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S-waves and P-waves. Created by Sal Khan.
Video transcript
What I want to do in this video is take a little bit about Seismic Waves One because they are interesting by themeselves But they are 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 are normal associated with earth quakes but they are any waves that travel through the earth That could be due to an earthquake or really just any kind of large explosion Or anything that really in a sense starts sending energy through the rock on earth Or really through earth itself Now there are two fundamentally different types of seismic waves And we are going to focus on one more than the other One is surface waves and the other is body waves Now surface waves are one that literally travel across the surface of something In this case we are 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 suface of water And there are two types of surface waves: Rally waves then 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 i'ts 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 if you are facing the direction of the wave movement Its moving to the left here, here its moving to the right, here its moving to the left, here its 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 are the fastest moving waves and these are also the waves that are used to figure out the structure of the earth So the body waves come in two varieties You have your P-waves or Primary Waves And you have your S-waves, or secondary waves And they are depicted right over here And these are actually energy that is being transfered 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 lets say that I have this rock over here and I hit it right over here with a hammer What that's immediately going to do, it is going to compress the rock That the hammer comes in touch with Its going to compress that rock But then that energy is essentially the molecules are going to bump into the adjacent molecules And then those adjacent molecules are going to bump into the molecules right next to it And then they are going to bump into the molecules right next to it So you are going to have this kind of compressed part of rock moving through the waves So these are compressed and those molecules are going to bump into the adjacent molecules So kind of immediately after that the rock will be denser right over here The first thing 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 now the compression will have moved and if you fast-forward it will have moved a little 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 the molecules are kind of going back and forth along that same axis They are going along the same direction as the wave So those are P-waves And p-waves can travel through air, thats essentially what 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 move at the speed of sound, 330 meters per second which isn't really slow by everyday human standards In liquid they'll move about 1500 meters per second And then in granite, which is most of the crustal material of the earth They'll move at 5000 meters per second, let me right that down or actually 5 kilometers per second if they are 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 kind 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 thats right above it would slowly be pulled down while this rock that initially hit will be moved back up So, you fast forward maybe a mili-second and now the next layer of rock right above that Maybe I should actually draw it like this 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 compression waves It is now moving perpendicular It is 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 is what an S-wave is And they move a little bit slower than the p-waves So if an earthquake 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-wave 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 transvesre 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 Lets think about it and hopefully it will make intuitive sense to you If I were to compress some of the water If I were to slam some part of the water here with like a big, i don't know, If I were to just compress it really fast it would do... The 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 So you would have a compression wave or a p-wave moving in the direction of my bump So p-waves it makes sense, and the same thing is true of air, sound waves And it makes sense that it could travel through a liquid But let's say that you, and remember we're under the water, we're nothing thing 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 slap the side of this volume of water here Well, essentially all that would do, 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 The water doesn't allow it to kind of, it doesn't have this elastic property Where something bounces that way it is going to immediately bounce back that way It is not being pulled back like a solid would So, s-waves only travel through solids So we are going to use, essentially our understanding of p-waves Which travel through air, liquid, and solid And our understanding of s-waves to essentially figure out what the composition of earth is