Download EARTHQUAKE WAVES

EARTHQUAKE WAVES
The source of an earthquake is a sudden break on a geological fault. Most of the energy stored in
the rock layers is used first to break the frictional forces holding the rock segments together.
However, a small fraction of this energy is released in the form of seismic waves radiating
outwards from where the rocks broke. When forces act on geological faults the rock blocks are
affected by compression (pressure) and shearing stresses. Because of this, seismic waves are
different kinds and travel with different velocities. There are two main types of seismic waves;
body waves which move throughout the Earth and surface waves which travel along the Earth's
surface. These waves travel at different velocities and reach a certain place at different times.
Seismologist use the difference in time arrival to locate earthquakes.
Body Waves
P – Waves: These waves are also called Primary waves. They travel like sound waves,
alternately compressing and expanding the particles of the earth, pushing them backwards and
forwards in the direction of travel. It's
like squeezing an eraser, as shown in the
diagram:
Their speed depend on how easily the
material can be compressed, how rigid it
is, and its density. They are the fastest waves, and thus the first to arrive, travelling at speeds of
4-7 km/sec in the earth’s crust. They sometimes hit houses with a boom of sound which rattles
windows, but they generally don't cause damage on the surface, except in very large quakes.
S-Waves are also called shear
waves because they move the
ground like a shaken carpet or rope.
The motion of individual rock
particles in an S wave is
perpendicular to the direction of
wave travel. The wave is transmitted
by a shearing motion between
particles: it's like bending an eraser
rather than squeezing it.
They travel slower than P-waves, so they will reach any point later. They are the second waves
to reach a seismometer. Moreover, their speed depends on the rigidity of the rocks, and because
liquids have no rigidity, they cannot travel through them. That's a very interesting property
which allows seismologists to locate the focus of earthquakes. Because of their side to side
motion, S-waves are very damaging to buildings. However, the surface waves which follow in
their wake can be even worse.
Surface Waves
As we have already said, surface waves differ from body waves in that they do not travel through
the Earth, but instead travel along the surface of the Earth. They result from the transformation of
P and S waves when they hit the surface, and they behave like S-waves in that they cause up and
down and side and side movement as they pass, but they travel slower than S-waves.
Love waves
Love waves start arriving just after S waves, or at the same time. They have a side to side
motion similar to S waves except
that they can only exist on the
surface. Love and Rayleigh were
both mathematicians who proved that
such surface waves could
theoretically exist, before they were
ever actually discovered on the
seismograph records.
Rayleigh waves
These ones, which come after love
waves, churn the ground in a vertical
plane like a wave breaking to the
shore. Rocks and soil are rotated in the
opposite direction to the travelling
wave as they are caught up and spun
about in it.
Coda
The dying of an earthquake is composed of a mixture of P, S, Love and Rayleigh waves that
have arrived along scattered paths through the complex rock structure, due to reflection and
refratction. The result is a series of surface waves, of several frequencies, which is called coda,
because it is like the final section of a musical composition
In a very large earthquake, seismic waves pass right through the earth and then continue to
bounce around inside it. In some cases vibrations can be reflected back and forth through the
earth for as long as a month. When sufficient energy is released in an earthquake it can set the
whole earth ringing like a bell. The 'tone' is a very low one, however. It takes the earth 54
minutes to complete one cycle of its lowest tone (compared to over 200 cycles per second for
middle C on a piano)