June 15, 2020

Instruments of Unusual Size

Posted by Shane Hanlon

A view of Ecuador’s Cotopaxi volcano and its long, cylindrical crater. After a sequence of eruptions in 2015, the crater floor dropped out of sight and the volcano produced strange sounds scientists dubbed tornillos. Credit: Silvia Vallejo Vargas/Insituto Geofisico of the Escuela Politecnica Nacional (Quito, Ecuador).

Volcanic craters could be the largest musical instrument on Earth, producing unique sounds that tell scientists what is going on deep in a volcano’s belly.

Chile’s Villarica volcano acted much like a gigantic horn when it erupted in 2015, created reverberating sounds that changed pitch as its lava lake rose to the crater rim. On the other hand, Ecuador’s Cotopaxi volcano has a deep, cylindrical crater that acts much like a massive organ pipe. The crater produced strange sounds scientists dubbed tornillos, the Spanish word for screw, when Cotopaxi began rumbling in 2015.

Jeffrey Johnson, a geophysicist at Boise State University, studies the unusual low-frequency sounds made by volcanic eruptions, earthquakes and avalanches. Understanding each volcano’s unique voiceprint could alert scientists to changes going on inside the crater that may signal an impending eruption, according to Jeff.

In this episode, Jeff describes how volcanoes and earthquakes produce infrasound – sound waves below the frequency of human hearing – and how the size and shape of a volcano’s crater defines the range of vibrations it can produce. Listen to Jeff recount the strange sounds geophysicists noticed during the eruption of Mt. St. Helens in 1980 and hear how earthquakes can make mountains ring like giant bells.

This episode was produced and mixed by Lauren Lipuma.

Transcript:

Shane Hanlon:              00:00                Hi, Nanci.

Nanci Bompey:             00:01                Hi, Shane.

Shane Hanlon:              00:03                Hi, Lauren.

Lauren Lipuma:             00:04                Hi, Shane. Hi, Nanci.

Shane Hanlon:              00:05                Do any of you or have any of you play or played a musical instrument at any point?

Nanci Bompey:             00:12                Yes. From third grade until ninth or 10th grade, I played the viola.

Shane Hanlon:              00:17                Oh.

Lauren Lipuma:             00:18                That’s an interesting choice.

Nanci Bompey:             00:21                It was an interesting choice.

Shane Hanlon:              00:24                What’s the difference between a violin and the viola?

Nanci Bompey:             00:26                Viola is like one string lower. So it’s a little deeper, a little lower, but I was so small. It was actually a violin strung as a viola, because the actual viola is so big. I think for little kids.

Lauren Lipuma:             00:37                Oh.

Nanci Bompey:             00:38                Yeah. I think I played it because there’s only so many people playing viola, everyone wants to do violin or maybe even cello. No one wants to play viola. So in my not goodness, I was the first viola.

Shane Hanlon:              00:50                You were the big fish in a small pond.

Lauren Lipuma:             00:53                The only fish in the pond, really.

Shane Hanlon:              00:55                Yeah, it’s quite brilliant. Welcome to the American Geophysical Union’s podcast about the scientists and the methods behind the science. These are the stories you won’t read in the manuscript or hear in a lecture. I’m Shane Hanlon.

Nanci Bompey:             01:08                And I’m Nanci Bompey.

Shane Hanlon:              01:10                And this is Third Pod From the Sun.

Lauren Lipuma:             01:15                Well, did you guys know that volcanoes can actually act as musical instruments?

Shane Hanlon:              01:20                So besides like the sound of-

Nanci Bompey:             01:23                Right, the sound of eruption. Exactly.

Lauren Lipuma:             01:25                Well, right. Yes, yes. They make noises when they erupt, but they also make sounds before they erupt and afterwards and scientists actually study them. And they’re kind of like the biggest musical instruments on the planet.

Jeff Johnson:                 01:46                Right. So my name is Jeffrey Johnson and I am a professor at Boise State University in the department of geosciences. And I study everything related to volcano geophysics and specialize in infrasound. Every volcano has a distinct personality in terms of the sounds that it produces. Those volcanoes that we refer to as open vent volcanoes, meaning that they’re actively de-gassing emitting material are producing sounds and volcanoes like to speak in low frequency. So they produce most of their most energetic sounds can be infrasound bank. And infrasound is basically just sound below the threshold of human hearing.

Jeff Johnson:                 02:27                So that means anything below 20 hertz. We’ve monitored the infrasounds so that we can understand, first and foremost is the volcano actively erupting? In many cases, we can’t observe the volcanic vent directly. Maybe it’s nighttime, maybe the weather is bad and the infrasonic record gives us a sense for whether or not the vent is open, whether it’s de-gassing and whether it’s exploding. So we’re able to record these signals from a long distance away. By a long distance, I mean anywhere from a few kilometers, to tens or even hundreds of kilometers and record an accurate time history of what’s happening up there in the volcanic vent.

Jeff Johnson:                 03:10                I began my career in geophysical research at University of Washington with professor Steve Malone, who was a famous seismologist active during Mount St. Helen’s 1980 eruption. And when I started to work with him and he said, “Oh, I’ve got this cool data that I would like you to take a look at. And it’s not sound data per se, but it is eruption data from Mount St. Helens.” And it is seismometers located throughout Washington state that responded to that eruption on May 18th, 1980. All of these seismometers hundreds of kilometers away around Mount St. Helen’s are receiving sound energy. So this was a story that needed an explanation. So we looked at these records and they were paper records at the time. And we concluded that we were observing sounds that were refracting both in the stratosphere, meaning 50 kilometers above Earth surface, and also in the thermosphere 120 kilometers above Earth surface before it being bent back to the ground in 20 minutes later, shaking the ground. And so that motivated me and Steve Malone to think about designing microphones that could more effectively record these pressure waves.

Jeff Johnson:                 04:28                One of my first volcanoes that I was able to study was Mount Erebus in Antarctica. And we recorded infrasound from that lava lake and the explosions that happened with some regularity at that lava lake. And this is back in 1998 when I went to my colleague and I said, “these infrasound signals appear to suggest that more than a thousand cubic meters of gas are being erupted with each explosion.” And that points to a bubble, a single bubble, that’s reaching a lava lake surface. That’s about the size of a small building. 20 meters by 20 meters by 20 meters. And I said to my colleague, “that doesn’t seem reasonable. Bubbles can’t be that big.” And this was Phil Kyle from New Mexico Tech who said, “yeah, bubbles are that big in some magnetic systems.” And a few years later, Phil Kyle and others were able to go to that volcano with video cameras and document the fact that these large bubbles, the size of big houses, small buildings are actually distending the surface of the lava lake, and then they are as a single unit exploding.

Shane Hanlon:              05:53                All right, I have to ask because the three of us, it’s hard for us to just to get mics working when we’re trying to record this podcast. How did they capture the sounds that the volcanoes are making?

Lauren Lipuma:             06:04                Yeah, so they go out and they deploy an array of microphones all around the volcano, usually a few kilometers away. And they do it usually near seismic equipment that’s put out to capture the ground shaking motion to see if the volcanoes shaking the ground at all. So they have a bunch of microphones. They try to have at least three, and that way they can use those to triangulate where the sound is coming from. They would love to have more microphones around a volcano if they can. And their goal is really to kind of record the sounds over time, to really get an idea of what the volcanoes voiceprint is. And then if they can maybe figure out if the volcano is getting ready to erupt.

Nanci Bompey:             06:38                The volcano voiceprint. I love that. So what does it sound like?

Lauren Lipuma:             06:43                Well, Jeff will tell you.

Jeff Johnson:                 06:49                One particularly interesting case study that I like to talk about is the eruption of Villarica in 2015, which is a volcano located in Chile. And on March 3rd, it erupted rather suddenly and with an impressive eruption that spewed material up to a couple of kilometers into the atmosphere above the vent. And it erupted fairly suddenly without too much warning. But upon reviewing the infrasonic records, what we observed was that during the course of the days, leading up to that eruption, the tones of these low frequency sounds changed systematically over time.

Jeff Johnson:                 07:29                And the fundamental change was both in terms of the frequency of those sounds and also the resonance or timbre of those sounds. And what we observed was that the frequency became subtly higher over time, over a few days, and this resonance, which was a manifestation of the volcano behaving like a musical instrument went away. So in the early days, while small explosions from Villarica would cause a little bit of reverberation in the hours leading up to this eruption that reverberation disappeared. So our hypothesis was that the lava lake within this active crater was rising systematically over time. Much like a trombone, a musical instrument. As you push the plunger up into the analog of a crater, the tones will change.

Jeff Johnson:                 08:36                So we observed that over time. And that led us to the idea that we were able to record the level of the lava lake in the crater. So a crater that is long and skinny might be analogous to an Oregon pipe, a musical instrument that has one open end and one closed end.

Jeff Johnson:                 09:09                And one thing that most people who listen to nice sounding musical instruments are aware of these resonant frequencies that those pipes are able to impart to the sound that you’re listening to. So in the case of that, very skinny, very deep crater, you get a lot of reverberation in the infrasounds you’re recording. And that reverberation sounds quite pleasing actually, when you speed that infrasound up to audio tones. When that pipe becomes short, you lose that reverberation and your explosions become more like funks than a funk followed by a decane oscillation. So that’s one thing we noticed when the lava lake started climbing up in this crater, it became more of a loudspeaker than an Oregon pipe, and we lost that reverberation.

Jeff Johnson:                 10:09                So this is a record from volcano Villarica in Southern Chile, which erupted in March 3rd, 2015, with a spectacular mile high fire fountain. What we’re looking at right now is a 10 hour record. That’s sped up by a factor of a hundred times so that we can actually hear the infrasound. That means one hertz infer sound tone becomes a hundred hertz audible tone. Starts off in the early morning with these discreet Strombolian explosions, small scale activity, nothing to be worried about. Just before 6:00 UTC time, the Strombolian popcorn type sounds morphed into continuous jetting.

Jeff Johnson:                 10:52                And this is when people start to get a little bit nervous. Those of us who are watching the volcano, and then shortly after 6:00, the paroxysm commences with an abrupt fire fountain reaches a mile high. And within an half an hour, it’s pretty much done for. Done for and over, except that on the Northeast side of the volcano, there’s this spectacular lahar infrasound signal that endures for some two hours following the eruption. And that’s what that high frequency tone sounds like. If you look at the wave form, you can see this decaying signal that’s indicative of a large, large mudflow.

Jeff Johnson:                 11:40                Villarrica was a really cool case study because we observed changing infrasounds. The frequency changed over time and the reverberations disappeared. Other volcanoes have their own unique sound prints, if you will. One volcano in Ecuador that has a particularly beautiful infrasound signal is Cotopaxi. And during the 2016 erupted sequence, this volcano was producing beautiful resonant signals that we called tornillos and tornillos is the Spanish word for screw. So what we were observing was some sort of event occurring within the crater and these oscillations or reverberations that were lasting more than a minute and a half.

Jeff Johnson:                 12:47                We think something was happening at the bottom of the crater to excite these resonances. But we don’t know precisely what that was. Now Cotopaxi is a fairly inaccessible volcano. It’s really, really high. Often the weather is bad. And so even though we were recording these signals regularly, we didn’t have much in the way of eyes on in terms of what was happening. But we do think there’s probably small explosions at the bottom of the crater. If for no other reason than this is what is exciting those resonances at other volcanoes, like for instance, Villarrica.

Jeff Johnson:                 13:25                Moreover, the fundamental frequency of these signals is about 0.2 hertz, which is far lower frequency than most other volcanoes. And so this beautiful tornillos like signal told us something about the size of this deep crater. So as with musical instruments, like in Oregon pipe, those larger pipes are responsible for the lower frequencies. So by that analogy, these giant deep craters such as Cotopaxi, and to a lesser extent, Villarrica are going to be producing even lower frequencies than musical instruments that humans like to listen to. In the case of Cotopaxi, we estimated that the distance or the depth of the crater from crater rim to bottom was around three to 400 meters. And that was capable of sustaining these very, very low types of frequencies we don’t see at other volcanoes.

Nanci Bompey:             14:35                The volcano stuff is so cool. I mean, is he just a volcano specialist or does he do recordings of other things on earth?

Lauren Lipuma:             14:43                Well, yeah, actually Jeff does recordings of other things too. He studies the sounds made by avalanches, which as you can imagine is pretty loud and, but actually earthquakes make infrasound too. Obviously when the ground shakes and buildings fall and things like that, that makes noise. But the earthquake makes other kinds of sounds too. And that can actually reverberate off of things like mountains and other features of the topography.

Jeff Johnson:                 15:11                Now earthquakes shake the ground and produce seismic waves. We all know that, but these seismic waves also impinge upon the free surface, the ground atmosphere interface, and they cause sound waves in the form of low frequency sounds to propagate up and into the atmosphere that we can then record with networks of arrays of infrasound sensors. We’re engaged right now in a very exciting project here in Idaho with the monitoring of an aftershock sequence from an earthquake that occurred a little bit more than a month ago. On March 31st, there was a large 6.5 earthquake in central Idaho. And since then many hundreds of earthquakes have occurred some as large as mid fours. And any of these earthquakes that are above, I would say about 2.5 are also producing infrasound signals.

Jeff Johnson:                 16:13                So my colleagues at Boise State University and I, we mobilized shortly after the main shock, deployed a network of arrays of sensors all around central Idaho. And we’ve been listening to these infrasounds and some of our preliminary results indicate these earthquakes are generating epicentral infrasound and they’re also generating infrasound associated with secondary sources. So what that means is an earthquake happens. The seismic waves come straight up to the surface and shake the atmosphere. And then those seismic waves also radiate out to further distances and shake the mountains that may be located at 20, 30, 40 kilometers. And those resonate and ring like bells, if you will producing secondary infrasound sources. Almost all the mountains in that area are ringing due to the passage of seismic waves. They seem to ring more than the valleys, the low lying regions, which for whatever reason that we’re still trying to figure out are not quite as loud in terms of the infrasound energy.

Nanci Bompey:             17:37                Were you surprised? What did you think when you realize this was going on?

Jeff Johnson:                 17:42                I suppose I was surprised to see or hear as much infrasound as we did. Folks who know a little bit about earthquakes are aware that earthquakes produce sounds, people who live close to earthquakes, feel the shaking. And they also can hear with their ears higher frequency sounds that are definitely coming through the air. They’re not coming through the ground or not coming solely through the ground. And so that points to the fact that these sound waves produced by earthquakes are really quite common.

Jeff Johnson:                 18:16                And they’re basically universal. And so if you have the right sensors, which are in this case infrasound sensors, you’re able to capture a whole world of low frequency sounds and understand where those sounds are coming from. Every day I realize how lucky I am to be able to both to travel and to do some really extraordinary field work. But at the same time, do some novel scientific study using this very powerful radiation from these geophysical sources. And every time you get a new dataset and you get to analyze it and tell a story, it’s a privilege.

Shane Hanlon:              19:04                Well, everyone does this change your opinion or revive, I guess your interest or dedication to playing or learning a musical instrument?

Nanci Bompey:             19:14                I have high aspirations, but it never really pans out in the beginning of this. At the beginning of the quarantine, I was like, I’m going to learn to play guitar. And I cut my nails and sat down for all of a five minute lesson with my boyfriend, decided my fingers hurt and I’m done. So that was that. What about you, Lauren, have you revived your skills?

Lauren Lipuma:             19:35                It does. It definitely revives me. I have a ukulele that I’ve been learning for six months, but I’m still pretty bad. Not really ready to play in front of people yet, but I enjoy singing to myself in my bedroom.

Shane Hanlon:              19:47                All right, folks. That’s all from Third Pod From the Sun.

Nanci Bompey:             19:51                Thanks so much to Lauren for bringing us this story and to Jeff for sharing his work with us.

Shane Hanlon:              19:56                This episode was produced and mixed by Lauren.

Nanci Bompey:             19:58                Don’t forget to check us out wherever you get your podcasts and to rate and review us on Apple podcasts. And of course you can always find us at thirdpodfromthesun.com.

Shane Hanlon:              20:08                Thanks all. And we’ll see you next time.