Northwestern Digital Learning Podcast: Episode 6, The Sounds of the Earth

Welcome to another episode of the Northwestern Digital Learning podcast, where each month we highlight an example of innovative teaching of learning across campus.

I'm Kelly Roark, a Faculty Support Specialist with Teaching & Learning Technologies at Northwestern University, and your guest host. In this episode we talk with Suzan van der Lee, a professor in Earth and Planetary Sciences, who is working on a new project involving a seismometer not far from the Evanston campus.


KELLY: How did this project get started?

SUZAN: For fun partially but also partially because we were working on a proposal for an outreach project and we wanted to come up with activities that people could do, STEM related activities that they could do around a seismometer that Northwestern owns that is installed in the Ryerson Conservation area so they can learn things about STEM fields in general via seismometer that we have installed there.

KELLY:  A seis… What did you say it was again?

SUZAN: A seismometer or a seismic station.

KELLY: Is that like a microphone?

SUZAN: Yes. The seismometers that we use for research is a combination of a microphone and a gravimeter.  

KELLY:  And is it in the earth?

SUZAN: Yes, you dig a hole and actually you make casing, like we used a sewer pipe you know vertical. And you pour concrete at the bottom because you want to level the seismometer so that it can measure, it is very sensitive, so they have to measure very small movements. And so you pour concrete so the sensor can be flat, so it needs to be level, it needs to be oriented toward the north or the east. And then you put a lid on that and then you cover that up with dirt. So it's a little bit of digging and then you open the vault. It's like a mini-vault and in it is the seismometer, so yeah. And one of those was the one in the conservation area, Ryerson Conservation area, and because that is close to Northwestern and because it's an area that is open to the public we thought that we should adopt that station.

KELLY: Is this one of the two or three meters ones?

SUZAN: An actual bulldozer dug a hole that's more than 2 meters deep like maybe five meters or so. And then grass grows over and slowly disappears from you know, you don't see that it was disturbed. If you know where it is you can see the heap.  But the way visitors can find it is because there's a solar panel nearby that provides power to the station and also there's an antenna to keep accurate time and a communication antenna that sends the data live to a server. These are seismic waves essentially because when you send sound into the earth it becomes seismic waves because pure sounds waves don't exist in solids because solids have a very high shear modulus. (Laughter)

KELLY: Suzan can see that she is losing me a bit but she gently explains how P and S waves are very low frequency seismic waves. This is one of the reasons I like her so much because even though I know basically nothing about her field she's so good at explaining it I feel like an expert when we're done talking.

SUZAN:  There's large earthquakes around the world that can move the ground here by centimeters like fractions or even by an inch. We never feel that because it's very very slow. So maybe it will move a half an inch over a period of an hour. So what we do with the seismic signals is we have this time series of ground motion or seismic waves that travel, you know, by our station, that is recording them. And then by compressing a whole time series signal in a shorter amount of time thereby increasing the frequencies of all the signals by the same factor, and then we can hear it. So we can take 24 hours of seismic waves and compress them into a minute and a half or so of actual sound. And so we can listen to what happens in a day just in a minute and a half.

Just the background noise sounds like birds and animals in the woods. So it's very fitting for that forest preserve to have that kind of sound come out of the seismic station. So yeah, squeaks and whistles and rustling leaves and wind and sometimes you hear rain sounds.

KELLY:  What actually are those sounds?

SUZAN: I don't k now yet. I have no idea. So that's why I'm getting more interested in this because I want to now be able to answer that question. I want to look at the data in different frequency bands and then make a movie that will show me what signal I'm listening to as I'm listening to it, and I haven't technically figured out how to do that.

KELLY: Were you shocked when you first sped up these sounds and compressed them? SUZAN: I was a little surprised. So there have been colleagues of mine throughout the country, throughout the world actually, they've done sonification of seismic data before but usually only of earthquakes because that's what we seismologists are interested in is the signals from earthquakes. So you can listen to those, and those are very interesting in themselves, but I had never listened, and I'd never seen anyone else do that, to look the whole day of background noise.

KELLY: And then something happened most of the files that you sent me that sounded like thunder at the end. Do you have any idea what that is?

SUZAN: Well, yeah. Things that sounds like thunder like “ka-boom boom” so, those are earthquakes. And they're over pretty fast.

KELLY: Earthquakes maybe in another continent?

SUZAN: Oh yeah yeah yeah. So you can hear earthquakes from Alaska and Oklahoma. So in sounds that you'll actually hear, we have actual earthquakes from Alaska. But you can hear a lot of earthquakes from around the world.  Any earthquake magnitude seven or higher or even sometimes magnitude six earthquakes. For example, the Alaska earthquakes are six point two, you can hear them very clearly. Oklahoma earthquake was magnitude 4.2, you can hear that very clearly.

KELLY: Suzanne continues to surprise me throughout our conversation and casually mentions that the sounds she listened to in July are quieter than in winter. Is the earth louder in the winter?

SUZAN: The oceans are more restless and create more noise. But I think it's also, the weather,  the freezing and thawing that affects what's going on inside the earth.

KELLY: I never thought it would be so beautiful I honestly thought the earth would sound very angry. (Laughter)

SUZAN:  Well it does when there's earthquakes!

KELLY:  By now you've probably figured out that the audio that's been playing under our conversation is not actually a recording of a forest, but the compressed sounds of that day as recorded by a seismometer. What you're listening to sounds from July 14m 2017. What's amazing is when you compress the seismic waves as Suzanne has done it sounds like birds singing and  animals chirping and wind blowing through the trees. On July 14th there was a 4.2 magnitude earthquake in Oklahoma which the seismometer in Illinois picked up. When you compress the sounds an earthquake sounds like thunder [thunder sound] - just like that. Suzanne and I listen to an earthquake together and we watch the peaks of the sound waves on her computer.

SUZAN: It's going to sound like thunder. And you can see there's a slight peak right in front of the bigger peak? So that will be the P wave that small peak, and then the bigger peak those are the surface waves, that are coming from the earthquake.

KELLY:  A P wave is…

SUZAN: It's the closest thing to a sound wave. And the surface waves [thunder sound] that’s the one earthquake and then [thunder sound] that’s the second. There’s all kinds of high frequency noise that’s also generated.  These data are recorded every day and stored in a data center. The data center in Seattle and it's part of incorporated research institutions for seismology consortium of universities and they store all kinds of research data for seismologists from all over the world. And so there's all kinds of changes going on in the earth and by listening to those who can say more about the actual physical properties of the E arth. So there's potentially lots of research applications.

KELLY: Suzanne really blows my mind by explaining that by listening to the earth we can learn more about the earth itself: about the soil and the tides. And that in something called forensic seismology you can study explosions and gunshots that could be used to track criminals. And then there's industrial activity from things like mining and fracking that causes little earthquakes. And she goes on to say that cultural noises get picked up by the seismometer as well - people waking up driving their cars and trucks, mowing the lawn, all of these things contribute to the readings as well.

Thanks for listening!