Seismic waves are mechanical vibrations that occur
inside the Earth.
They are caused
when rock breaks or is deformed. When this happens, waves of energy are
released and if strong enough, cause the surface of the Earth to tremble
noticibly. Many Earth tremors are of such low magnitude they are not noticed
The classic, standard
model of seismic waves dictates that seismic waves occur
along fault lines.
Why do the
Great stress is
placed on rocks along these boundaries. If the stress becomes too great,
the tectonic plates slide along each other
or slip under each other, releasing the built-up energy in waves. Faults
are weak and only need a small stress to cause them to move. One possible
cause for earthquakes lies deep under the surface. Far below the surface
of the Earth, temperatures and pressures are tremendous. These quantities
can become so great that one mineral can be transformed into a new, denser
mineral. If this process happens over a large enough area, the old area
loses its base of support and drops down. This action would cause tremors.
The 'slash' across
the crop rows in the photo on the right show where a probably fault line lies. (Note how the crop
rows are shifted)
Types of Seismic
are several types of seismic waves. The three that you need to know are
P-waves and S-waves (both BODY WAVES - travelling through the body of
the Earth) and L-waves (which are SURFACE WAVES - travelling allong the
All of them travel faster the denser the matter they travel through (cf.
sound in Y8)
waves move out from the focus of the quake in all directions.
Body waves come
in two main forms: S and P waves.
- They are
Primary waves - also called
- They travel
the fastest - Phastest (sorry!
- sad I know, but if if helps you remember...)
- They travel
by particles vibrating Parallel
to the direction the wave travel (they are longitudinal Push
- They travel
through solids, liquids and gases - all Physical
P waves are compressional
or longitudinal waves; that is, the medium vibrates parallel to the direction
that the wave energy is traveling.
A P wave travels
fastest and arrives first at a detector. For this reason, these waves
are called primary waves (hence the letter "P"). A P wave can travel
through liquid and gas.
- They are
Seconday or Shear
- They travel
Slower than P-waves.
- They travel
to right angles to the vibration of the particles - they are tranSverse(see
cannot pass through liquids or gases - only travel through solids
- Solely Solids.
An S wave is slower
and arrives at the detector second. For this reason, S waves are called
secondary waves ('cos they arrive second!).
These waves are transverse waves. In transverse
waves, the medium vibrates perpendicularly to the direction of enegy travel.
As these travel
through the 'crust of the Earth' (lithosphere). I had through that they
got their name from that! But they actually get their name from the scientist/mathematician
who discovered them! - A.E.H. Love. There is some erroneous information
on the WWW about this!
A Love wave is
a surface wave. It is the surface waves that are most damaging as they
cause the earth's crust to undulate. (R-waves are also surface waves but
we don't need to know about them!)
waves travel along the surface of the earth from the point directly
above the quake or epicenter. If large enough,
they may actually cause ripples on the surface. These waves are the ones
that cause most of the damage.
like light or sound waves can be reflected when the waves strike a boundary.
In this way, geologists can detect where boundaries between rock lie.
use the idea of refraction to locate rock boundaries. Refraction occurs
when a wave passes from one medium to another. The result is that the
wave bends. The wave bends toward the normal if the new medium is denser
and away from the normal if the medium is less dense.
By knowing the
initial direction of the seismic wave and where it ends up, geologists
not only learn that a different type of rock layer is present but also
learn the relative density of each layer. Geologists can also use this
information to tell where fault lines lie. Using these ideas, scientists
have learned that the Earth's crust and much of the mantle are solid.
Rays 'bend towards
the normal' when they 'slow down'. In solids the denser the material,
the faster the vibrations travel. Therefore in A the wave is speeding
up and the grey area must be denser rock. In B it is the other way round!
The Earth's structure
waves also tell us a great deal about the core. Recall that P waves can
travel through liquids whereas S waves cannot. When an earthquake occurs,
both S and P waves radiate from the focus. Because the rocks get denser
as depth increases the path of the waves bends - see the diagram.
The S waves are
detected over a little more than one-quarter of the earth's surface (103° to be exact) . Beyond that, no S waves are seen. This tells us then that
for some reason, the S waves do not travel through the core. Hence,
the core must be made of liquid. A large, quiet S wave shadow zone
is created on the other side of the Earth.
In contrast, the
P waves are detected on the opposite side of the Earth as the focus. A
shadow zone from 103° to 142° does exist from P waves, though. Since waves
are detected, then not, then reappear again, something inside the Earth
must be bending the P waves and bending them towards the normal. From
this evidence using waves, we can tell that part of the core is liquid
(S wave shadow) and part (the inner part) must be solid with a different
density than the rest of the surrounding material (P wave shadow zone
due to refraction). In actuality, the inner core is thought to be made
of solid iron and nickel.
an Earthquake Epicentre
To find the epicenter
of an Earthquake you need to use at least three seismograms. Each earthquake
centre can work out how long it took for the P waves to reach the detector
by looking at the difference in the time it took for the P and S waves
to arrive. From that information they can then work out how far away the
epicentre is. But they do not know the direction that the vibrations travelled
from. So they use the information from three centres to pinpoint the only
point the vibrations could have eminated from. Where the three cross is
you need to be able to determine the arrival times of the p-waves and
the s-waves.(Remember that p-waves will always reach the seismograph first
because they travel faster and that s-waves only travel through
solids). Then calculate the time difference of arrival of the two wavetypes
and use that and the speeds of the waves to work out how far away the
epicentre must be.
University Site - Earth Sciences Virtual Lab
L. Jones (September 2002)