 It's 1 o'clock on a Monday afternoon, so you must be watching Think Tech Hawaii's research in Manoa. I'm your host, Pete McGinnis-Marc, and every week we bring you some exciting science from the University of Hawaii at Manoa, focusing on both the Earth as well as the planets. And we've got some really exciting science this time from around Hawaii. So my guest today is Rhett Butler, who is a geophysicist and a seismologist at the University of Hawaii's HIGP. So first of all, Rhett, welcome back. Thank you. You've been on the show before. Happy Easter. Number four. Number four. And it's great to have you here, and today we are going to be talking about earthquakes. Yes. And in particular, earthquakes around Hawaii. Yes, little ones. Little ones. All right. And in preparation for today's show, I started looking a little bit about Hawaiian earthquakes. Right. And there are lots of them, right? Quite a few. Actually, most people don't realize how many earthquakes we get here if you compare it to California or even Japan. We get quite a few. Yeah. And this number is primarily large because most of the quakes are relatively small. Well, it's actually whether you count big ones and small ones, it really doesn't matter. It's really the big island is where the action is. Most of our earthquakes happen there, and the rate of seismicity on the big island is comparable to both California and Japan in terms of per unit area. Per unit. And your job description is both a seismologist who presumably studies earthquakes and a geophysicist. The two are related, presumably. Well, you know, physics of the earth is geophysics, and the tool or my tool is typically seismology, which is from shaking. Seism is the Greek word for shaking. Okay. And we've had people like Niels Grobe, who is a geophysicist, and Gerard Friar, who studies earthquakes. Oh, yeah. So you're a combination of two of our previous guests, right? Something like that. Something like that. Well, I have broad interest. Let's put it that way. All right. Now, you brought along the first slide, and I think this will get across to our viewers a little bit more of some of the scale of the earthquake. So describe to us here, we've obviously got a map of the Hawaiian islands. I think Gerard Friar actually made this map years ago, but this shows the major earthquakes that have happened since the 1850s. So as you see, most of them are on the big island. The larger the diameter, the larger the earthquake, we've had earthquakes up into the upper sevens, seven, seven, seven, eight, and the smaller ones there are like sixes. But you can see over time, there's quite a number of events, not so many out on Oahu where we are, but more concentrated. And the numbers on this diagram relate to the numbers at low left. So for example, number two was a one eye earthquake, 6.8. Yeah. That number two is a 6.8. But actually, when it happened, it jarred the foundations and partially busted up Punahou School years. This is back in the day. Really? And number six, June 28th, 1948, a 5.2 just off Waikiki? Yeah. What are you shaking things up? Well, I was not around here then, but there's some uncertainty of the location. But yes, it would have really shaken up Waikiki. That would just be a regular earthquake or a submarine landslide? No, that would be, that would just be a regular earthquake. All of these are regular earthquakes. Although it's really clear we have submarine landslides, if you just look around the bathymetry of the islands, we've not really had one which we've clearly seen on the seismic record that this is a landslide. So we suspect them, we've not seen them. All right. And there's a lot around the Big Island which are apparently quite large, right? Yeah, they get up to the 7s. The 1975 Kalapana earthquake, 7.5. Oh yeah, these are huge. There's actually a 7.7. We listen to the news and when you have, say, a magnitude 7 quake in Japan or Indonesia, that is pretty big news. Oh yeah. It appears as a potential sonomular and then they always announce that it's too small, but yeah, it's always recognized that you get in the upper 7s, you can get sonomaginic effects quite easily. All right. And what we were seeing there were perhaps the top 12 earthquakes which in sort of since Western contact have occurred within the Hawaiian islands. Is that the sum total or how often do we get an earthquake? The larger ones, we haven't had a really big one since the event in, I believe it was 2006 in October. Mm-hmm. So that was a 6.8 or a 6.9 and it caused a fair amount of damage. And it woke me up. Yeah, I never experienced earthquakes. I was, of course, on the mainland while this happened and called in information from California as opposed to being here to actually feel it. Oh, I see. Yeah. And the second slide, I think, will elaborate a bit on this point. Just how many earthquakes take place? And this, I believe, we get this illustration from Hawaiian volcanoes. Yeah. I get a view of this in my office at UH and we're getting earthquakes all the time, but most of them are really small. They're the ones and twos and threes. And so if you're right on top of it, you can certainly feel it, but it's harder to see those things up with the island tree. And what we're seeing here again is of the eastern Hawaiian islands and the colored symbols, whether it's the white circles or the green triangles or, yeah, they are different times. Yes. Okay. Well, some of them it's depth. It depends on the color scale. So it looks like this one is... This is from the oldest quake, which is on the list on the right, is from March 15th of this year. Okay. So that implies that we might have had a dozen measurable earthquakes of magnitude two or three, which we can't feel, correct? If you're on the big island, potentially you'll feel it. But you won't feel them up here in Oahu, because we're just too far away for the energy to come up here. So why is the big island suffering so much from earthquakes? Well, it's got some of the largest volcanoes on the planet. Okay. Okay. And as it builds the volcanoes up, the internal stresses are such that eventually things fail and you get an earthquake. We have the local volcanic effect earthquakes. And then we have the earthquakes where you pile up a huge mountain and it presses right down on the lithosphere of the planet. And then you get earthquakes that we call deeper ones here, where they're 30, 40, even 50 kilometers down just from the massive weight of the islands. All right. So it sounds as if there are two different ways in which we could get earthquakes. On the big island, obviously we have killer whale active today and monolore may erupt a year from now sort of thing. But as the magma comes from deep below the Earth's surface, that can actually break rock forces. So that's one type. It's one of the ways you can tell an element pending eruption if you see seismicity coming up to the top. Yes. Yes. I've heard some of the HBO geologists describing how they might be able to predict monolore erupting again. And then the second style is that because of the weight of the lava flows already on the surface, then it's like pushing down on a pillow and basically it's giving way under a certain weight. It causes stresses to be accentuated in certain places and those can then become earthquakes. Okay. And we see in that slide there might be dozens of earthquakes of relatively small scale every day. And you mentioned earlier how does this compare to places like on the U.S. and California where we always think California's seismically challenged. I mean we really do have quite a number of earthquakes here. And when you're looking at the little ones you have to remember that every time you go down a magnitude and you go from a three to a two, you get ten times more twos than you do get threes and you get ten times more ones than you get twos. It's basically so most of the energy, I mean most of the numbers of earthquakes, the small ones happen a lot more often. So even though on that one plot which goes back to about 1850, well there's a lot of them on there but if you counted the number of small events it would be huge. Okay. And the quakes which we saw on the first slide where we were basically seeing several of our Maui one off Oahu, is that due to the same sort of subsidence of the islands? Exactly. You're still seeing, the shallow events are still more involved in the structures of the islands themselves as they were built into volcanoes and fractures within that structure whereas just the weight of the island creates deeper events. And those are the ones we typically see, we see them around the islands, actually we've seen them even here on Oahu, little ones that are substantially deep. But as your jobs are seismologists this must be a wonderfully fertile area of the planet to actually do seismological research. Oh yeah, this is a fantastic question. What kinds of things do you try and understand? Is it how the weight of the islands can cause quakes or can you learn anything about the interior? Well yeah, I mean if you, small events it basically tells you things have happened but you can't really look at the details because they're so small you only see them a couple places, larger events you can look at the whole mechanism of the earthquake which tells you the state of stress around that point and gives you an idea of how the weight of the island is is interacting with the stresses for the whole Pacific plate as it's moving around. So it's a great interplay between the two. We've had Bin Chen on the show in the past and Bin does high pressure mineral physics looking deep into the earth's interior. What kind of seismology can that actually learn something about the Hawaiian islands or the ocean plate upon which the islands are going? When you look at these you're always trying to put them in the context of other bits and pieces of knowledge. So it'll tell you about the velocities which can tell you something about the mineralogies. If you see absorption of energy such that the sounds don't travel as far that tells you something about temperature and once again about mineralogical states so although you have a focus on you've learned something because of the earthquake you try to relate that to other things you know about the planet and so it's part of a whole picture. And where you say temperature presumably you could detect whether or not you can identify the plumbing system of one of the whole things. Yeah that would be the help. You could bang the chambers on the ground will be hot so maybe Right. Transmit seismic energy. So it changes the velocity of which sound propagates it changes how fast it attenuates with distance all of these things have three-dimensional structure associated with it. So this is sort of a good introduction for the audience on seismology in Hawaii but obviously you're doing active research in seismic studies and I think the third slide will actually show us as we get ready for the break we can go to look at this third slide and let's just sort of walk some of the views around the scale bar should be a hundred kilometers down at the bottom right but we're seeing here what well I mean last year and many of us remember that there were some earthquakes felt in Kailua on that side of the island and they were little they were a little magnitude four earthquakes bigger than threes and twos of course and a little red balloon there number seven was one which occurred on March the 9th basically a year ago and it was a four point seven and it was well well felt around in Molokai and on the on the Wahoo and so I went and looked at that particularly that just out of curiosity because you could feel it you could talk about it you go look at it and so ultimately I ended up finding some very interesting things about that earthquake so I went back and said well let's look at the rest of them that happened in that area because I mean there have been a number of these things that have been happening recently so you pull up the events out of the data and we have a seismic station at the aloha cable observatory right the logo there so right so this is a observatory on the seafloor on seafloor this is the deepest one on the entire planet they said about 4,800 meters it's really deep and it's connected to a Wahoo by a retired telecommunications cable that the university acquired from AT&T and then put all the equipment connected to the cable so this observatory is about 80 kilometers north of a world about a hundred about a hundred okay and just to help the viewers a lot of it more we're looking at the structure of the ocean floor north of Molokai and yeah a Wahoo the numbers represent where you found right it each one is a different earthquake and but they are all have occurred since 2013 and they seem to cluster that those blocks on the ocean yeah there's a massive landslides that are probably a million years old and so yes there's surface features but the earthquakes themselves are very between 10 to 30 kilometers deep so they're much deeper than the surface fish and finally just as we get ready to take the break that the KIP on a Wahoo oh that is another size seismograph right yes we are blessed with one of the global seismographic network stations on the planets right here in the center of the island and near Kapapa Gulch which after which it's named and so we have a standard reference station to compare what we're learning on the sea floor with what we're learning from a standard so so when we come back you're going to describe a bit more of what you've discovered with these yes earthquakes okay so it's time for us to take a break so let me just remind you all you're watching think tech Hawaii research in Manawa I'm your host Pete McGinnis Mark and my guest today is Dr. Vett Butler who is a seismologist and a geophysicist and when we come back you'll tell us all about these Molokai earthquakes so see you then hey baby that's you I want to know will you watch my show I hope you do it's on Tuesdays at 1 o'clock and it's out of the comfort zone and I'll be your host RV Kelly see you there hey aloha standard energy man here on think tech Hawaii where community matters this is the place to come to think about all things energy we talk about energy for the grid energy for vehicles energy and transportation energy and maritime energy and aviation we have all kinds of things on our show but we always focus on hydrogen here in Hawaii because it's my favorite thing that's what I like to do but we talk about things that make a difference here in Hawaii things that should be a big changer for Hawaii and we hope that you'll join us every Friday at noon understand the energy man and take a look with us at new technologies and new thoughts on how we can get clean and green in Hawaii aloha and welcome back to think tech Hawaii's research in Manoa I'm your host Pete McGinnis Mark and my guest today is Dr. Brett Butler who's a seismologist and a geophysicist at HIGP at UH Manoa so we set the stage before the break on trying to study specific earthquakes north of Molokai so maybe you can sort of lead us through the next slide gets into some data which I've no idea at all what it means you can help me here if we can go to the next slide oh these are these are what we call seismograms okay words that if you starting at the left and then going to the right that's time moving forward so you can see it's a very flat line in front of the left one there and then all of a sudden it happens and you see a bunch of different things that are happening you see arrivals that are coming in at different times some of them are going straight to the seismic to the station some of them are bouncing in the ocean some of them are different velocities and but let's explain I think what we're seeing is the left-hand diagram that's right from station aloha right deep on the ocean floor that's right it's a hydrophone listening to the sound of that earthquake and KIPZ is the monitoring station on a wahoo right it's the Z what we use Z for vertical components so it's measuring vibrations vertically okay now why are they different it's the same earthquake yeah but here they're well they're different they are sampled at different rates the one on the left the hydrophone is actually sampled at a 24 kilohertz which is incredibly fast 24,000 samples per second the one on the right's a hundred samples per second so there's a lot more high frequency information the one on the right KIPAPA that's a standard seismic station if you won't go into all the detail but because it's on the island you're not getting the reverberations in the water on the left one you can see a lot more arrivals that's because lots of the energy is reverberating in the water so the energy is coming from down beneath the crust 10 or 30 kilometers and it's coming straight up to our seismic station then once it hits the bottom the sea floor just bounces up and down after that so we're primarily interested in just the direct energy because it's the quote simplest part of the whole path and the thing that made this one particularly interesting is when I got the in got the the recording it was the extraordinary high frequencies that were in this and what I mean by high frequency is typically if you're in seismology almost all the sound that you get from earthquakes is below human hearing humans hearing is about 20 Hertz so most of the noise is really deep okay and you can't hear it whereas this particular earthquake these Molek high earthquakes you could hear if you think about a keyboard you could hear all the way up to Middle E in other words really high frequency up to about 170 Hertz which is very unusual so that it's like the difference between you walk into a room and you hear these deep bassy notes versus listening to the whole orchestra and so it's rather extraordinary to get to see that right now for more background for our viewers we've had Milton Garces on recently talking about infrasound right which is the same low frequency sounds but in the atmosphere so seismologists would do infrasound through the earth's crust well it's the same principle I mean the vibrating atmosphere is a fluid okay and it only has one velocity the acoustic wave velocity vibrating solids have many many vibrational modes and so when you're looking at seismic waves you're looking at a whole bunch of different things that are not expressed in either the air or the water I love your analogy with say a pianist a pianist right who's playing the entire keyboard yeah it's it's rather remarkable that you can hear those kinds of high frequencies and you're hard pressed to find a a an example out there anywhere else on the planet where you see these kinds of high frequencies so I was rather struck what I saw is that because there aren't many station aloha scattered around well that's part of it part of it is that we have extraordinarily few seafloor observatories and so just this kind of recording is not available to most scientists you measure things mostly on land when you're going deep sea side deep when you're doing global seismology there are very few actual stations in the ocean so the land confuses the signal or there's so much background no no it's noisy noisy the surface of the earth is a noisy place if you go deep into the earth it gets a lot quieter if you go to the bottom of the seafloor it's still a lot quieter if you're at the surface with the wind is blowing and the thermal thermals are happening and you the waves are crashing into the side of the island it obscures what you can hear and so you don't get to hear these high frequencies you get to hear just noise so we got it so this was kind of like a picture into the sound of what it would really be like if you didn't have all of this noise at the surface and I know you've brought along another fascinating slide if we go to the next slide trying to visualize what these well what I what I was looking at in this and this is what we call a spectrogram yeah vertical axis is frequency so it goes up to 200 Hertz okay 200 Hertz is about middle C approximately okay and the bottom of there is is around it goes below a few Hertz and that's in other words you're off the bottom of the piano there you can't hear anything and so you're seeing the full range of this one signal now to the far left that's the background noise and there's just colors that that's the solid that's the noise and so if we just subtract off the average noise and then you basically paint it all blue where you it's where it's noisy you can have the signal sticking out here which is really clear this very clear at zero time because it's very close we're very close to it and and the colors and in particular I guess you're looking at the red yeah red is power so there's strong right how strong the signal is and it is as you go up in frequency the signal diminishes but relative to the blue background you can clearly see it above the noise and so I looked at that I thought wow I've never seen anything that high before so that and that then that was the impetus to go look at nine other earthquakes in the area is this a unique event or is this is a technique that's commonly used by seismologists and well more people that work in the oceans use these spectrogram techniques okay it's used it's not unheard of it's not routinely used and what do you learn by looking at this well this basically it tells you what a frequency content versus time so you can get how much energy there is it's the color which you used to paint it with and then the time and the frequency it gives you informations about what you're actually seeing from that earthquake source and the power is not instantaneous does this mean that you might have slowly cracking rocks propagating an earthquake or that is at different depths or it all depends you know the earthquake source is a ultimately are measuring the release of stress yeah and it's catastrophic in the sense that it does just start it starts and it propagates and you the further it goes the bigger the earthquake is and as it turns out the way that that rupture propagates actually makes an important role in how much energy you get out of it typically for earthquakes when we're seeing it we're seeing something that it's rupturing at a very it's the the velocity of that wave front is changing all the time it's it's you can think of it as a jitter and as it jitters it creates all those high frequency noises so at low frequencies you can see that in other words of all those notes in the piano that you can't hear I mean they're below the piano hearing you're looking at that jitter but one thing we've learned out of this is if we look at the higher frequencies now we can get a different view of what's happened and I think you mentioned you looked at nine different quakes let's look at the next slide which will show us first law yeah this is a cutoff figure so on the left is what we call p waves those are the fast things that go through the earth okay and the s waves are slower remember I said that there's a couple there are at least two velocities of solids yeah they travel at different speeds and they both tell us different things about the earthquake now we're showing this as a function of frequency and the left side shows what we have seen always before we've never seen the high frequencies and the numbers so the left side is like seven ten four there they're there they're the numbers that were on that earlier I got it so each of those you can look at them they're all basically falling off of the same rate that's omega squared it make it of the minus two what that means is that as frequency increases the amplitude is decreasing so it's it's losing energy at a certain rate okay and this is what you would see just from anywhere around the planet all right if you were looking at earthquakes just this kind of decay rate in the spectrum and I think what you found by looking at station aloha is shown in the final slide yeah which should go next slide please so uncover the part there we go and we've basically not seen this part of the earthquake spectrum and what happens is you see that because we have these very quiet conditions for these very small earthquakes we're actually seeing information in the earthquake source that we've not seen before now I mentioned that at lower frequencies for the larger events the velocity is jittering as the earth is cracking and there's this wave spreading it's jittering it's not moving smoothly if it just propagated exactly the same speed it'd be very little energy it's that jitter that creates all the energy so with the layperson in the street would she be worried or excited about this discovery I wouldn't be worried about it it just it's an interesting phenomena and I think from my perspective it's not so much that the that we're looking at something unique about the earthquake source we're looking at something that's unique about hawaii's attenuation of the energy in other words we can see the energy so therefore it hasn't been attenuated here and we've seen that something has changed at about 50 hertz so it's once again you go up your keyboard here and you go up an octave or so all of a sudden the way that the earthquake is expressing its energy has changed from like if the slope has changed so what it means is part of the energy is dependent upon the jitter and the rupture itself but at higher frequencies you're looking at something much smoother and that's what's shown in the actual and so presumably seismologists like yourself get a much better picture of the way earthquakes propagate through the the line primarily because we've got station aloha and the university having this fiber cable to a real-time monitoring station yeah it's a great resource for we get to see something that unique location close to a wahoo very interesting results and he says something very directly about the earthquakes we're looking at but also the crust that it exists in and you you've omitted the fact that we've got seismologists who know how to interpret the data yeah i'm sorry we're running out of time so let me just thank you again i know you've been back on the show a number of times i'm looking forward to seeing you again soon and let me just remind the viewers you've been watching think tech hawaii research in manoa i've been your host pete mcginnis mark and my guest today has been dr red butler who is a geophysicist and seismologist within the university of hawaii's higp so until next week goodbye for now and see you in a week's time bye