Download Earthquakes 4 Using Quakes1 Earth Structure

Using Earthquakes 1
Using Seismograms
In the 1800s, seismographs became more
commonplace. Seismograms of a single
earthquake could be collected from around
the world so they can be carefully inspected.
Using Seismograms
Scientists learned some important facts:
1. P-waves are faster than S-waves.
2. S-waves cannot go through fluids.
3. The density of Earth’s material changed
(Discovered in 1909 by Mohorovičić. This
changes the speed and refracts the wave).
4. The interior of the Earth has a specific
structure.
Using Seismograms
Scientists noticed that s-waves never went
much past halfway around the globe.
Scientists also noticed that there were wide
areas that would never record p-waves.
Regardless of how big the quake was.
It was known that s-waves could NOT travel
through a fluid. Also, material of different
density would refract p-waves . . .
. . . so the explanation was easy.
Using Seismograms
The Earth has a molten core in its center!
Using Seismograms
They called this the
shadow zone.
Because the area
always seemed to
be at an angle of
110̊ away from the
earthquake, they
calculated how big
the core is.
Using Seismograms
By making a graph of the seismic wave speed vs.
the depth of the Earth, scientists were able to see
other features.
Using Seismograms
With improved seismographs, we are able to see p
and s-waves bouncing off the inside of the Earth!
These are called ss or pp-waves when they have
reflected once. (ppp-waves for three reflections.)
In the shadow-zone, there are no direct p or swaves, but there are pp, ss, ppp, and sss-waves.
P-waves refracting in the core are called k-waves.
When they re-emerge, they are called pkp-waves.
K-waves that penetrate the solid, inner core are
called i-waves.
Using Seismograms
Scientists have re-created how waves move through
different types of rock (at different pressures and
temperatures) in laboratories.
By comparing these recreations to what is seen
from actual seismograms, we have deduced the
structures that make up the Earth’s interior.
Earth’s Layers
Earth’s layers can be viewed in two different ways:
• Compositional Layers and
• Mechanical Layers
Earth’s Layers
Compositional Layers
These are layers divided by
what they are made of.
–Crust: Made of felsic and
mafic rock, 0-65 km deep.
–Mantle: Made of liquid and
solid rock, 65-2890 km deep.
–Core: Made of iron, nickel,
and sulfur 2890-6396 km
deep.
Earth’s Layers
Mechanical Layers
Divided by how P and S waves move through them.
– Lithosphere:
• Crust and top-most part of mantle. Most
earthquakes start here P and S waves move through
them.
– Asthenosphere:
• In the mantle, P and S waves move through them, but
will slow down and refract due to material change.
– Mesosphere:
• Where the mantle becomes more solid and more
Earth’s Layers
Mechanical Layers
Divided by how P and S waves move through them.
– Outer Core:
• Liquid iron, nickel and sulfur. P waves slow down, S
waves stop.
– Inner Core:
• Solid iron and nickel. P waves speed up and refract
due to state change.