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Seismic waves | Earth geological and climatic history | Cosmology & Astronomy | Khan Academy


What I want to do in this
video is talk a little bit about seismic 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 figured out what the
different 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 the Earth. They could be due to
an earthquake, or just really any kind of a large
explosion, or anything that really essentially
starts sending energy through the rock on Earth,
really through Earth itself. Now, there’s two
fundamentally different types of the seismic waves. And we’re going to focus
on one more than the other. One is surface waves. And the other is 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, rally waves, and 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 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
essentially the ground shifting left and right. So here it’s not
moving up and down, but here it’s moving,
if you’re facing the direction of the wave
movement, 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 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’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 ways come
in two varieties. You have your P-waves,
or Primary waves. And you have your S-waves,
or Secondary waves. And they’re depicted
right over here. And 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 it right over here
with a hammer or something. 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 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 go bump into
the adjacent molecules. So kind of immediately
after that the rock will be denser right over here. The first things
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 so now the compression will
have moved, and if you fast forward it will have moved
a little bit 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 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 can
travel through air. Essentially 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
essentially, move at the speed of sound, 330
metres per second, which isn’t really slow by
every day human standards. In a liquid they’ll move
about 1,500 meters per second. And then in granite,
which is most of the crustal
material of the Earth, they’ll move at 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 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 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 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 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 the compression waves. It’s now moving perpendicular. It’s now 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’s what an S-wave is. And they move a little bit
slower than the P-waves. So if an earthquake that
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-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 can 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 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, 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 would just compress it really fast. A 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. And so you would have a
compression wave, or a P-wave, moving in the
direction of my bump. So P-waves waves it makes
sense, and the same thing is true with air or sound
waves, that it make sense that it could travel
through a liquid. And remember, we’re
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 take that hammer and kind of slapped
the side of this little volume of water here. Well, essentially
all that would do is 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, because the water doesn’t have
this elastic property where if something bounces
that way it’s going to immediately
bounce back that way. It’s being pulled back
like a solid would. So S-waves 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-waves to essentially figure out what
the composition of Earth is.

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