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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.
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:
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r 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.
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 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.
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.
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)
