Download S Waves - new-technologies

1
Earthquakes occur due to movements that take place within the Earth.
Large pressures build up between huge plates of land both above and
below the sea. The release of these pressures is accompanied by huge
releases of energy as one land mass slides against another.
2
Earthquakes occur due to movements that take place within the Earth.
Large pressures build up between huge plates of land both above and
below the sea. The release of these pressures is accompanied by huge
releases of energy as one land mass slides against another.
Shock waves
travel out from
the “epicentre”.
They are also
called Seismic
Waves.
This is the Earthquake.
3
Shock Waves
There are three types of shock waves called L, P and S.
L waves only travel close to the surface so they do not tell us
much about the structure of the Earth. They travel through a thin
surface called the crust.
P waves travel and penetrate below the crust.
S waves travel slower then P waves
and they too penetrate
below the crust.
We shall only concern ourselves with P and S waves.
4
P waves
It is useful to think of these as “push” waves.
They are longitudinal waves.
Compression
Rarefaction
It seems to push here.
The motion of the particles in a longitudinal wave is
parallel to the direction of the motion of the wave.
It is important to remember that a P wave (which is
longitudinal) can travel through solids and liquids.
5
S Waves
It is useful to think of these as “shake” waves.
They are transverse waves.
It seems
to
“shake”
here
The motion of the particles in a transverse wave, is
perpendicular to the direction of the motion of the wave.
It is important to remember that an S wave (which is
transverse) can travel through solids only.
6
Summary
P waves
•It is useful to think of these as
push waves.
•They are longitudinal waves
S Waves
•It is useful to think of these as
shake waves.
•They are transverse waves
•The motion of the particles in a
•The motion of the particles in a
longitudinal wave, is parallel to the transverse wave, is perpendicular
direction of the motion of the wave. to the direction of the motion of the
wave.
P waves can travel through solids S waves can travel through solids
and liquids.
only.
7
S waves
Epicentre
The waves spread out
through the Earth and are
received at the surface.
We assume that the
shadow region
forms behind
something that the S
waves cannot travel
through.
Cross-section of
the Earth
Liquid Core
There is a huge
region where no S
waves are received
at all. A shadow
region perhaps?
S waves can’t
travel through a
liquid
8
P waves
We also suspect a solid inner core.
Again there is a shadow region.
But the
longitudinal P
waves also pass
through the core.
9
The Structure of the Earth
There is a thin CRUST
between 5 and 50 km
thick.
There is a sharp change
in density below this
The change occurs
as we pass to the
MANTLE, a very
hot region.
Temperatures reach
1,200oC.
10
The Structure of the Earth
There is a thin CRUST.
The MANTLE is a
very hot region.
The OUTER CORE is
LIQUID with
temperatures ranging
from 2,000 to 5,000oC.
The SOLID INNER CORE
is very dense - perhaps 3
times that of the crust.
11
THE MANTLE
THE INNER CORE
THE OUTER CORE
THE CRUST
12
More about Seismic
Waves
It is clear that the waves change direction below the surface of the
Earth.
They do this due to a change in speed and the process is called
refraction.
If the speed changes gradually,
the path is gently curved.
Fast
Slow
Fast
Slow
13
More about Seismic
Waves
The speed change is caused by a change in density of the material.
The more dense the material, the faster it travels.
Note that light slows down in more dense materials which is exactly
the opposite to Seismic Waves and sound waves.
The Seismic Waves behave as if they have
speeded up as they travel deeper into the Earth.
You should be able to convince
yourself from the path the seismic
waves take, that the Earth must be
getting more dense the deeper you
go down.
Earth
14
More about Seismic
Waves
You will have noticed that when the Seismic Waves reach the
surface again, they come up from almost directly below.
This has important consequences for the way in which
earthquakes shake buildings.
S wave
Shake wave
Transverse
P wave
Push wave
Longitudinal
15
More about Seismic
S wave
Shake wave
Transverse
Waves
The shake waves
cause the
buildings to
move from left
P wave
to right - in the Push wave
horizontal
Longitudinal
16
More about Seismic
S wave
Shake wave
Transverse
Waves
The push waves
cause the
buildings to
move up and
P wave
down - in the
Push wave
vertical
Longitudinal
17
And Finally
P waves are really called PRIMARY waves
S waves are really called SECONDARY waves
And L waves travel the slowest
Can you guess which is which on the seismograph below?
S waves
displacement
P waves
time
L waves
18
This seismometer detects vertical vibrations
It detects P waves
The drum rotates.
When there is an
earthquake, the heavy
mass tries to keep still
whilst everything
around it vibrates up
and down.
spring
pivot
But to us it seems as if the mass moves
and then the pen leaves a trace on the
drum.
Earth
Earth
19
This seismometer detects vertical vibrations
It detects P waves
The drum rotates.
When there is an
earthquake, the heavy
mass tries to keep still
whilst everything
around it vibrates up
and down.
spring
pivot
But to us it seems as if the mass moves
and then the pen leaves a trace on the
drum.
Earth
20
This seismometer detects horizontal vibrations
It detects S waves
The drum and moving heavy mass
is mounted so that it can move
horizontally
21
22