Download Seismic Waves File

Waves can travel through solids as well as liquids and gases. Here are just a
few examples:
 You can often feel fast cars and heavy lorries
passing close to a house;
 It was said that American Indians used to put their
ear to the ground to hear approaching horse riders;
 By putting a glass nest to your ear and pacing it on
an adjoining wall, you can hear what is being said in
the nest room.
The ability of the wave to travel through the material depends very much upon
its properties. By study Earthquake waves, seismologists are able to
indirectly ‘see’ into the depths of the Earth and tell us about its structure.
Earthquakes
Earthquakes happen when rocks
suddenly fracture and move. The
tremendous energy released during
the fracturing process makes the
ground vibrate and it is these
vibrations that travel away from the
focus of the quake.
Obviously the passage of the
earthquakes can cause serious
damage. Often the ground can be
seen moving, if enough movement
occurs then building can fall and
mountains can break up.
Earthquakes can also occur under the
sea. The ground movements form big
waves that get even higher as they get
closer to the coast.
These vibrations are picked up by seismometers, which measure the
strength of the waves. Seismographs show that there are three main types of
waves:
1.
P-Waves Primary waves are the
fastest waves and are therefore the first
to be recorded.
They are longitudinal push-pull
waves and can travel through
liquids as well as solids.
2. S-Waves- Secondary waves are slower
than P waves.
They are transverse shake and
shear waves and can only travel
through solid materials.
Surface Waves- These waves are the
slowest waves and only travel along the
surface or crust of the Earth.
There are two types of surface waves –
longitudinal and transverse waves. These
are the waves that do the most damage
when the earthquakes occur.
The black dots show
the distribution of
the focus of
Earthquakes around
the world.
The pattern of
Earthquakes is
evidence of the crust
of the Earth being
split up into tectonic
plates.
Questions
The following sketch shows a seismograph record of an earthquake.
Start of P
Waves
Start of S
Waves
Start of surface
Waves
Time in Minutes
P Waves
P and S
P+S+Surface
1. Which wave arrived first? ______________________________________
2. Both P and S waves travelled through the Earth to arrive at the station.
Which wave travelled faster? __________________________________
3. What was the time delay between:
(i)
P and S waves? _____________________________________
(ii)
P and Surface waves? ________________________________
(iii)
S and Surface waves? ________________________________
4. An elapse time of 1 minute corresponds to a distance of approximately
800 km between the epicentre (point on the Earth’s surface directly above
focus) and the recording station.
If the recording station records an elapse time of 3 ½ minutes, how far
away is the epicentre?
_________________________________________________________
_________________________________________________________
_________________________________________________________
5. Why is it not possible for one seismograph to pinpoint the epicentre of an
earthquake?
___________________________________________________________
___________________________________________________________
________________
6. How is it possible to find the epicentre of an earthquake?
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
7. A powerful earthquake
has struck somewhere in
Europe. The relief
agencies want to get help
to the area as quickly as
possible, but they have
not located the epicentre
yet. Help them to locate
its position on the map
from the information
gained by some recording
stations.
Elapse times between P
and S waves:
 London 1 ½ min,
London
Madrid
Naples
0
0
400
800
1 min
1200
1600
2 min
2000
2400 km

Madrid 2 min,

Naples ¾ min.
3 min
Time between arrival of P and S waves
7. Seismic waves are caused by __________________________________
8. Seismic waves start in the Earth’s ___________ from a point called the
__________
9. The point directly above this on the Earth’s surface is called the
______________
10. Seismic waves are detected using a ____________________________
11. The _____ waves are the fastest and can travel through _________ and
_________
12. The _____ waves are slower, they can only travel through ___________
13. Which regions of the Earth get the most earthquakes and why?
___________________________________________________________
___________________________________________________________
___________________________________________________________
Seismic records have led geologist to believe that the Earth has a layered structure, and
can be thought of as being a bit like a cracked egg.
The solid crust is broken up into
smaller bits, called plates, which float
on a dense mantle. Parts of the
mantle are molten liquid and
movements in this liquid cause the
plates to drift into one another. It is
the meeting of the plates that causes
the earthquakes.
The density of the material forming
the Earth increases with depth.
Nuclear reactions within the solid
inner core maintain the high
temperatures of the body of the
Earth.
The outer core is liquid and large
convection currents within this layer
give rise to the Earth’s magnetic field.
P-Waves
The speed of both types of waves
increase as they travel through the
Earth’s interior.
However, the speed rapidly changes
when they cross between layers of
material.
S-Waves
The speed of the p waves decrease
considerably when encountering the
outer core, but S waves are stopped
altogether.
A
B
Direct
P-wave
shadow
zone
The diagram above shows the paths taken by P and S waves as they travel through the
interior of the Earth.
 Point A - Both P and S waves are gradually refracted as they travel and therefore
follow curved paths as they spread out from the epicentre. This indicates that the
density of the material within a layer gradually increases with depth.
P and S waves are detected up to angles of 1030 from the epicentre.

Point B - At certain points the refraction becomes more pronounced as the
waves meet a definite boundary between layers.
No S waves can travel through angles greater than 1030 hence the boundary at B
must separate solid from liquid. This is the evidence that the outer core is made of
a dense liquid.

P waves entering the liquid outer core slow down and are focused (like waves
travelling through a lens) into the region at the bottom of the diagram (angles greater
than 1400). A shadow zones for direct travelling P waves exists between 1030 and
1400, however, weak P waves are detected in this region due to the refraction of the
inner core.