Seismic waves and how we know earth's structure
Seismic waves S-waves and P-waves
- 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
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