Earthquake waves are also called seismic waves. These waves are of three types. These are:
1. Primary or P waves are push and pull waves. They are also called longitudinal waves. These waves resemble sound waves, since both are compression-dilatation or compression rarefaction waves.
In these waves each particle vibrates to and fro in the direction of propagation. P waves pass through gases, liquids and solids in the same manner. These waves travel outward from the point of disturbance in all directions in Straight lines.
They are the fastest of all earthquake waves. Their average velocity is 5.3 km a second and a maximum of 10.6 km per second. P waves are the first to reach the epicenter. The path followed by these waves through the earth is concave.
2. Secondary, S or Shear Waves are also called transverse wave. In these waves the particles vibrate at right angles to the direction in which they travel (the direction of propagation). S waves pas only through solids.
They cannot pass through liquids. It is interesting to learn that in the same kind of rock the speeds of travel of P and S waves are different because they depend on different properties.
The velocity of P waves is governed by the density and compressibility of the rock, whereas that of S waves depends on its density and rigidity.
As a matter of fact, P waves travel at about 1.7 times the speed of shear waves. However, shear waves closely follow the P waves. Even though the velocity of S wave is less than that of P wave, the former (S wave) is more destructive. P and S waves cause the rocking motion of the earth.
3. L or Surface Waves reach the earth's surface after P and S waves. Surface wave travels with a lower velocity than the other two around the surface of the earth. Surface wave is very destructive.
There are two types of L waves: (i) Raileigh Waves (ii) Love Waves. Raileigh waves are characterised by the motion of particles in elliptical orbits in the plane of propagation. In the second kind of waves i.e. love waves, the motion of particles is horizontal and at 90° angle of the direction of their movement.
Both of these waves provide very valuable information for distinguishing between the continental and oceanic types of crust. Besides the above named three major waves i.e. P, S, and L, there are some other minor waves called 'microseism'.
It is worth remembering that the epicenter of an earthquake can be located when its distance from the three conveniently placed stations are known. By a close analysis of the record of P and S waves, the thickness of the earth's crust and its variation in different parts of the earth can be calculated.