Kinds of Waves
Some seismic waves travel through the earth's interior while others travel only over the earth's surface. There are two types of body waves. The primary, or P wave, is the faster of the two. P waves always travel at higher speeds than S waves, so whenever an earthquake occurs, P waves are the first to arrive and to be recorded at geophysical research stations worldwide. These waves are transmitted through all parts of the earth. S waves, which are slower than P waves, are known as the shear waves. This type of wave causes lateral vibrations. S waves are not capable of travelling through liquids. S waves have not been detected in any part of the earth deeper than about 1,800 mi, which indicates that below this depth the earth is molten.
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