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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. And joining me is my colleague and co-host Jen Kalt, director of Humboldt Baykeeper. Hey, Jen.

And we are talking about earthquakes today. Joining us is our queen of earthquakes, professor emeritus of geology at Cal Poly Humboldt, Lori Dengler. Hey Lori, I'm so thrilled to have you on today's show. I, I think that if you live in Humboldt, the earthquake of December 20th really was quite startling and you probably have a lot of earthquake questions and I hope that we will answer them in today's show if people have been living under a rock or we have listeners from across the United States, Lori, do you want to talk to us about the December 20th earthquake? Just kind of give us some rough facts about it, and then we can drill into it.

LORI DENGLER:

First of all, we have to be careful in Humboldt County when we say a December 20 earthquake because we had one in 2021 as well as the one this year. In fact, we are in the most seismically active region of the lower 48. Nearly 46% of the entire earthquake energy budget in the co terminus us comes from Humboldt County and the adjacent offshore area.

So what happened this year's December 21st was not unprecedented, but for many people it was more than just a wake up call. It was truly frightening. The earthquake originated on a fault just offshore of false Cape and then that rupture proceeded to grow to the east north east actually towards us. It ruptured approximately 10-12 miles. It didn't break the surface. That rupture was entirely beneath the surface and when it was done it came up with a magnitude of 6.4. The earthquake was felt very, very strongly in southern Humboldt, Fortuna and certainly felt more strongly by many people in the Humboldt Bay up through McKinleyville area than any recent historic earthquake. I've talked to many people who said that was the strongest earthquake I've ever felt. Well, they may have forgotten 1980 or 1992 because we tend to forget those things.

But certainly it was very strong and the fact that it occurred at 2:34 a.m. exacerbated the effects, let's say the dramatic effects. If you're shooting a movie to make that happen in the dark when you're sound asleep is definitely much more frightening than if it's happening on a nice sunny afternoon and you happen to be outside. So those are the sort of bare facts of the earthquake. We have a number of different instruments that recorded it. We have seismographs and we also have strong motion instruments and the strong motion instruments are particularly important. They measure acceleration and when it comes to earthquakes, acceleration is the important thing, not magnitude, certainly epicenter is important, but acceleration is really, really important.

So to understand what we mean when we're talking about acceleration: You're sitting in your car at a dead standstill, you very slowly put on the gas and it takes you an hour to get to 60 mph. Nobody in that car is going to feel any forces on them at all. It's going to be so slow if you have anything kind of sitting on your windshield, even on the top of the car, if you've left your purse on the top of the car, your phone on the top of the car, it's still going to be there when you get up to whatever speed you're going to. Let's now do that exercise again and go from 0 to 60 in one second. That's going to slam you against the back of your seat, bye-bye cell phone on top of the car. You're being hit with the force because force is mass times acceleration, not just how fast you're going, it's how quickly the rate of change of the motion is.

Now, earthquakes can produce some pretty amazing accelerations. Obviously the ground is at zero and then it very quickly will accelerate upwards and then get back to zero or accelerate Side words, we have both vertical and horizontal motions the faster it goes from zero to some peak speed and then stops and goes in the other direction. The more force is going to exert, We can measure that in terms of the acceleration of gravity or one G. Everyone is familiar that you happen to be a jet pilot and you experience six or seven gs acceleration as you're diving around. That might knock you out because that's really hard on your brain. The same thing happens in earthquakes. If we have accelerations that are approaching one G, that's making you weightless, that's going to make you nauseous, that really is going to affect you. The sort of level at which people can experience an earthquake is way down at about five hundreds of a G. The earthquakes that we often feel in Humboldt County and people say, oh well that was kind of fun. Those are about five hundredths of a G. By the time you get up to about two-tenths of a G, that's when we start seeing damage to structures. Large earthquakes rarely produce accelerations much above about half a G to two-thirds of a G. That's sort of the standard peak acceleration for a big earthquake.

The earthquake that happened on December 20th at 2:34 a.m. produced a whopping 1.46 G in Rio Dell. It was just under one G in Fortuna. Those are huge numbers and if you look at the all-time list of largest accelerations in California earthquakes, we find number one is 1992 the station at Cape Mendocino that experience over two G for just a spike. Again to give you a feel for what that means. There was a caterpillar D nine tractor trailer that was very close to where that acceleration station was. It had been sitting there all winter mired in mud up to its hubcaps, that earthquake caused that cat to bounce up into the air, move over and come back down about a foot away from where it had been before, without leaving a scratch in the mud. That is access Celebration in excess of one G. You're throwing things up into the air. Do you look in the annals of old earthquakes? There's one in the 1870s in the Owens Valley which knocked every mule off its feet. That's a strong earthquake.

WHEELER:

I know folks are familiar with the Richter scale acceleration. Does it have a name in regards to earthquakes? Is there any other scale to measure them that people could look this up?

DENGLER:

Its acceleration? P. G. -- a peak gravitational acceleration is the term. We actually don't use Richter scale anymore. We haven't used it since the 1970s, folks that still use Richter scale are the media. It's kind of interesting. We've managed to educate almost everybody else, but the media still insists on throwing out Richter. Richter's idea was great. It was in the 19 thirties 1935 act. Actually when he proposed that you could look at the biggest wiggle wood Anderson seismograph long obsolete and convert that into a number that roughly went from 1 to 10. It's logorhythmic. It's open ended. It was a great idea. We have improved upon it as we've gotten much newer seismic instruments and a much better way to handle the actual physical dimensions of an earthquake. Now, I just prefer to call it magnitude. Forget Richter.

The number scale still roughly corresponds to how Charles Richter defined it back in the thirties, a magnitude three earthquake is still going to be little. If you're close to it, you'll feel a little short jiggle. A magnitude eight earthquake is probably going to do damage if you're near it. We now have negative earthquakes. We have instruments that can detect a minus 0.5 tiny earthquake about the dimension of my desk. The really large earthquakes which the old Richter scale could not handle at all. It really saturated out at about 7.3. The only way you can really measure the really big earthquakes is with our new newer magnitude scale getting up into the nines. So magnitude is a measure of the total energy release By the source. It's not telling you anything about how that energy was released. So, we have some earthquakes that are huge. That may have 40, 50, 60, 100 mile long, false. Yet the rupture is very slow. People don't feel it very strongly.

An example of that was in September 1992. There was an earthquake. I think the magnitude was in the eight range in Nicaragua. The rupture was so slow that many people didn't feel it at all. And people who did feel it felt it very, very rolling unfortunately, that earthquake ruptured beneath the ocean and it produced a very large tsunami, There were many deaths. There was no damage due to the earthquake shaking, but the tsunami was very large, tsunamis are caused by the displacement of the sea floor.

So it's perhaps the most important lesson of our 20 December 2022 earthquake is don't confuse magnitude with the damage potential or how strongly you feel it. The fact that it was only a 6.4 says nothing about its real capacity to do damage. Now. 6.4 means that it's going to be contained the day damage is going to be concentrated very close to the source. Had it been a magnitude eight with those kinds of accelerations we would have seen damaged like that over hundreds of miles. But what you experienced, especially in southern Humboldt, was as strong an earthquake as you will probably ever feel,

WHEELER:

Wow. Well, that's incredible. Rio Dell obviously suffered what seems like the largest damage. It was not the closest jurisdiction to the epicenter of the earthquake. How does that work? Why is Rio Dell feeling this so much more significantly than Ferndale, which was some miles closer.

DENGLER:

Also Petrolia, where it was very weak, even though the map may show it as a single spot that's only telling you where the rupture began. A better way to think about an earthquake is imagine a rock hit your windshield, that's the epicenter and then the crack proceeds to grow all the way across your windshield. Or maybe it proceeds to grow in two directions because that sometimes happens with earthquakes. Sometimes the rupture starts at one end, sometimes it starts in the middle, but it always starts at one spot beneath the Earth's surface. In this case it was about 12, 13 miles beneath the surface and then it proceeded to grow primarily east northeast along a line that sort of took it towards Hydesville -- started at False Cape, then headed towards Hydesville. As the earthquake is rupturing, it's producing those P waves and especially the S waves the whole whole time. When you crack a stick, you'll hear that snap and that snap is the sound waves that that snap has kicked in the air.

Well, in rock we have both the sound waves -- we call those p waves. You can think of them as pressure waves or primary waves. Rock also resist shearing. And so when it breaks, we also get an S. Wave, a secondary wave, that is a side-to-side shearing kind of motion. We don't get that in the air because air has has no resistance to shearing. If it did, I would truly always be double speaking you because you first hear the audible p wave and then you'd hear the secondary wave and boy, our brains would have evolved differently. But in the Earth we always have those two types.

And then we also have the slower surface waves. They don't tend to do nearly as much damage as the rupture is breaking, It's pumping out the p wave that's pumping out the S waves. What you feel at any particular place is a function of how close you are. Not to the epicenter but to the fault. If you look at where the fault is, although the epicenter is further away from Rio Dell, if you look at the projection of the fault, it actually comes pretty close to Rio Dell because it's sort of heading in that direction. But there are other things to the angle you are relative to the fault. When that fault breaks, you tend to get the maximum, the strongest shear waves in the direction perpendicular to the fault or along it. And then there are areas in between where the shaking is much less. You also enhance the shaking depending on if the rupture is moving towards you, which it certainly was in Rio Dell or moving away from you.

It's the Doppler effect. If you've ever heard a train or even a large truck as it's coming towards you, the sound gets higher and higher and higher. And if they're moving away from you it gets lower and lower and lower. That frequency. That sound is telling you about how closely packed those waves are or how stretched out they are for single family houses. It's those closely packed higher frequency waves that really are susceptible to the earthquake motion.

And then there's also the geology. Now it turns out Rio Dell is in a place on the edge of the Eel River basin, big sediments up against bedrock, we tend to get amplification just like when you see waves hitting against a hard rock, you tend to amplify the motion in those low. The main issues in Rio Dell for this earthquake, it was just not in a very good location. It has nothing to do with the fact that the homes are not built really any differently than how the homes are built in Loleta or Fortuna or other community. Certainly we have a number of older homes. Older homes are more more susceptible. They tend not to meet the current building codes.

And one of the really exciting lessons about this earthquake is that structures that did meet the current codes, for example, those highest accelerations were in the ground right next to a freeway overpass, there was no damage at all on that overpass, they did not have to shut the road. It performed exactly as it should, even though the accelerations were huge. Redwood Memorial Hospital did just fine. The accelerations on the third floor of that hospital were huge. They're stronger in a building than they are in the ground and yet the hospital performed fine. We've learned how to build structures that resist struck actually resist strong ground shaking. The most vulnerable structural problem that we have is older homes built on post and pure foundations or cripple walls. The problem is when the shaking starts, you want the structure to be married to the ground, You want the structure to be all moving together as the ground moves. Post and pier, you're going to have each little part of the structure moving a little differently relative to that pier. If you've cross braced them, it will tie them together a little better and it certainly helps. But by far and away, the best thing is to have a perimeter foundation. The next best is a slab foundation with the structure firmly bolted to it.

WHEELER:

Yeah, I have a post and pier and I have been wondering what I should do about it. So cross bracing, trying to make it all move as one. That is the best thing probably that I can do to within my budget.

DENGLER:

That's the first step, I'm hoping. And I think California has a program to help people put in better foundations. It usually starts at the beginning of the year. It is so much better to have a perimeter foundation. I tell people when they ask me about earthquake insurance, I say if you have a lump of money together, it's much better to redo the foundation on your home with that money than just pay through the nose every year to get insurance and that's not to say insurance can't be used. Having a good foundation is the best way to get a good night's sleep. My son recently bought an 1890s farmhouse in Arcata. First thing we did in the remodeling was to put in a perimeter foundation ...

JEN KALT:

And we'll put in some links to these types of things in the show notes because this is really important for a lot of people...

DENGLER:

... and, and it's even more true of mobile homes. I've got a photo in Rio Dell that shows one trailer off of its foundation and another trailer. That is just fine mobile homes. If you don't restrain them, they're absolutely terrible getting a decent restraint system. And the problem is that the mobile home seems fine without a restraint until there's an earthquake? I mean, you know, it's okay, whereas you've got to have some foundation on a house, you know, it's not going to sit there without some foundation. Mobile homes are always problematic in earthquakes. They can do fine, but you have to go out of your way to get a proper restraining system.

WHEELER:

So some of the folk wisdom that I hear after earthquakes is, oh, that's great. We've let off some of the pressure. We've released some energy. This is going to forestall or make the next earthquake less damaging. Is there any truth to this or is this just stuff that we say to ourselves to make us feel better.

DENGLER:

There is no truth to that, particularly with this earthquake. In fact after a big earthquake, you're much more likely to have more strong earthquakes because you're in the midst of this aftershock sequence in the immediate hours, days and weeks, right after a large earthquake. That's actually when you're most likely to feel more earthquakes. And some of those aftershocks are certainly capable of producing additional damage. But the other thing is to realize that we have this complex geologic orientation of plates and false people say, oh, well we had this earthquake on the phone a year ago and now we had another one on the Mendocino triple junction fault this year. And I'm going, well, wait a minute. We have so many faults in the vicinity of the triple junction and onshore and offshore as well. The geology department used to sell t shirts that said Humboldt Bay geology. Proud of our faults. Most of our faults don't have names, many of them are offshore. Many of them are just sort of slivers. So this triple junction area, the earthquake that occurred, both this December and last December are certainly related to things that are going on at the triple junction where we have these three huge plates on the earth's surface. We have the pacific plate, the north american plate and the Gorda plate that are all kind of grinding together. They're all moving in different directions. And so we have this 2030 mile diameter zone, I'd argue it's even bigger than that. Where everything is being stressed between a rock and a hard place and then some because the geologic forces that are driving the plate motions around it, they're not stopping just because we had an earthquake, they don't stop there. Just continuing to grind. We are as likely to have another magnitude six earthquake today As we were two weeks ago. Some would argue that we might even be more likely. In fact, the usgs does these probability studies of aftershocks and in the week following, they estimated a one in 2000 chance that we would have had another magnitude 6.2 or larger earthquake in that week. One in 2000 is not very big, but it is slightly elevated over non earthquake likelihood. I mean, we live in this beautiful, unique geologic setting and we always have the small but real likelihood of a match magnitude six or seven and even larger earthquake at any time. There's nothing about this earthquake. That means we are more or less likely to have the really big earthquake that we talk about the cascadia subduction zone earthquake because that's being driven by different forces than what are happening at the triple junction. It's still a possibility it could happen this afternoon. It could happen 10, 15 years from now. I'm pretty sure it will happen in the next 100 years. This earthquake was really unrelated to the dynamics of the subduction zone at least given what I know in my perception that said, when that subduction zone earthquake goes, I will be very surprised if anyone experiences accelerations as high as what we're experienced in Rio Dell last week.

KALT:

The only bigger earthquake I felt was the 1989 Loma Prieta earthquake, but the fact that I was asleep and that I was jolted awake by the USGS MyShake app right at the split second when the shaking started was pretty terrifying, and then to have the power and the internet down... The first thing I did was text a friend in Petrolia, Thomas Duncan. who texted back right away and said it wasn't too bad here and I thought, okay, so Petrolia is still on the face of the earth. What I wanted to ask you to talk about is how people can be more prepared, especially when we're asleep to jolt up into the middle of the night to an earthquake like that. What should people do, since we do spend a big chunk of our lives in our bed. We need to be prepared to do the right thing, right, other than run around screaming or whatever. Whatever. I noticed my dog went right under the coffee table that I have in a closet and I thought she got the training right? You're supposed to go under a table and cover your head and hold onto the table. Is that what people should do?

DENGLER:

It's more nuanced than that, and certainly more nuanced when that earthquake is waking you up in the middle of the night, whether or not you get the MyShake alert seconds before or seconds during it.

Let's first talk about earthquakes at night. You're going to be in bed, hopefully asleep. You want your bedroom to be the safest place in your house. You take that mirror down that you have over the bed. Not a good thing to have in Humboldt County. Take that huge potted plant that's on the bookshelf next to your bed and put it down where it's low. If you have bookshelves behind your bed, have a nice little wire restraining. Because my first experience, I was in my parents bed, the Imperial Valley earthquake happened and all those books came down on top of me. I was seven years old. Maybe that's why I got into studying earthquakes. I don't know.

You really need to have that eye of what can move and what can topple by far and away. The majority of injuries in our earthquakes are because of things loose flying around in the house. So both of our bedside lamps fell over. We were fortunately out of town. Obviously when something falls down that can break, then you get up in the dark and maybe you don't have a flashlight nearby, you're gonna lacerate your feet. So before today, tomorrow, this week, while this is still on your mind, really go over your bedroom, your Children's bedrooms and make sure those are the safest places in your house. Having a crib on wheels, that's a terrible thing. The earthquake motion is just going to cause that crib to go slamming into a wall.

Now is a really good time to make sure that you have a flashlight or a cell phone close at hand. I'm always accused of having my cell phone on my body fused to me. But in times like this it's it's actually useful. It's always tethered to its cords and I know exactly where it is. So it's very easy to grab. I suggest that you just take a little sack and put a flashlight, some slippers or shoes that have decent soles and a pair of gloves and tie it to the side of your bed. If you happen to have a flashlight sitting up on a counter, who knows where that's going to be after the shaking. If you have a drawer put it in a place where it can't go flying. Having light is huge. Really, really important in the middle of the night.

Those sorts of things of making sure your sleeping place is as safe as possible if you have any heavy furniture that might slide, secure it to the wall.

Now let's talk a little bit about the MyShake app. My shake work for me too. I was in the third floor of a hotel near Sacramento and that's how I learned about the earthquake, which actually was somewhat fortunate because I was in a place with power and I could then find out about it and then go on the air with Lauren Schmidt at three a.m. Came out. You came on K. Um also does a good job. So radio is your friend in any disaster. Make sure you have a portable radio with batteries and then just toggle the dial until you find somebody that happens to be on the air that is by far and away. Your best way of getting information. Don't try to make your phone work unless you have really life threatening situation. You want to stay off your phone. Texting is okay because the texting will be kind of marshaled. You really feel you have to communicate. Go ahead, text, that's fine. But please don't call.

Not everybody probably knows what MyShake is. California has an earthquake alerting system. We used to call it earthquake early warning ... I prefer to say "alert" because it's not predicting the earthquake, it's detecting the earthquake in the very first few seconds after that rupture begins. So that rupture began offshore of False Cape. And then the closest seismic stations, there's one of Petrolia. There are a number that we have on the north coast and as soon as four seismic stations picked up those first seismic waves and algorithm goes into effect makes an estimate of the location and the likely magnitude -- I think the first magnitude for this was 5.6 on that automated system -- and then it calculates the area where people are likely to just feel it intensity three. Intensity three. If you were awake you'd kind of go, oh was that an earthquake combination larger than magnitude 4.5 and intensity three.

And then that message is pushed out to that large area was over a million people. It included Sacramento, it included some people in the San Francisco Bay area. Many of us didn't feel it in those areas, but it went to you to the message will say earthquake, earthquake drop cover and hold on if I'm in bed in the middle of the night here and I get that I am not getting out of my bed because I recently had knee replacement surgery and I'm not getting down on the ground.

Anyway able bodied people. If you feel like getting next to your bed on the ground, that's fine. Hopefully your bedroom is safe, your bed is safe. I would just curl up and make myself as small as possible and put a pillow over my head and stay there. Why am I not going down the stairs or going under the Because I don't know if I'm going to have one second, 10 seconds or 15 seconds before that ground shaking starts and if you're fairly close to the epicenter like you were you're going to actually get the notification and start to feel it at the same time. So the further you are away, the more likely that alert will come before the ground shaking starts. It's useful because just having the mental mindset, oh shaking maybe coming really helps to control that adrenaline rush, earthquakes can frighten people to death. And we see that in almost every earthquake and sadly we saw it in this earthquake. The adrenaline surge that will affect your heart can affect other conditions can put you into kind of a state of shock and unless you get medical care instantaneously it can kill you. And of course a big earthquake, you're not going to get medical care instantaneously so anything you can do to lower that adrenaline rush, that fear response is really worth doing for me.

I think the MyShake certainly helps last year's December earthquake. I was here, I got the alert, the alert came about 34 seconds before we felt shaking and it was wonderful. I mean it was like oh earthquake come. Yeah, there it is, of course it didn't shake nearly as strongly as this one did. The other thing I try to do is I try to get myself counting as soon as I feel the beginning of an earthquake because that gives your brain something to focus on and if your brain is focused then again that adrenaline will just kind of lay low, so it will help remind you to breathe, it will help remind you to pay attention to get under something if it's nearby.

WHEELER:

Well, I've learned so much and I've also learned that I have so much more to learn about earthquakes. So we will have to have you on in 2023 to teach us more about geology. And we will have in our show notes, which you can find on the lostcoastoutpost.com, links to the MyShake...

DENGLER:

MyShake app.

WHEELER:

... MyShake app, we'll have links to the MyShake app. And Lori has also produced a wonderful series, informative op EDS in the time standard and other places. So we'll have links for you to learn more if you want to. So go to the Lost Coast outpost dot com, where you can find more information.

Lori, thank you so much for joining the Econews Report, and look forward to talking to you in 2023.

DENGLER:

Happy New Year to you.

WHEELER:

Thanks, everyone. And I will talk to you in the New Year. Join us again on this time and channel next week for more environmental news from the north coast of California.