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What I want to do in this video is take a little bit about Seismic Waves

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One because they are interesting by themeselves

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But they are also really useful for figuring out what the actual composition of the earth is.

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You've seen my video on the actual layers of the earth

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and seismic waves are crucial to actually realizing how people figured out what the different layers of the earth are

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And just to be clear seismic waves, they are normal associated with earth quakes

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but they are any waves that travel through the earth

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That could be due to an earthquake or really just any kind of large explosion

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Or anything that really in a sense starts sending energy through the rock on earth

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Or really through earth itself

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Now there are two fundamentally different types of seismic waves

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And we are going to focus on one more than the other

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One is surface waves and the other is body waves

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Now surface waves are one that literally travel across the surface of something

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In this case we are talking about the surface of the ground

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And this right here is a depiction of surface waves

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And these really are more analogous to the type of waves we normally associate with the suface of water

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And there are two types of surface waves: Rally waves then love waves

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We won't go into a lot of detail but you can see that

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Rally waves are kind of the ground moving up and down.

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Right here the ground is moving up, here i'ts moving down, here it's moving up, here it's moving down'

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So you can kind of view it as kind of a ground roll

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The love waves are essentially the ground shifting left and right.

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So here it's not moving up and down but if you are facing the direction of the wave movement

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Its moving to the left here, here its moving to the right, here its moving to the left, here its moving to the right

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In both cases, the movement of the surface wave is perpendicular to the direction of motion

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So, we sometimes call these transverse waves

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And these are essentially analogous to, as I said, kind of, what we see in water waves

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Now the more interesting thing are the body waves, because the body waves...

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First of all they are the fastest moving waves

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and these are also the waves that are used to figure out the structure of the earth

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So the body waves come in two varieties

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You have your P-waves or Primary Waves

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And you have your S-waves, or secondary waves

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And they are depicted right over here

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And these are actually energy that is being transfered through a body,

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So it's not just moving along the surface of one

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And so, here in this diagram that I got from Wikipedia,

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Which I think Wikipedia got from the US Geological Survey

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We have a hammer being hit on some rock or whatever

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And what you see is right when the hammer gets hit at this end of the rock,

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And I can zoom in a little bit

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So lets say that I have this rock over here and I hit it right over here with a hammer

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What that's immediately going to do, it is going to compress the rock

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That the hammer comes in touch with

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Its going to compress that rock

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But then that energy is essentially the molecules are going to bump into the adjacent molecules

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And then those adjacent molecules are going to bump into the molecules right next to it

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And then they are going to bump into the molecules right next to it

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So you are going to have this kind of compressed part of rock moving through the waves

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So these are compressed and those molecules are going to bump into the adjacent molecules

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So kind of immediately after that the rock will be denser right over here

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The first thing that were bumped those will essentially bump into the ones right above them

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And then they will kind of move back to where they were

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And now the compression will have moved and if you fast-forward it will have moved a little forward

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So, you essentially have this compression wave

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You hit the hammer here and you essentially have a changing density

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That is moving in the same direction of the wave

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In this situation, that is the direction of the wave

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And you see the molecules are kind of going back and forth along that same axis

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They are going along the same direction as the wave

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So those are P-waves

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And p-waves can travel through air, thats essentially what sound waves are, compression waves.

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They can travel through liquid

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And they can obviously travel through solids

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And depending, in air they'll travel the slowest

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They'll essentially move at the speed of sound, 330 meters per second which isn't really slow by everyday human standards

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In liquid they'll move about 1500 meters per second

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And then in granite, which is most of the crustal material of the earth

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They'll move at 5000 meters per second, let me right that down

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or actually 5 kilometers per second if they are moving through granite

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Now S-waves are essentially, if you were to hit a hammer on the side of this rock

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So, let me draw another diagram since this is pretty small

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If you were to hit a hammer right over here what it would do is it would

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Temporarily kind of push all the rock kind over here it would deform it a little bit

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and that would pull a little bit of the rock back with it and then this rock thats right above it

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would slowly be pulled down while this rock that initially hit

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will be moved back up

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So, you fast forward maybe a mili-second and now the next layer of rock right above that

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Maybe I should actually draw it like this

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the next layer of rock right above that will be kind of deformed to the right

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and if you keep fast forwarding it the deformation will move upwards

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And notice, over here, once again, the movement of the wave is upwards

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But, now the movement of the material is not going along the same axis

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that we saw with the p-waves or compression waves

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It is now moving perpendicular

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It is moving along a perpendicular axis or you could call this a transverse wave

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The movement of the particles is now on a perpendicular axis to the actual movement of the waves

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And so that is what an S-wave is

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And they move a little bit slower than the p-waves

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So if an earthquake were to happen you'd see the p-waves first

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and then at about 60% of the speed of the p-waves you would see the s-wave

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Now the most important thing to think about

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especially from the point of view of figuring out the composition of the earth

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is that the s-waves can only travel through solid

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And you might say, "Wait! I've seen transvesre waves on water that look like this."

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But, remember, that is a surface wave

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We are talking about body waves

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We're talking about things that are actually going through the body of water

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And one way to think about this is...

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If I had some water over here

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So, let's say that this is a pool

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I'll draw a cross-section of water, I could have drawn it better than that

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If I have a cross-section of water right over here

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Lets think about it and hopefully it will make intuitive sense to you

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If I were to compress some of the water

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If I were to slam some part of the water here with like a big, i don't know,

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If I were to just compress it really fast it would do...

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The p-wave could transmit because those water molecules would bump into the water molecules next to it

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which would bump into the water molecules next to that

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So you would have a compression wave or a p-wave moving in the direction of my bump

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So p-waves it makes sense, and the same thing is true of air, sound waves

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And it makes sense that it could travel through a liquid

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But let's say that you, and remember we're under the water, we're nothing thing about the surface

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We're thinking about moving through the body of the water

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Let's say that you were to kind of take that hammer and kind of slap the side of this volume of water here

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Well, essentially all that would do, it would send a compression wave in that direction

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It really wouldn't do anything it wouldn't allow a transverse wave to go that way

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The water doesn't allow it to kind of, it doesn't have this elastic property

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Where something bounces that way it is going to immediately bounce back that way

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It is not being pulled back like a solid would

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So, s-waves only travel through solids

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So we are going to use, essentially our understanding of p-waves

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Which travel through air, liquid, and solid

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And our understanding of s-waves to essentially figure out what the composition of earth is