lab here. So they're basically microscopic algae. They have little silica shells and the silica shells make them very resistant to breaking down. So they're recorded throughout the sediment record over thousands of years. And they're really useful because they're very sensitive to changes in tidal inundation or salinity or even the substrate or the sediment that they live in. And so they can be really good recorders of these environmental changes that are happening through time during the earthquake cycle. And so we can use them both in kind of qualitative ways to say, you I can see that this changed from a high marsh environment to a tidal flat when the land dropped down based on the change in the soil to the mud and the diatoms that are contained in those sediments. And then we can also get more quantitative with them and actually give a precise elevation of where in the intertidal frame those sediments all accumulated. And so we can say in the high marsh environment, you were at mean high water based on what diatoms are living there. And then post earthquake, you were in a tidal flat that was one and a half meters below mean high water. And that change can then tell us about how much the land subsided. And this can be used to inform many different things from kind of modeling, know, the modeling people do to try to simulate what the next earthquake will look like. So the substance can tell you how much slip to put offshore on the fault. And then that's going to help inform, you know, where the shaking is going to be, where tsunamis might be generated, et cetera. And they can help inform basically how much subsidence we can expect in these communities along the coast as well.

Travis

So it sounds like the diatoms are kind of like the witnesses to the crime that you're investigating at this point.

Tina

Yeah. mean, the sediments themselves reveal a lot. You know, there's a big change from a soil to a mud. But having that It's what we call a proxy or kind of like another indicator that's in there that's like telling you about the environment beyond just the sediment is very powerful because you can get different, you know, changes in sediments with different processes. And so it's nice to be able to say like with certainty that you went from, you know, something that was fresher, lessened and dated to something in a title plot.

Travis

Yeah, I'm just trying to completely milk this detective analogy for all that is worth.

Tina

Love it. Love it.

Travis

Well, I know an important part of your work and some of the recent grants that you have is being able to take your research and translate it into just actionable information for the general public, for people like me. And so I'm curious, what all are you all doing to help communicate what you're finding to people in ways that can help better their lives?

Tina

Yeah, so it's been exciting to think about how the geology and kind of this information that to us is super interesting, to someone in the public, they're not that interested in the mud that I love to play in and think about. But trying to take the geologic data that we can gather in these studies and turn it into something meaningful for hazards preparation and for the public is really exciting to me. So we've been trying to do more of that. And one of the things we're working on now is basically mapping out where subsidence is going to occur along the subduction zone. So based on our geologic knowledge of how much the land went down in the last earthquake, try to sort of simulate that in modern communities today and try to be able to inform them about how much the floodplain is going to change in their community. in some of these places, we haven't had an earthquake in over 300 years. And so we have built in areas that potentially are not great to have built in. They are near mean high water there in places that can be inundated multiple times a year. When the land subsides one to two meters, those communities are going to, in some cases, be in near mean tide level. But definitely, it's going to expand the floodplain. So places where people need flood insurance is going to expand. And we've found that basically expands by hundreds of kilometers squared along the subduction zone. If you add up all the different estuaries and think about how the floodplains are going to expand. if you kind of project this subsidence onto these communities. And some are more effective than others. And so I think it's just an important component of kind of hazards preparation that is not being considered yet that we hope to bring a little bit more awareness to. And having that sort of our detective, you know, our results of our detective investigation to be able to share with people and, you know, people often think about the tsunami, which is going to be a you know, have a huge impact on the communities. But post tsunami, know, tsunamis recede and there's going to be destruction. also post tsunami receding, you're going to have a land that is now one to two meters lower. And that's going to really change. It can change ports. can change, you know, wastewater treatment plants, just airports. There are a lot of airports in Cascadia that will be in the floodplain post earthquake. And so these are things obviously that need to be considered as we sort of prepare for the next earthquake.

Travis

Yeah, and I guess it sounds like it's both earthquakes and tsunamis. think everything I know about tsunamis, I think I learned from a movie that I can't remember the title of, but I'm not sure if there's a ton of tsunami movies, so you probably know what I'm talking about.

Tina

Is it the one about the Indian Ocean tsunami?

Travis

Yes, I think so. I think Ewan McGregor was in it.

Tina

Yeah, I think it's called The Impossible, I think. Some wild...scenes in that looked pretty scary. I would not want to experience that.

Travis

No, I've watched that movie with my now wife and she still talks about it because of just people and kids and being in a bad situation. And so like it left. So she's terrified of tsunamis. So I think we probably both are at this point.

Tina

We should all have a healthy fear of tsunamis, but we can prepare and have a plan. Not here. When Blacksburg, we're totally fine. So.

Travis

Well, that's one of the things I was going to ask you. What should the average person that maybe lives in one of these areas or really any coastal area, what what types of things should they be paying attention to?

Tina

Definitely a have a tsunami evacuation plan. And so, you communities in Washington, Oregon, Northern California have tsunami inundation maps that the public can access and, you know, have a plan for if you feel earthquake shaking, first you're gonna follow the guidance of the drop, cover and hold. If you feel if the shaking lasts beyond 20, 30 seconds, that's gonna be a extremely large earthquake and you should be evacuating from this if you're in the tsunami inundation zone. And that can be found online and interactive maps or just like static maps of the different neighborhoods and kind of the different routes to take out of there. So even when vacationing in places like this, it's important to just be aware of that and find that resource and see what you need to do.

Travis

Yeah, that makes a ton of sense to me. I think the people in that movie were on vacation too. So, yeah, they should have listened to this podcast before they went. Yeah, we can go back in time. We can fix that.

Well, one of the things I like to ask people who work in a variety of different spaces is what about your work and your research? What gives you hope?

Tina

I think just bringing awareness to folks about empowering them with the knowledge to actually do something in the event of an earthquake or tsunami. That brings me hope just because I feel like there's a lot of fear around it and a lot of people are only exposed to these things through movies like you mentioned where all you see is the catastrophic side of it. And that it is going to be a catastrophic event, but I think with preparation, if people have kind of their earthquake go bags with food, water, medicines, and things they need, and a plan for post earthquake evacuation, because the tsunami is going to arrive, it could be in some communities, it's within 15 or 20 minutes. And in those communities, have people are thinking more about vertical evacuation in Cascadia, in a place like Southern Washington, there's actually not enough time to evacuate the area that's going to be inundated. And so people are thinking about doing vertical evacuation. So building towers that people can or identifying buildings that they think are tsunami, I guess, resistant, I don't want to say proof, but a place where everyone can gather on a certain floor of a building or a vertical evacuation towers that are built to withstand inundation. And that's really the only way these communities can reach safety because the tsunami will arrive faster than you can drive or walk or run out of there. In other places in Oregon, are, you know, driving routes or even walking routes to higher ground. And so just being able to, you know, let people come up with a plan, you know, give them the resources and knowledge to do that is a really rewarding part of the job and not focusing so much on just the scaring people part, but more like arming them with some information that they can help themselves with.

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Travis

And thanks to Tina for sharing her expertise at the intersection of subduction zones, earthquakes, and tsunamis. If you or someone you know would make for a great curious conversation, email me at traviskw at vt.edu. I'm Travis Williams, and this has been Virginia Tech's Curious Conversations.