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WHEELER:

Welcome to the Econews Report. I'm your host this week, Tom Wheeler, Executive Director of EPIC. And joining me is my friend and colleague, Scott Greacen, Conservation Director of Friends of the Eel River. Hey, Scott.

GREACEN:

Hi Tom. Overdramatic, Tom.

WHEELER:

Joining us is our friend and repeat show guest, Jay Patton. Hey, Jay. Hi. I'm glad to be here. And Jay, we're going to have you introduce yourself. All right.

PATTON:

I'm an engineering geologist at the California Geological Survey, and I work for the Seismic Hazards Program and the Tsunami Unit.

WHEELER:

All right, so we are going to be talking about the Cascadia Subduction Zone, sea level rise, subsidence, and the big one on today's show. This is inspired by a paper that recently came out, which you can find a synopsis of on the Lost Coast Outpost and in the show notes to this show.

GREACEN:

We could shorthand it as Dura et al., for the author of the paper. The title is "Increased Flood Exposure in the Pacific Northwest Following Earthquake-Driven Subsidence and Sea Level Rise." So it's about that interplay between seismic effects and sea level rise.

WHEELER:

To start the conversation, we should have an understanding of plate tectonics and our own local plates, and the way that everything comes together, and I— Wait, wait.

GREACEN:

I'm not gonna need calculus for this, am I?

WHEELER:

I don't think so.

PATTON:

I hope not.

WHEELER:

People I'm sure have heard about the triple junction and I've heard about the Cascadia subduction zone and the big one. But we probably don't have a great understanding. So Jay, tell us about Humboldt County and the plates of our area and why we are so seismically active here. All right.

PATTON:

So I hope we have about 120 minutes to go over this. No, I'm just joking. Plates are those really thin layers of the earth that move around, and they move around relative to each other. So at their plate boundaries, they could move towards each other. Those are convergent plate boundaries. They can move side by side. Those are transform or strike-slip plate boundaries, and they can move away from each other. Those are called divergent plate boundaries. The Cascadia subduction zone is a convergent plate boundary where oceanic plates are going down into the earth beneath the continental plates. We have the North America continental plate, and here in offshore, we have the Gorda oceanic plate. Those plates, when they're moving side by side, they get stuck, and that earthquake fault gets stuck. And as soon as the stress on that fault is greater than the strength of the fault, that fault slips, and that's what we call an earthquake.

And then to the south of us, from basically Baja, California, all the way up to basically Honeydew, is the San Andreas fault, and that's a strike-slip fault that moves side by side. And then offshore of Cape Mendocino, going westwards, is a Mendocino fault, also a strike-slip fault moving side by side. And we had, on December 5th last year, a strike-slip earthquake on that fault.

GREACEN:

So it's fair to say this is a pretty complicated seismic scenario here.

PATTON:

It is really complicated here, and these plate boundary systems and the faults associated with these plate boundary systems interact in ways that we're still learning about, and there's still lots to learn. But where these three faults meet, it's not where the three plates meet, it's where the three faults meet. So the Cascadia Subduction Zone fault, the Mendocino fault, and the San Andreas fault, where those three fault systems meet, we call the Mendocino Triple Junction. It's really kind of a region, and we're still learning about that. And so we can be exposed to earthquakes along the subduction zone.

The San Andreas fault, for example, the 1906 San Francisco earthquake generated strong ground shaking, landslides, and liquefaction here in Humboldt Bay and the Eel River Valley. The Mendocino fault, we felt that last December. The subduction zone, then there are earthquakes in the Gorda Plate. For example, the 2022, December 2022 earthquake, that was an earthquake within the Gorda Plate, and that was really a strong shaker. Then there can also be earthquakes in the faults in the North America Plate, like the faults that generate lots of the hills around here, the Little Salmon Fault, the Mad River Fault Zone, that has some multiple faults in it.

GREACEN:

Don't let him get too close to the ground, folks. He'll find fault with it. Yeah, right. But to focus in, though, on the Cascadia subduction zone and the earthquakes it generates, that's really the focus of this paper and a lot of other research along the Oregon, Washington, British Columbia coasts, as well as here in California. We are often treated as an afterthought in the stories about these studies, but it seems to me that we ought to be placed really in the center because, as I understand it, those Cascadia subduction zone quakes start here. And sometimes they go all the way north to Seattle and Vancouver Island, but sometimes they don't. But they always hit here if they happen. Is that right?

PATTON:

That's generally the case. If we look at the paleoseismology, which is the study of prehistoric earthquakes, so the paleoseismology data from Cascadia along the entire margin, it goes from here all the way up into offshore Vancouver Island, of the last 10,000 years, there's only been one that we have evidence for so far. We could always make new discoveries, but there's only been one earthquake out of about 44 that ruptured only in the northern part of the margin. For all of those other earthquakes, the southern part of the Cascadia subduction zone appears to have been involved in those earthquakes.

So it is true that most of the Cascadia subduction zone earthquakes include Northern California, but we're not sure exactly where those earthquakes start and stop. So the subduction zone is broken up into sections, and in the southernmost section, we don't know if the earthquake starts in the northern part of the section or the southern part of the section. That's really an academic question and not so important for people, because when you're in earthquake country, you wanna be earthquake prepared. We don't know when the next earthquake's gonna happen. It could be today, it could be tomorrow, it could be 100 years from now. So it's always good to have that go bag and to be prepared.

Let me quickly go over, look, what we might expect during a Cascadia subduction zone earthquake in general. You just can imagine the Gorda plate is stuck with the North America plate. Where that plate is stuck is being pulled downwards. The plates, generally, they behave like elastic, like my waist and my underwear that I need to replace. It wears out, and so when the plate gets pulled down, the conventional wisdom is that during an earthquake, when that plate, that fault slips, it would go up, and that's basically what we find in earthquakes around the world, subduction zone earthquakes around the world. That's also part of our model of when we're looking at the paleoseismology of the Cascadia subduction zone. Lots of the evidence infers what type of motion happens vertically during that earthquake.

GREACEN:

Just say a little bit about what forms those evidence takes. How do scientists make those inferences? Yeah.

PATTON:

So I mentioned when you're being pulled down, you go up. Well, if you go further to the east, the crust isn't being pulled down, it's being flexed upwards. And then during the earthquake in those locations, it goes down during an earthquake. And so you can imagine that the areas that are further to the west, closer to the fault, those are the areas that are being pulled down now that will go up. And the areas further to the east from the fault, those are the areas that are going up now that would go down during an earthquake. The subduction zone fault in Northern California, Humboldt Bay, is really close to us. Whereas up in Oregon, the fault is further offshore.

And in Washington, the fault is further offshore. So in Oregon and Washington, that's where this evidence was first developed to understand what happened during past earthquakes. I'll describe that now. And so imagine you are a happy tree living close to sea level along some bay or estuary. And you're just above high tide. And then a cascade subduction zone earthquake happens and the ground goes down. Now the surface of the earth is below sea level. So your roots are being inundated by saltwater once or twice a day. And you're no longer a happy tree. And in less than a year, you die. That soil that you're rooted into gets buried by mud that comes in with the tide. And over time, more and more mud fills in and the tectonic deformation, you start going back up again. As evidenced by the 1964 earthquake in Alaska, which was a subduction zone earthquake, in about 10 years, we would expect that the ground surface would be close to where it was before the earthquake.

GREACEN:

Just to run that back that 64 quake in Alaska resulted in about two meters of land subsidence in some areas, right? Yeah, five six feet in some places. Yeah, and that's in minutes during the quake Yeah, and that's the kind of motion that the Dura paper is talking about really but then afterwards it took about ten years for that same landscape to spring back

PATTON:

For it to spring back and for mud to fill in and to raise that elevation back to what it was.

GREACEN:

There's an element of just natural processes creating some land there.

PATTON:

Sedimentation and the elastic part of the crust. So that's what Brian Atwater in 1987 published the first paper in Science about the evidence that they found of this co-seismic, that's like cooperate, co-with, so with earthquake co-seismic subsidence. What they found in Central Washington, the coastal of Central Washington, were these buried soils, these soils that had marsh plants growing in them, and they were buried in mud. And because those earthquakes repeat over time, there's basically like a layer cake, layers of these buried soils.

GREACEN:

So freshwater wetlands being replaced by saltwater soils.

PATTON:

Exactly, exactly, and then again and again and again. So that's what established our understanding of what the Cascadia could generate earthquakes. At that time, there was still a debate about whether or not Cascadia could even generate earthquakes. We were not really sure. There was lots of evidence for both hypotheses.

GREACEN:

No Japanese records of tsunamis generated by those quakes, but yeah

PATTON:

That came to play, yeah, we learned about that later. The orphan tsunami that was generated by the last Cascadia subduction zone earthquake in 1700, that led to a tsunami that traveled across the ocean in Japan, because they also have earthquakes and tsunamis. Usually they are associated with each other, but when they get a tsunami without an earthquake, they call it an orphan tsunami. So the 1700 tsunami is one of those orphan tsunamis. And so that was later part of the story that was developed. Yeah, so before that, like there are no earthquakes in Cascadia on the megathrust. We've had 4.9, 4.5 in the mid 2000s in Central Oregon, but basically the megathrust is very quiet. And so there were lots of reasons why we thought that it would not generate earthquakes. But after Brian Atwater and others started finding this evidence along the coast, it became more clear that Cascadia could. Now let's come down to Humboldt County. And so we'll look at, we'll compare and contrast Crescent City with Humboldt Bay.

GREACEN:

They have a prison, we have a university. Yes.

PATTON:

And their prison has a great educational program now. And my neighbor in Manila, she worked for years as an advocate for the prisoners. So I really appreciate her for that. Right now, Crescent City is going up. So Crescent City is going up so much, it's going up faster than sea level rise. So if you look at the tide gauge in Crescent City, the sea level is actually going down. The relative sea level. Yeah, the sea level, if you're sitting there at the coast and observing what the water is doing, the water level is going down over time. Because it's going up, what do you think is going to happen during an earthquake in Crescent City?

GREACEN:

Given the description you just advanced, Tom suggests it's going to go down.

PATTON:

Yeah, and that's what we find. So Eileen Hempel-Haley, who helped develop these tools, these investigation tools, she studied the layers in Crescent City and has found that there's evidence for co-seismic subsidence, lowering of the ground during earthquakes.

GREACEN:

And again suddenly

PATTON:

Lowering the

WHEELER:

So when we talk about sudden, how sudden is sudden? Like it's, is this the ground kind of drops out from under?

GREACEN:

you within the first minutes is what's over a few

PATTON:

Yeah, it takes a few minutes. If we go back in time to the 1964 earthquake, back then, people who were doing what you all do, do radio shows, they would bring around a recorder with them all the time so that they could record themselves for their radio shows. So there was a reporter who was up there at the time, and if one does an internet search for recording, audio recording of the 1964 earthquake, he recorded his observations during the earthquake, and it lasted over five minutes. We can imagine that the slip on the fault is going to take several minutes, and so that lowering is going to take several minutes. In the paper, it talks about after slip. So the fault, most of it slipped during a few minutes, but it could continue slipping for weeks, days to weeks after that, or maybe even longer. That might be difficult to tell from the layers of mud whether or not the layers were generated by the main earthquake or this after slip, because it's still, in geologic time, it's still instantaneous, even though it's weeks or maybe months later, it's still like the evidence you look at later, centuries later, you might not be able to tell.

WHEELER:

Yeah, basically. Yeah. You are listening to the Econews Report. We're talking about the Cascadia subduction zone and land subsidence as a result of the Big One.

GREACEN:

So, Crescent City's going up now, and thus we expect it to go down when a quake hits. Humboldt County, though, is going down now, mostly we think.

PATTON:

Yep, yep, yeah. Based on our work looking at tide gauge data, so Jeff Anderson worked with us. So in 2010, Todd Williams and a group of us, we got some information for the surveyors in Caltrans who had identified a region around Humboldt Bay that was going down and they were confounded by that. So contacted Todd who had been working, had done a study using GPS or now called GNSS data to look at the tectonics. And based on what Caltrans provided us, we developed the Humboldt Bay Vertical Reference System Working Group and we had a meeting in October of 2010 where we brought together over 30 federal, state, local agencies and academia and consultants to look at the existing data at the time so that we might, as a group, identify a problem.

And we indeed, we all looked at the data and concluded that, yeah, it looks like Humboldt Bay is going down. The tide gauge data at the North Spit tide gauge, which is where the Coast Guard Station is out on the southern part of the Samoa Peninsula, showed that sea level was rising faster than the global sea level record. And so what we did was we applied for some money from the U.S. Fish and Wildlife Service and we got some funding to fund this research. And so Jeff Anderson, a collaborator, he deployed some tide gauges. So we deployed tide gauges in historic tide gauge locations. We worked with Reid Burgett, who's now at the Department of Geology and Mineral Industries in Oregon. They're basically Oregon Geological Survey. And then Todd Williams and I looked at the GPS or GNSS data and we found that Humboldt Bay was subsiding really fast at a fast rate. So- Oh, fast. Southern Humboldt, like hooked and slew, it's going down at about five or so millimeters per year. If you look at the...

GREACEN:

is about the same rate as sea level is coming up.

PATTON:

Well, the sea level rise rate in the Northeast Pacific is about one and a half, 1.8 millimeters per year. And so the change in sea level rise is different globally. There are different reasons why the sea level is changing at different rates in different places due to the shape of the earth, due to the thermal expansion that's affecting the sea level rise significantly. So there are lots of reasons, but basically what we discovered at that point, it was the highest rate of sea level rise on the West Coast. Since then, we've identified other people. We, the Royal We, have identified some other places where there's really high sea level rise rates because the ground is going down.

But at the time, that was the first place that we identified this really large sea level rate. So yes, so Humboldt Bay, like Mad River Slough is going down and it varies within the bay. It's about two millimeters a year in Mad River Slough, the mouth of Mad River Slough where Samoa Boulevard crosses the slough there, just West of Arcata, the North Spit. It's in between Hookton Slough data in Mad River Slough. So let's step back and look sort of like at a global view of subduction zones. And back to our concept that we talked about of the elastic nature of the crust. And so as you alluded to, because it's going down, one might expect that the Humboldt Bay would go up during a cascaded subduction zone earthquake.

Now, there are a few cases around the world where the earth was going down before the earthquake and went down during the earthquake. And one of those places was in 2011, the Great East Japan, the Tohoku-Oki earthquake that generated a tsunami that came here, that the East Coast of Japan was subsiding before the earthquake and it subsided during the earthquake. There are some differences between our plate boundary and the plate boundary there. Here, the Gorda Plate is maybe about 5 million years old where it's beneath us, 5 to 7 million years old, whereas it's 80 to 90 million years old in the Northwest Pacific where it's subducting beneath Japan. Japan had a really high slip, so there are estimates that the amount that the fault slipped during that earthquake was over 100 meters, which is the highest slip that we've ever observed.

Now, we've only been observing earthquakes for a very short time. And so there are lots of reasons why that that happened there. But conventionally, the places that you're over where the fault is locked, those areas are going to go down between earthquakes and up during earthquakes. And there's a whole bunch of other geologic information that supports that hypothesis. Then on the other side, we have, because Gary Carver was colleagues with Brian Atwater, Gary Carver was a professor at Humboldt State, a geologist, studied earthquakes. And he, in the late 80s, he saw what Brian was doing and so he went out with his tools to go study mud and he collected cores along Mad River Slough and he found this same type of data that Brian Atwater had found, these layers of marsh soils that got buried by tidal muds.

GREACEN:

And there's been similar work on the mouth of the Eel.

PATTON:

Yes, yes, Wenhao Li, who was one of Gary Carver's students, finished his master's thesis in 1999, and he collected the most cores of any master's student at Humboldt, and he used little critters called foraminifera to study how much the ground went down. Foraminifera are single-celled organisms that live in the water, and just like Eileen Hempel-Haley, she used diatoms that are also single-celled organisms. And these organisms live in restricted zones based on basically salinity, and so there are different critters that live in the tidal mud than live in the freshwater wetlands. And so you can compare the types of critters that live in the soil versus the mud above the soil, and you can make an estimate of how much the ground went down based on the species distribution of those critters that you find in the mud.

GREACEN:

I guess I want to take a crack at summarizing the import of the Dura paper that sparked us into this conversation, which is, as I get it, that the consequence of the seismic uplift and subsidence could well be to dramatically expand the flooding zone right after an earthquake, especially in places like Humboldt, where we've already got a significantly growing zone that's subject to flooding. They call it the 1% zone. And I guess I want to hear from you, Jay, a little more about how subsidence and the actual, like, the physical movement of subsidence during a quake affects, if we know anything about that, how the water moves in a tsunami.

PATTON:

Okay, great. So this is a fantastic paper. Tina and all of all of their co-authors are world-class geologists who have been studying Cascadia and other subduction zones. Some of these people have been, like me, been studying these subduction zones for over, for about 30 years. One of the main findings that they present is that when the ground goes down, it subsides co-seismically. If you were sitting there at the coast, just like the tree saw that the water level went up. If you were right at the edge of high tide before the earthquake, after the earthquake, high tide is going to go further inland.

GREACEN:

So if your house in King Salmon's at high tide now...

PATTON:

You'll be below high tide after the earthquake if that's what happens. And we don't know what's gonna happen in Humboldt Bay. It may go up, it may go down. This is one of the most important unanswered questions that we have yet to really resolve is whether or not Humboldt Bay will go up or go down. But if it goes down, the high tide will reach further inland. Just like sea level rise, it's just basically the same type of effect that we would get over that might take centuries of sea level rise would happen in a few minutes. And speaking of sea level rise, that was another thing that they included was how will the tsunami hazard areas change with sea level rise.

And that's something that we haven't done yet, but I got some funding from the Office of Public Research from the state of California. And I'm working with my collaborator, Pat Lynette, at the University of Southern California. We're looking at the changes in the impact of tsunami hazards to the built environment based on sea level rise. And we're looking at one half one and two meters sea level rise, and we're applying probabilistic tsunami sources. So that probabilistic means like how probable things might happen. So we've heard of the hundred year flood. Well, we have a hundred year tsunami, a thousand year tsunami, 3,000 year tsunami model. So we're applying those different models at those different sea level rise amounts. Within the next year, I'll be happy to come back and tell us all about what we found.

GREACEN:

Tom, you have to describe Jay's t-shirt.

WHEELER:

Well, it's the coolest t-shirt I think I've ever seen. It is the 1992 seismograph as recorded at Humboldt State University, showing the earthquake and all of the aftershocks. And Jay, where can I get that damn shirt?

PATTON:

So Lori Dengler prepared this shirt after the 1992 Cape Mendocino earthquake. The geology department has these, well it's the Redwood Coast Tsunami Work Group and you can donate to the Redwood Coast Tsunami Work Group and get one of these shirts of your very own.

WHEELER:

That's super fly. So one of the questions since, since I have you here, people will often say, Oh, I feel like we're due for the big one. It's been X number of years. It seems like maybe there's some sort of a pattern.

GREACEN:

Because the paper says we've got a 15% chance now,

WHEELER:

do we, how do we know how, how freaked out should we be that we here in this room are going to experience the quote unquote, big one in our lifetimes?

PATTON:

We should not be freaked out about anything.

WHEELER:

You should not be freaked out about it, because it sounds very freaky, because we're talking about minutes of shaking here.

PATTON:

Yeah, and the reason why we shouldn't be freaked out is because we all know what to do during earthquake drop, cover and hold on. And if you're in the tsunami hazard area, get to high ground as soon as you can. So with that information, most of us are going to do quite well during that earthquake. The reason why we shouldn't be freaked out is because we never know when it's going to happen, but we know what to do. There's a whole bunch of other stuff. If you go to the internet and search for living on shaky ground at the Redwood Coast Tsunami Work Group, you can get this brochure that will help you learn how to be more resilient during earthquakes. It may be that it's been a long time, but it may be a long time until it happens. It may not happen during our lifetimes, but everyone could be prepared.

WHEELER:

The one thing that Lori Dengler said after the 2022 earthquake she came on the show was that the violence of that earthquake was probably going to be more violent than a subduction zone earthquake in terms of like the sheer gravitational change in that earthquake. That a subduction zone earthquake will be a lot of energy released, but it's going to be released over a lot longer of a time. So at least prepare me, what can I know that I'm going to experience during this big one?

PATTON:

Yeah, Laurie's correct that that 2022 earthquake shook really strongly. In fact, the Strong Motion Instrument Program that is at the California Geological Survey, most of the seismometers, USGS, you look up the earthquakes in California, come from the Strong Motion Instrument Program, but there are other networks that contribute as well. That recorded the SMIPP, Strong Motion Instrument Program, that earthquake generated the strongest ground shaking ever recorded by the Strong Motion Instrument Program. The difference between an earthquake like that, that ruptured the oceanic crust, which is really strong, it's made out of a rock called peridotite, which is really strong, generate what we call large stress drops.

The stress drop is how much stress is released over time. And so it released lots of stress and so generated strong ground shaking. It was high frequency, so the ground went back really fast. Whereas a subduction zone generates strong ground shaking, but lower frequency, longer period shaking, and it lasts longer. The shaking is still going to be violent and strong, but probably won't be as intense as the 2022 earthquake, but it will last quite a bit longer, five minutes or more. I hope that helps.

WHEELER:

Yeah, well, I guess that helps.

PATTON:

You know what, Living on Shaky Ground is a really great tool and a very little bit of information goes a long way to not only feel better about what you're doing, but also to actually be better.

WHEELER:

You'll find a link to Living on Shaky Ground on the show notes on thelostcoastoutpost.com. Check it out there. Jay Patton, thank you so much for joining the Econews Report. Thanks for having me. Thanks, Jay. Join us next week on This Time and Channel for more environmental news from the North Coast of California.