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TOM WHEELER:

Welcome to the Econews Report. I'm your host this week, Tom Wheeler, executive director of EPIC, the Environmental Protection Information Center. Joining me is my friend and co-host Scott Greacen of Friends of the Eel River. Hey Scott. And we are talking about earthquakes, which is probably something that y'all in the listening audience are thinking a lot about after the most recent roller that we went through.

And we have a fantastic guest to help us understand earthquakes and what to do in the case of an earthquake and what our future earthquakes might look like. And that is Jay Patton, adjunct professor of geology at Cal Poly Humboldt. Hey Jay, how are you?

JAY PATTON:

Hi, Tom. Great. I'm glad to be here.

WHEELER:

Well, so we recently had a nice size roller. It was at least big enough to alarm us, to get us to remember that earthquakes are part of the natural environment here in the North Coast. Hopefully we can learn from this experience and become better prepared just in case something bigger than this last one comes around.

So Jay, do you want to go through and tell some of the important statistics or talk to us about this most recent earthquake from a geologist's perspective?

PATTON:

Sure. So we live in earthquake country, as everyone probably already knows. That's because we live near a whole bunch of different sources of earthquakes. We have different types of plate boundaries that all meet in the Cape Mendocino area in a region called the Mendocino Triple Junction, and that's because it's where three major plate boundaries meet. The Cascadia subduction zone, which is a convergent plate boundary where plates move towards each other. The San Andreas Fault, which is called a transform plate boundary where plates move side by side during earthquakes called strike-slip earthquakes. And the Mendocino Fault, which is also a strike-slip or transform plate boundary.

The earthquake that we had last week, the magnitude 7.0 earthquake happened. And so each of those plate boundaries are potential sources of earthquakes. But we can also get earthquakes on faults within the plates. So we have the Gorda plate, the oceanic plate that is diving beneath the North America plate, Cascadia subduction zone. And so there are faults within the Gorda plate that can generate earthquakes like we had in 2005, 2010, and 2014. On December 20th, 2022, that was a Gorda plate earthquake, even though it was deep beneath Ferndale. Then we could also have faults within the North America plate that can generate strong ground shaking. And we haven't had any large North America plate earthquakes in a while. The 1992 Cape Mendocino earthquake, 7.2, was a North America plate earthquake, which generated a tsunami.

The earthquake we had last week, the magnitude 7, we've had similar earthquakes of that magnitude on that fault. For example, in 1994, there was a magnitude, now a magnitude 7.1 earthquake further to the west of our earthquake from last week. It sort of fits preconceived ideas about that earthquake fault, what type of earthquake and what size of an earthquake can happen.

Basically, so for a strike-slip fault earthquake, it's usually a fault that's sort of straight, and the crust moves on either side of the fault, side by side. Yeah, side by side. And basically, we don't think that those earthquakes generate very large tsunami like a subduction zone earthquake might generate, because the subduction zone, which generate what are called thrust or reverse earthquakes, they cause the seafloor to move vertically. And when that seafloor moves up, it lifts the seawater up, and then the seawater is above sea level and potential, and wants to fall back down. So it falls back down, and then with momentum, it falls below sea level. And then that water goes up and down, just like as you're splish-splashing in a bathtub, that water goes up and down. And that oscillation is what generates the energy for a tsunami to radiate outwards.

So for a strike-slip fault earthquake, the crust isn't generally moving up and down much, it's just moving side by side. So when tsunami are generated, and the quake did generate a tsunami, they are usually much, much smaller than subduction zone tsunami.

WHEELER:

So Jay, a lot of people have said to me that this was a 7.0, that it kind of surprised them that this was a 7.0, that they've been through a lot of earthquakes and this didn't feel like that significant of an earthquake, especially in comparison to the 2022 December earthquake, which was of a smaller magnitude. So I think that this leads me to a question about what is magnitude and what are we actually measuring here when we're talking about magnitude? Because it doesn't seem like it sometimes lines up with our lived experience.

PATTON:

So the earthquake magnitude is basically the amount of energy released during an earthquake. And that's calculated by multiplying the area of the fault that slipped by the amount that the fault slipped by something about the elastic properties of the crust. But basically the amount of the fault that slips, the area, times the amount that it slipped. And that doesn't change no matter where you are. The magnitude of the earthquake is the magnitude of the earthquake. Now in terms of intensity, which is what we feel during an earthquake, I just have a question for you. There was an earthquake in Nevada yesterday. Did you feel that earthquake?

WHEELER:

I did not.

PATTON:

Why don't you think you felt that earthquake?

SCOTT GREACEN:

Well, that is some distance off.

PATTON:

That's right, you were too far away to feel that earthquake. So the intensity, what you feel during the earthquake, gets smaller further away you are from the earthquake. So that's really important to recognize. The 7.0 earthquake was further offshore, further away from people, so that had something to do with how the ground shaking, the intensity was lower.

But also the 6.4 earthquake was really interesting. Looking at the seismometers that recorded the ground shaking, the intensity, in Ferndale and Fortuna and Rio Dell, had really high measurements of ground shaking. One of the ways that we measure ground shaking is in the units of g, or gravity, it's called a gal, it's a measure of gravity. So at the earth's surface, at sea level, the amount of gravity is one g. So if an earthquake generates ground shaking of more than one g, that's what can make things lift up in the air. If you have ground shaking more than one g vertically, your house can get lifted up off of its foundation if it's not attached to the foundation.

And that happened during the December 2022 earthquake. There were houses in Rio Dell that were lifted off of their foundations. Those three locations, Ferndale, Fortuna and Rio Dell, all recorded acceleration of greater than one g. I think Rio Del recorded 1.42 g, which is the highest ground acceleration ever recorded by the Strong Motion Instrument Program from the California Geological Survey. So these are really strong. Ground shaking recorded by this magnitude 7 was closer to 0.3 g, so much smaller.

Now why was the intensity different? The 2022 magnitude 6.4 earthquake ruptured the Gorda crust. It's composed of a rock called peridotite, which is a really strong rock. Whereas the magnitude 7.0 earthquake on the Mendocino Fault, on one side of the fault we have the Gorda plate, which is this peridotite composed of peridotite plus other rocks like basalt. But on the other side of the fault, on the south side of this east-west oriented fault, on the south side of the Mendocino Fault, is an ancient subduction zone where the rocks are sedimentary rocks that are not as strong as the rocks and Gorda crust. So it may be, now I'm just hypothesizing here, I don't know because there's no movie that takes us down into the Mendocino Fault to see what's going on down there. But my estimate is that this ground shaking was less because on one side of the fault are sedimentary rocks instead of this really strong peridotite.

GREACEN:

So Jay, you explained a little while ago that you calculate magnitude by looking at the length and the depth of the rupture multiplied by this factor for the flexibility of the crust. So it sounds like that's the issue here. Well, the magnitude... How variable is it different between these two earthquakes?

PATTON:

I was talking about the difference in the shaking intensity, so the magnitude isn't affected by that.

GREACEN:

But it's that that variable you were talking about. Yeah.

PATTON:

Yeah, so the properties of the crust on the south side of the Mendocino Fault, so that appears to have affected the shaking intensity, and it could affect the magnitude estimate. I'm actually not exactly sure how they come up with the magnitude estimate, how the USGS determines that, but there are lots of other ways. I just described a basic way of calculating the magnitude, so I'm not exactly sure how they characterize the elastic modulus, so that's the parameter, the modulus, it's an elastic modulus, and so I'm sure that the crust on the south side of the Mendocino Fault has a different elastic modulus as the Gorda Plate does.

GREACEN:

So all this leads me to an observation. Tom, you used the word roller a couple times when we started this show, and it leads me to think a little bit about the different words we have for quakes and how some of them are kind of descriptive, like a roller, but like the 2022 quake was more of a bouncer. Anyway, I'm thinking like in the spirit of the observation that the people of the North have dozens of words for snow, that we might usefully explore how to more precisely express the nature of these things in ways that are a little more accessible than maybe the technical terms, because I'm not sure we're going to hear on the street in Eureka, well, that sure felt like a strike slip to me.

PATTON:

Yeah, I think people just use the words that come to their minds about how they felt it. Yeah, and those words are qualitative. I'm not sure we'll ever really get a good sort of dictionary set of words that are the best words to use.

WHEELER:

So in reading the comment section, I saw someone say something along the lines of after this earthquake happened and they got the tsunami warning that they knew it was all BS because they've been through so many earthquakes before. And they felt that based on the, the rumbling that they experienced that this couldn't have produced a tsunami. That doesn't sound like that really is good science. Can you talk to us about what can cause a tsunami? I know we talked about the types of earthquakes, strike slips versus subduction, but a 7.0 itself is capable of producing a tsunami, right? Yes.

PATTON:

Yes. Well, living on the North Coast here, I moved here in 1991, so I'm a newcomer, but I've felt lots of earthquakes. We come to think that we, having felt all these earthquakes, we here on the North Coast are exposed to more earthquakes than anywhere else in the state of California.

The most earthquakes that are sensible, that we can feel. It's possible that that might mislead us a little bit because we haven't had Cascadia Subduction Zone earthquake in over 300 years. We may not have the totality of experience that we think we might have. I would caution people to try not to be too bold in their conclusions. But I understand what that person who wrote that comment on social media, I understand what they're coming from because it didn't feel that strong.

I live in Manila, I live in the sand dunes, and it lasted a long time. I was in a meeting and I told everyone, hey, hey everyone, I got to drop, cover and hold on. I went under my desk because that's what we all should do. And it was scary because I didn't know how long it was going to last. As soon as I got under the desk, the ground shaking got more intense. Like, oh, I hope this is not a Cascadia subduction zone earthquake. But for a 7.0, they definitely can generate tsunami.

And so the National Tsunami Warning Center, who is the authoritative source on tsunami alerts and warnings and advisories and watches for the West Coast of the U.S., they have about between five and 10 minutes to analyze the earthquake and to generate any notifications that they have to send out. This happens before the location of the earthquake is really certain, before we really know what type of an earthquake it is. They are under a lot of pressure to get this information out for people to, so the emergency managers, the first responders, the sheriff's departments, the police departments, that they can all start taking action.

WHEELER:

You are listening to the Econews Report, and we're talking about earthquakes.

GREACEN:

Why is that time pressure so intense?

PATTON:

Those are the procedures we have we need to get how long...

GREACEN:

Will it take a tsunami to hit the coast from a quake possible location we're looking at here?

PATTON:

So, that's more of a more complicated question because well because there's a difference between local tsunami and distant tsunami. Okay. And these notifications from the national tsunami warning center that gets sent out if it's a there are there are four levels -- there's a notification which is the lowest level there is a watch there's an advisory and there's a warning which is the highest level. The warning which means there may be tsunami greater than one meter or one yard in size those go to everyone's cell phones through what's called a wireless emergency alert the WEA. And so everyone in the county if if your county is in a warning everyone in the county gets a WEA notification.

So everyone in in Humboldt County got a WEA notification when that when that happened. Now here in Humboldt County it was a local event for us, whereas in San Francisco it was a distant event for them. And these and these alerts from the NTWC these are for distant events -- distant tsunami -- but it was a local tsunami for us. So for a local tsunami people need to people respond to the natural warning and the natural warning is the earthquake or if you see the water retreating from the beach. These different natural warnings that we have, whereas the official warnings those are our cell phone alerts the WEA notifications those are the U.S. Coast Guard or the Civil Air Patrol flying over with loud speakers telling people to evacuate from the beach or the sirens going off or the TV and radio notifications NOAA weather radio. These are all official warnings not natural warnings.

So here we felt the earthquake and what people need to do when they feel an earthquake that lasts unusually long and however unusually long that's up to you to decide might be 20 seconds 30 seconds. We don't really want to put a number there so we just say if it feels unusually long and you're in a tsunami hazard area the THA which you can look up at tsunami.ca.gov or you can go to the Redwood Coast Tsunami Work Group web page and look at the tsunami evacuation maps. If you're in the yellow part of those maps the THA maps you're in the tsunami hazard area and you feel an earthquake that's unusually long you should drop cover and hold and then as soon as you can walk you should walk outside of the THA get to high ground, and it's up to you as an individual to respond to a natural warning.

However because people the cell phones worked the earthquake wasn't big enough that it wiped out all of our towers, people got those WEA alerts and so they so they got both the local and the distant experience and so that was really complicated and that's something that was new for this like I had thought like in my mind like okay well but we hadn't really discussed how to address the difference between a local and a distant.

GREACEN:

And you make a really important point, though, Jay, I think, that in the event of a Cascadia subduction zone quake, which many of us see as not if but when, we're likely to lose the cell phone network in at least some areas. And so we don't want to become conditioned to this idea that if I don't get a cell alert, it's not an issue. If it feels like a big quake, it feels like a big quake.

PATTON:

Yeah, and so that's why, like, if it shakes strongly for an unusual amount of time, everyone, it's up to you. You're the one who can protect yourself and get to high ground. So back to magnitude 7.0 earthquake, is it going to make a tsunami or not? Well, in 1992, the Cape Mendocino earthquake, 1992, a magnitude 7.2 earthquake, that was not a strike-slip-fault earthquake. It was a reverse or it was a compressional type of an earthquake in the Petrolia area near Cape Mendocino, and it actually generated a small tsunami that was recorded up and down the coast. It was recorded in Oregon. It was recorded all along California. It was even recorded in Hawaii. Now, it was not a large tsunami. It didn't generate flooding on land, but it probably did generate strong currents in the water. So even though the tsunami may not inundate the land, well, you never want to go and try to surf a tsunami.

In 2022, in January 2022, we had a tsunami generated by a volcanic eruption in Tonga in the southwest Pacific, and people in the Santa Cruz area, Monterey area, they went surfing to surf the tsunami, and they had to be rescued. So first responders, lifeguards, had put their own lives at risk to rescue these surfers who got stuck out in the water because it turned out that they probably shouldn't have actually done that.

But also, another possibility is that a magnitude 7.0 earthquake, that could trigger a submarine landslide, and that submarine landslide, now, no one's going to feel the landslide, but that submarine landslide could generate a really large tsunami. Now, it's going to be localized, so it's possible that only Humboldt Bay and Trinidad and Shelter Cove would have gotten impacted by a submarine landslide. San Francisco would not have been impacted by a submarine landslide associated with this earthquake, but we just don't know. And so, it's foolish to think that just because you haven't seen a tsunami from a 7.0 earthquake, to think that it wouldn't cause a tsunami that could be dangerous. This 7.0 earthquake did generate a small tsunami that was recorded at the Arena Cove. There's a tide gauge at Arena Cove, which is by Point Arena, and it did generate a small 10 centimeter size tsunami.

Now, that's not a dangerous tsunami on shore. It didn't generate any inundation, but it does show that an earthquake, a strike-slip earthquake, can generate a tsunami.

GREACEN:

Right after the earthquake, PG&E camera up on Kneeland got a striking image of Humboldt Bay that many people will have seen that appeared to show the bay emptying out, essentially.

PATTON:

Yeah, this was a really fascinating observation. I was actually paying a contractor who was fixing up a house that I'm selling, and they mentioned that they live in the Arcata Bottom, and they mentioned their friend who was up on Fickle Hill, I think, who had made that same observation.

And at first, in my mind's eye, I looked back at the tied gauge from the North Spit tide gauge, which is located where the Coast Guard station is out on the South Peninsula. And it's kind of difficult to say whether or not there's a tsunami in that tide gauge record. The waves before and after the earthquake, they're not any bigger. So most people don't think that a tsunami was recorded there.

Yet, we have video and photo evidence from many different sources, including Bob McPherson, who lives in Bayside. He got some really good views of Arcata Bay, emptying out and filling back in. This is a really interesting observation, because the water surface elevation, so the elevation of the top of the water, went up and down several feet in Arcata Bay. But if you look at the North Spit tie gauge, it doesn't go up and down several feet.

So this is a great mystery that we're working on. And what we really need are observations, if anyone has any observations of the South Bay, of what was going on in the South Bay. Was the South Bay going up and down? And what this will help us learn is what phenomena, what happened. So our leading hypothesis, we don't know if we're correct or not, is that this was called a seiche, S-E-I-C-H-E, a seiche. And a seiche is basically when the water within a container, like the Humboldt Bay or your bathtub, when it sloshes back and forth with a certain resonance. So I'm sure everyone who can do a bathtub in the water, you can move your body back and forth at the same frequency as the water in the tub. Your body is in resonance, is resonating with that water. What can happen is the ground motions from an earthquake, the surface waves, the seismic waves from an earthquake, if the frequency of those seismic waves relates to the frequency of the body of water, those seismic waves can set up a resonance. And then that water body can slosh around and resonate at that frequency. And so this can also happen with wind. Wind shear can generate seiche.

But lots of us are familiar with earthquake generated seiche. In fact, my boss to the agency that I work with, he lives south of Sacramento and his pool in his backyard was sloshing around. And that was a seiche. And so there are lots of reports of people in the Sacramento area who had pools with the water sloshing around. Those are seiches. And we think that what happened Humboldt Bay may have been a seiche. But we don't know. And if we had observations in the South Bay, and if the water was going up in the South Bay when the water was going down in the North Bay, or the opposite was happening, that could help us understand if this was a seiche or not. But if they were both going up and down, then maybe the water was going in and out the mouth. We just need more observations to really figure out what was going on. It was really, really strange.

GREACEN:

Salmon residents. Paging Salmon residents.

PATTON:

Exactly. King Salmon. Yep. College of the Redwoods. Table Bluff. We need your observations.

WHEELER:

So I think it'd be foolish for us not to cover some basic ground rules for keeping ourselves safe in the event of an earthquake. And we've covered a couple of those already. So let's bring those back together. The takeaways, what are, what should we do when we start to experience an earthquake?

PATTON:

Well, the first thing to do is to remember to keep safe. And usually that means stay where you are. The moniker is drop, cover, and hold on. And if you do an internet search for living on shaky ground, the Redwood Coast Tsunami Work Group has a brochure that's online. And you can download a PDF, and you can get a hard copy. And there's lots of information about there about how to prepare your house, how to prepare yourself for earthquakes. And drop, covering, and holding on, that's how we all want to respond to an earthquake. Because you never know if it's going to get bigger. Just drop, cover, and hold on. And basically what you need to do is get underneath a desk and cover your neck, and then wait until the shaking stops.

In the Living on Shaky Ground website, you can find other information. For example, if you're in a wheelchair, what to do. Because you can still stay safe in a wheelchair. There are lots of other strategies for people who can't dive underneath their desk or a table. And then on the North Coast, if you're in a tsunami hazard area and the ground shakings for unusual amount of time, you drop, cover, and hold on. And as soon as you can, evacuate to high ground. Because for a Cascadia Subduction Zone tsunami, the tsunami might arrive at the coast while the ground is still shaking. It may take 5 or 10 minutes, but it might arrive at the coast while the ground is still shaking. So we really need to take care of ourselves.

You now are an expert on what to do. Drop, cover, and hold on. And then evacuate to high ground as soon as you can. So those are the two main things. There are lots of other details. Like if you're in bed, stay in bed. Don't go to a door. That's a myth. Like a long time ago, the door was the strongest part of the house. But now houses are built that they're strong everywhere. And so don't go to a door frame, because the door could slam shut and hurt you. Get underneath a table or desk. A brochure can help people learn more.

WHEELER:

All right, Jay, thank you so much for joining the EcoNews Repoert. I have certainly learned a ton in this episode and I hope our listeners did too. And join us again next week on this time and channel for more environmental news from the North Coast of California.