[MUSIC] So I've shown the principle of a seismometer. It's simply the concept that weights have inertia, so the weight stays in place while the frame, which is attached to the earth, moves around. There are two ways in which seismologists set up these instruments so that they measure different types of motion. One is by using what's called a horizontal motion seismometer, and that's basically what I just showed here. That is, that it consists of a weight hanging from a frame. The frame is anchored to the earth. And when an earthquake happens, the frame moves as does anything that's fixed to it, while the weight, because of its inertia, does not. Now, instead of having a paper that you pull through the way I've done it, in a real seismometer, the paper is rolled around a cylinder. The cylinder is attached to the earth, and the earth, when it vibrates, moves a cylinder underneath the paper. All the while, the cylinder's revolving, and as a consequence, the pen doesn't just trace back lines in the same place, but rather traces the wave like shapes that we saw on the sheet of paper. Now seismologists also want to measure motion that's happening in the vertical direction, because sometimes the ground motion of an earthquake is back and fourth, sometimes the ground motion is up and down. To measure the up and down motion, they set up what's called called a vertical motion seismograph, and that differs only in the configuration of where the weight and the spring are. The cylinder, in the case of a vertical motion seismograph, is vertical, and the pen is made horizontal, so the pen's held in place. And as the ground goes up and down, it traces out a line. Now modern seismometers aren't actually simply weights and pens anymore. And they don't use paper anymore. Rather, they are electromagnets that move inside an electric coil. So when the seismometer moves, the electromagnet stays in place, and it creates an electrical signal which can be recorded digitally. One thing to keep in mind is, a seismometer is going to record any vibrations in its environment. Now some of those vibrations might be due to earthquakes, but others might be due to traffic going by, or heavy trucks, or trains, or even trees swaying. So in order to maximize the signal to noise ratio, meaning, in order to maximize the signal that's due to real earthquakes and try to distinguish them from the noise that's just due to the environment, seismologists will tend to bury their seismometers in vaults so that they're below the surface of the ground. Here's a typical record out of a modern electronic seismograph. To the left, there's no earthquake taking place. The little jitters of the trace are the daily noise that happens in the environment where the seismometer is placed. Suddenly, an earthquake comes in, and you can see that the signal jumps. There's suddenly a very large wave, and as the earthquake energy tapers off, that signal gets smaller and smaller as time passes. Today, seismometers are deployed worldwide. There are seismometers on all continents, and in many continents there are many, many seismometers. In the United States in particular, during the last several years, there's been a big experiment taking place called the Earth Scope Experiment in which an array of seismometers has been set up across the country. At any given time, this array extends from the Canadian border to the southern edge of the United States, and it includes hundreds of seismometers arranged in a grid. As time passes, technicians remove the seismometers from the west side of the grid and place them on the east side of the grid. So the grid as a whole migrates slowly across the country like a caterpillar. But each instrument stays in place for about two years or so. This array, called USArray, is providing a huge amount of insight into the way earthquake waves pass through the earth. And it's giving us information about the nature of the interior of the Earth. [MUSIC]
