1. Ambient Seismic Noise: What we can learn from what we can’t feel
Celeste Labedz
Massachusetts Institute of Technology
Incorporated Research Institutions for Seismology
Summer Internship 2014
Final Draft

2. What can ambient seismic noise give us?
Clues about the inside of the earth
A “telescope” for looking down
Earth’s structure
Earth’s history
Earth’s hazards
wikipedia.org
What can ambient seismic noise give us? – A lot! Imagine if you were blindfolded, but wanted to figure out where you were. You would listen carefully to the sound around you for clues; if you heard birds chirping, you’re outside; if you hear cars going by, you’re near a road; if you hear a sink running and a refrigerator humming, you’re in a kitchen. It’s the ambient noise, the sounds in the background, that provide the hints.
Clues about the inside of the earth – I use ambient seismic noise in the same way: to look for clues about what I can’t see. For me, though, “what I can’t see” is the inside of the earth. This is all about learning what’s beneath our feet!
A “telescope” for looking down – In order to think about what’s below us, let’s think about what’s above us. How do we learn about things above our heads? If it’s not that far up, we can go there ourselves with a ladder, hot air balloon, or rocket; but some things, like faraway planets, stars, and galaxies, are beyond our reach, so we use a telescope to observe them. Looking into the earth is the exact same way; we can go a little way down by digging, but to get really deep, we need a better tool. Just like you can use a telescope to focus light from a distant star into something we can understand, I use sensors and computers to focus ambient seismic noise into something we can understand.
Earth’s structure – Now, let’s think about what we can learn from our “telescope” of ambient seismic noise: Earth’s inner structure. As shown in this diagram, Earth has layers: the crust, mantle, outer core, and inner core. There are also more subtle changes in chemistry and structure, layers within the major layers. Ambient seismic noise lets us see the boundaries between these, so we can map out what the earth is like at different locations and depths. There are a lot of things we can learn from discovering the subtleties of Earth’s structure.
Earth’s history – For example, we can use our structure map to find out about the history of our planet. Let me explain: Earth’s outer shell is made of big chunks called tectonic plates that move very slowly over time. Sometimes, one plate gets pushed under another plate in a process called subduction, shown in this picture. After being subducted, a plate will melt, but that happens extremely slowly over millions of years. As long as it’s not totally melted yet, we can find it with our ambient noise because it’s different than the material around it. Finding old tectonic plates like this can help us deduce what our planet was like millions of years ago.
Earth’s hazards – Understanding Earth’s inner structure is also important because Earth can be a dangerous place! Knowledge of what’s below our feet can help us understand the workings of, and possibly triggers for, hazardous events such as earthquakes and volcanoes. Learning to predict such disasters, even if we can only get a few minutes warning, could potentially save a lot of lives.
Transition – Now that you know all of the cool things we can learn from ambient seismic noise, I’ll show you what it is and how it actually works.
geo.arizona.edu

3. What does “seismic” even mean?
Earthquakes!
Movement of waves through the earth
Big waves
Little waves
Ambient seismic noise
What does “seismic” even mean? – First we need to go over what ambient seismic noise is. The strangest word here is “seismic,” so let’s think about what that means.
Earthquakes! – Of course it means earthquakes. If you’ve ever heard the word “seismic” before, there’s a good chance it was in a news report about an earthquake.
Wave Movement – Wave movement: this definition of “seismic” is a little more textbook. By “waves,” I mean motion going from one place to another, like an ocean wave rolling in toward the shore or a sound wave speeding into your ear. In a seismic wave, though, material in the earth is doing the moving. That may still sound a lot like “earthquake” to you, but let me broaden things:
Big – There are big waves that move through the earth occasionally; they can be triggered by earthquakes, asteroids, or bombs, and sometimes they’re even big enough for you to feel them shaking the ground below your feet.
Little – There are also little waves that are too small for a person to feel; we only know they happen because we have equipment that is much more sensitive than any person. Lots of things can cause these smaller movements: ocean waves pounding on the shore, big storms in the sky, a loud rock concert, trucks driving over roads, or even you jumping up and down. Waves like these are moving through the earth all the time.
Ambient seismic noise – Ambient seismic noise is all of those little waves going on when there are no big waves. It’s just like the ambient sound in a room; even if we’re all really quiet, you can still hear things in the background, like the air conditioner humming or a cricket chirping.

4. How do we get ambient seismic noise?
Seismometers
…LOTS of seismometers
usarray.org
How do we get ambient seismic noise? – Now that we know what ambient seismic noise is, how do we get it?
Seismometers – Since ambient seismic noise is too little for humans to feel easily, we use an instrument called a seismometer, which is a really precise motion sensor that we bury in the ground. Seismometers the first part of our “telescope” for looking down, sensing the movement of the earth and recording it for us.
…LOTS of seismometers – To get a really good picture of what’s below our feet, we need a really huge amount of data.
Figure – My research uses data from the Transportable Array, a giant project that put a big network of seismometers all over the United States over the course of ten years. Here’s a map of all of their stations. I used an east-to-west stripe of stations running from northern California to New England for my research.

5. How do we turn ambient seismic noise into something we can use?
wikipedia.org
Correlation between stations
Analyzing the correlations
How do we turn ambient seismic noise into something we can use? – Having all of that data from all of those seismometers is great, but it takes a few more steps to turn it into something we can really use to see Earth’s insides.
Correlation between stations – First, I take the data from pairs of seismometers and correlate it. Correlation takes two sets of data and returns a new set that shows how similar the two are when you match them up with each other at different times in the data. This diagram on the right shows how it works; you compare the blue data and the red data at different places, like at the bottom of the picture, then it gives you the black data that shows their similarity. I use a computer to correlate the data from every possible pairing of seismometers. For each pair, I get a squiggly correlation like this one. This process of correlation is the second part of our “telescope” for looking down, “focusing” the seismometer data into something more useful.
Analyzing the correlations – Once I have a bunch of those correlations, I’m ready to start looking for waves. There are lots of ways I use computers to analyze the correlations, but they’re basically all different methods of doing the same thing: looking at all of the correlations when we arrange them based on their distance apart.

6. A plot of correlations:
Figure – That arrangement looks something like this. Each one of those vertical squiggles is a correlation from a pair of seismometers. The lines’ place on the graph is based on the distance between those seismometers, with the closest together on the left and the farthest apart on the right. How far down the line you go is the time difference between the two seismometers’ data in the correlation.
Each line gets a big squiggle when the time difference between the seismometers matches the time it takes for a seismic wave to move between them, so that stripe of big squiggles from top left to bottom right is actually showing us a seismic wave! Even though it’s made of weak little background waves too small to feel, we can still see it when we put a LOT of data together. This graph is made of an entire year of ambient seismic noise from twenty different seismometers.

7. How does seeing waves let us see inside of the earth?
Seismic waves bend and bounce
Different paths
Different times
aeic.alaska.edu
How does seeing waves let us see inside of the Earth? – Great, now we’ve found a wave, but how does that let us see what’s below our feet?
Waves bend and bounce – Seismic waves can bend and bounce, just like waves of light bend through a magnifying glass or bounce off of a mirror. Inside of the earth, seismic waves bend toward the surface as they go deeper, because the material through which they’re moving is increasing in density; and seismic wave bounce when they hit a sudden change in composition or structure, like at the boundary between two of Earth’s layers.
Different paths – With all of that bending and bouncing, there are a LOT of ways that a seismic wave can get from one place to another! You can see in this diagram a few examples of the many paths that a seismic wave can take through the inside of the earth.
Different times – Some of those paths are shorter and some are longer, so two seismic waves taking different routes can end up at the same place at very different times. We can use those time differences to make educated guesses about the paths that the waves took.

8. The Big Idea: Look at the arrival times of seismic waves
Figure out the paths they took
Turn the paths into a model of the inner structure of the earth
Use the model to observe layers, subducted plates, hazard zones, and more
The big idea – The big idea is to (1) use the waves’ different arrival times to (2) figure out the paths they took, because knowing how much they bent and where they bounced can tell us about what the earth is made of and where that composition changes. We can use that information from a lot of different paths to (3) construct a model of what the earth is like on the inside. This model can then be used to (4) observe the inside of the earth in reference to things like structure, history, hazards, and beyond.