December 25, 2019 9

The mohorovicic seismic discontinuity | Cosmology & Astronomy | Khan Academy

The mohorovicic seismic discontinuity | Cosmology & Astronomy | Khan Academy


The first realization
that there were actually distinct layers of the earth
came from this guy right over here, Andrija Mohorovicic. And I apologize ahead
of time to any Croatians for butchering any
of the pronunciation. And he was a meteorologist
and a seismologist. And he was the
first one to notice, in 1909, when there
was an earthquake. There was an
earthquake in Croatia, a little bit
southeast of Zagreb. So the earthquake was
roughly over here. And lucky for him and
lucky for us, before that earthquake there
was actually a bunch of seismographic stations
already in the area. And all these
seismographic stations are are, essentially,
instruments were installed so that if there
was any essentially seismic waves
passing, they would be able to measure it
when the waves got there. And what was
interesting about this, Andrija realized that if
the entire earth was just kind of a uniform materials–
let’s draw that scenario– it would get denser
as you go down. And so you would have
kind of this refraction, this continuous refraction, or
these curved pats, happening. But he realized that, let’s
say we had an earthquake right over here, so this
is the uniform case. Uniform. Uniform layer, only one layer,
although it does get denser. Then the closer you
are to the earthquake– so waves would get
there first, then waves would get over there, then
waves would you get over there– and these are the body waves. These are the ones
that are traveling through the earth’s crust. But in general, the further you
are away from the earthquake, or the time it takes for
the waves to get to a point, is going to be proportional
to the distance that point is away
from the earthquake. So you would expect to
see something like this. So if you were to plot
on the horizontal axis, if you were to plot distance,
and on the vertical axis you were to plot time, you
should see something like this. You should see a straight line. And that’s just
because it’s traveling roughly the same velocity
along any of these arcs. It’s maybe getting a little bit
faster as it’s getting deeper. But roughly the same
velocity as it’s traveling along these arcs. And the distance
of these arcs are proportional to the
distance along the surface, along the distance
of the surface. So essentially, the time is,
they’re all traveling roughly at the same velocity,
and their just traveling different distances,
so the time it takes is just going to be
proportional to the distance. But he noticed
something interesting. When he actually measured when
the waves from that earthquake reached different
seismographic stations, he saw something interesting. So this is in the
theoretical, if we had a kind of this
uniform one-layered earth. But he saw something
interesting. So once again, this
is the distance, and this right
over here is time. And at 200 kilometers,
at 200 kilometers away from the earthquake–
so until 200 kilometers, he saw exactly what you would
expect from a uniform earth. It was just the time took was
proportional to the distance. But at 200 kilometers, he
saw something interesting. All of a sudden, the waves
were reaching there faster. The slope of this line changed. It took less time for
each incremental distance. So for some reason,
the waves that we’re going at these farther
stations, the stations that were more than 200
kilometers away, somehow they were accelerated. Somehow they were
able to move faster. And he’s the one that
realized that this was because the waves
that were getting to these further stations must
have traveled through a more dense layer of the earth. So let’s just think about it. So if we have a
more dense layer, it will fit this
information right over here. So if we have a layer
like this, which we now know to be the crust, and then
you have a denser layer, which we now know to be the mantle,
then what you would have is– so you have your earthquake
right over here, closer by, while you’re still
within the crust, it would be proportional. It would be proportional. And then let’s say
that this is exactly, this right here is
200 kilometers away. But then if you go any further,
the waves would have to travel. They would travel, so
they would go like this. And then they would get
refracted even harder. So they would get refracted. So they would be a little
bit curved ahead of time. But then they’re going to
a much denser material. Or it’s not gradually dense,
it’s actually kind of a all of a sudden a considerably
more dense material, so it will get
refracted even more. And then it’ll go over here. And since it was
able to travel all of this distance in
a denser material, it would have traveled
faster along this path. And so it would get
to this distance on the surface that’s more
than 200 kilometers away, it would get there faster. And so he said that there
must be a denser layer that those waves are
traveling through, which we now know
to be the mantle. And the boundary
between what we now know to be the crust
and this denser layer, which we now to be the mantle,
is actually named after him. It’s called the
Mohorovicic discontinuity. And sometimes this is
called the Moho for short. So that boundary between
the crust and the mantle is now named for him. But this was a huge discovery,
because not only was he able to tell us,
based on the data– based on, kind of, indirect
data, just based on earthquakes happening, and measuring
when the earthquakes reach different points of the
earth– that there probably is a denser layer. And if you do the math,
under continental crust that denser layer is
about 35 kilometers down. He was able to tell us
that there is that layer. But even more
importantly, he was able to give the clue that
just using information from earthquakes,
we could essentially figure out the actual
composition of the earth. Because no one has
ever dug that deep. No one has ever dug into
the mantle, much less the outer core or
the inner core. In the next few
videos, we’re going to kind of take this insight,
that we can use information from earthquakes,
to actually think about how we know that there
is an outer liquid core and that there’s an
inner core, as well. And then, obviously,
you could keep going and think about all
the different densities within the mantle
and all of that. I won’t go into
that much detail, but I’ll see you
in the next video.

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