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a seismograph, with the waves being traced on a rotating drum. The trace is called a seismogram.
Seismographs are set up to form networks. The network of seismograms produced can be used to calculate the magnitude of an earthquake, when and where it occurred, the depth of the earthquake focus, and many other properties.
Geologists group the waves produced by an earthquake as those that travel through the rocks, which are called body waves, and those that travel along the ground, which are called surface waves. Body waves provide more information about the earthquake, but surface waves usually have the strongest vibrations and cause most of the damage done by earthquakes.
During an earthquake two types of body waves are produced. One type, known as p waves, or primary waves, is similar to sound waves in that they consist of a pattern of pressure waves radiating through the earth. They can travel through any kind of material, liquid or solid.
The second type of waves are known as s waves,
or secondary waves, which are similar to the waves produced by shaking a rope. They are also called shear waves, and they displace material at right angles to their path. They can travel only through solids. Shear waves travel somewhat more slowly than P waves.
A seismogram shows P waves arriving first, often as a very short burst of waves felt as a kind of thud or jolt. The S waves follow a few seconds later, giving a kind of swaying feel to the ground. Then some seconds later the surface waves arrive, and they make the ground pitch
(Below) This is a typical trace, or seismogram. It shows the arrival
of the small P and S waves. They
are the waves that provide most information about the nature of
the rocks through which they pass. A few minutes later the much larger surface waves arrive. They provide limited information about the rocks but are the main cause of property damage, injury and death. Two types of surface waves are commonly experienced; together they produce a rolling and pitching motion. They are known as love waves and rayleigh waves.
Surface waves
2500 3000 3500
P wave 0
S wave
8
0 500 1000 1500 2000 Time in seconds
Magnitude of wave