Seismic Waves

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Seismic Waves : S-waves and P-waves

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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