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Current time:0:00Total duration:8:38

Video transcript

what I want to do in this video is talk a little bit about seismic waves size Micke 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 figure it out what the different of 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 that travel through the earth that could be due to an earthquake could be due to an earthquake earthquake or just really any kind of large explosion an explosion or anything that really essentially starts sending energy through the rock on earth or really through through earth itself now there's two fundamentally different types of seismic waves and we're going to focus on one more than the other one is surface waves one is surface waves and the other is body waves 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 Rallye waves and love waves we won't go into a lot of details but you can see that rrally 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 the essentially the ground shifting left and right so here it's not moving up and down but here it's moving if you're if you're going if you're facing the direction of the wave movement it's moving 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 that we've got sometimes called 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 waves come in two varieties you have your P waves or primary waves you have your P waves and you have your S waves or secondary or secondary waves and they're depicted right over here and these are these are actually 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 and I hit it right over here with a hammer or something what that 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 that 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 bump into the adjacent molecule so kind of immediately after that the rock will be denser right over here the first things that we're 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 moved a little bit forward so you essentially have this compression wave if you hit the hammer here and you essentially have a changing density that is moving that is moving in the same direction of the wave in this situation in this situation that is 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 P waves can travel through air that's what essentially sound waves our compression waves they can travel through liquid they can travel through liquid and they can obviously travel through solids and depending and they'll travel the slowest will essentially move at the speed of sound 330 meters per second which isn't really slow by everyday human standards in a liquid they'll move about 1500 meters per second and then in Granite which is most of the the kind of crustal material of the earth they'll move it 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 a granite now S waves S waves are essentially if you were to hit a hammer on the side of this rock so let me draw another 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 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 maybe I should draw it actually let me draw it like this the next layer of rock right above that will be kind of deformed 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 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 not moving along a perpendicular axis or you can 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 wave so if an earthquake that were to happen you would 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 S waves can only only travel through solid 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 you're talking about body waves we're talking about things that are actually going through the body of water and a one way to think about this is if I had if I had some water over here so let's say that this is a pool I'll draw a cross-section of water if I draw a cross-section of water I could have drawn it better than that let me 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 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 cules next to that and so you would have a compression wave or a pea wave moving in the direction of my bump so P waves it makes sense and the same thing is true with air or sound waves that it makes sense that it could travel through a liquid but let's say that you let's say that if I remember were 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 let's say that you were to kind of take that hammer if you were to take a hammer and kind of slap the side of this little volume of water here well essentially all that would do it would send a compression wave it would sent a compression wave in that direction it really wouldn't do anything it wouldn't it wouldn't allow a transverse wave to go that way because it's not going to the the the water doesn't allow it to kind of it doesn't have this elastic property where something bounces that way it's going to immediately bounce back that way it's not being pulled back like a solid wood so S waves only 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 ways to essentially figure out to essentially figure out what the composition of earth is