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S-waves and P-waves. Created by Sal Khan.
Video transcript
What I want to do in this video is talk a little bit about seismic waves. One, because they're interesting by themselves, but they're also really useful for figuring out what the actual composition of the Earth is. You've seen my video on the actual layers of the Earth, and seismic waves are crucial to actually realizing how people figured out what the different layers of the Earth are. And just to be clear, seismic waves, they're normally associated with earthquakes, but they're any waves that travel through the Earth. They could be due to an earthquake, or just really any kind of a large explosion, or anything that really essentially starts sending energy through the rock on Earth, really through Earth itself. Now, there's two fundamentally different types of the seismic waves. And we're going to focus on one more than the other. One is surface waves. And the other is body waves. Now, surface waves are ones that literally travel across the surface of something. In this case, we're talking about the surface of the ground. And this right here is a depiction of surface waves. And these really are more analogous to the type of waves we normally associate with the surface of water. And there's two types of surface waves, rally waves, and love waves. We won't go into a lot of detail, but you can see that rally waves are kind of the ground moving up and down. Right here the ground is moving up. Here it's moving down. Here it's moving up. Here it's moving down. So you can kind of view it as kind of a ground roll. The love waves are essentially the ground shifting left and right. So here it's not moving up and down, but here it's moving, if you're facing the direction of the wave movement, to the left here. Here it's moving to the right. Here it's moving to the left. Here it's moving to the right. In both cases, the movement of the surface wave is perpendicular to the direction of motion. So we sometimes call these transverse waves. And these are essentially analogous to, as I said, kind of what we see in water waves. Now, the more interesting thing are the body waves, because the body waves, first of all, they're the fastest moving waves. And these are also the waves that are used to figure out the structure of the Earth. So the body ways come in two varieties. You have your P-waves, or Primary waves. And you have your S-waves, or Secondary waves. And they're depicted right over here. And this is actually energy that's being transferred through a body. So it's not just moving along the surface of one. And so here in this diagram that I got from Wikipedia, which I think Wikipedia got from the US Geological Survey, we have a hammer being hit on some rock or whatever. And what you see is right when the hammer gets hit at this end of the rock, and I can zoom in a little bit-- so let's say I have this rock over here and I hit it right over here with a hammer or something. What that's immediately going to do is it's going to compress the rock that the hammer comes in touch with. It's going to compress that rock. But then that energy, essentially the molecules are going to bump into the adjacent molecules. And then those adjacent molecules are then going to bump into the molecules right next to it, and then they're going to bump into the molecules right next to it. So you're going to have this kind of compressed part of rock moving through the wave. So these are compressed, and those molecules are going to go bump into the adjacent molecules. So kind of immediately after that the rock will be denser right over here. The first things that were bumped, those will essentially bump into the ones right above them, and then they will kind of move back to where they were. And so now the compression will have moved, and if you fast forward it will have moved a little bit forward. So you essentially have this compression wave. You hit the hammer here, and you essentially have a changing density that is moving in the same direction of the wave. In this situation that is the direction of the wave, and you see that the molecules are kind of going back and forth along that same axis. They're going along the same direction as the wave. So those are P-waves. And P waves can travel through air. Essentially sound waves are compression waves. They can travel through liquid. And they can obviously travel through solids. And, depending, in air they'll travel the slowest. They'll essentially essentially, move at the speed of sound, 330 metres per second, which isn't really slow by every day human standards. In a liquid they'll move about 1,500 meters per second. And then in granite, which is most of the crustal material of the Earth, they'll move at around 5,000 meters per second. Let me write that down. So 5,000 meters per second, or essentially 5 kilometers per second if they're moving through granite. Now, S-waves are essentially-- if you were to hit a hammer on the side of this rock-- so let me draw another diagram since this is pretty small. If you were to hit a hammer right over here what it would do is it would temporarily kind of push all the rock over here. It would deform it a little bit, and that would pull a little bit of the rock back with it. And then this rock that's right above it would slowly be pulled down, while this rock that was initially hit will be moved back up. So you fast forward maybe a millisecond, and now the next layer of rock right above that will be kind of deformed to the right. And if you keep fast forwarding it the deformation will move upwards. And notice, over here, once again, the movement of the wave is upwards. But now the movement of the material is not going along the same axis that we saw with the P-waves, or the compression waves. It's now moving perpendicular. It's now moving along a perpendicular axis, or you could call this a transverse wave. The movement of the particles is now on a perpendicular axis to the actual movement of the waves. And so that's what an S-wave is. And they move a little bit slower than the P-waves. So if an earthquake that were to happen you'd see the P-waves first. And then at about 60% of the speed of the P-waves you would see the S-waves. Now, the most important thing to think about, especially from the point of view of figuring out the composition of the Earth, is that the S-waves can only travel through solid. And you might say, wait, I've seen transverse waves on water that look like this. But remember, that is a surface wave. We are talking about body waves. We're talking about things that are actually going through the body of water. And one way to think about this is if I had some water over here-- so let's say that this is a pool. I'll draw a cross section of water. I could have drawn it better than that. If I have a cross section of water right over here, let's think about it, and hopefully it'll make intuitive sense to you. If I were to compress some of the water, if I were to kind of slam some part of the water here with like a big, I don't know, some type of-- I would just compress it really fast. A P-wave could transmit, because those water molecules would bump into the water molecules next to it, which would bump into the water molecules next to that. And so you would have a compression wave, or a P-wave, moving in the direction of my bump. So P-waves waves it makes sense, and the same thing is true with air or sound waves, that it make sense that it could travel through a liquid. And remember, we're under the water. We're not thinking about the surface. We're thinking about moving through the body of the water. Let's say that you were to kind of take that hammer and kind of slapped the side of this little volume of water here. Well, essentially all that would do is it would send a compression wave in that direction. It really wouldn't do anything. It wouldn't allow a transverse wave to go that way, because the water doesn't have this elastic property where if something bounces that way it's going to immediately bounce back that way. It's being pulled back like a solid would. So S-waves only travel through solids. So we're going to use essentially our understanding of P-waves, which travel through air, liquid, or solid, and our understanding of S-waves to essentially figure out what the composition of Earth is.