It is known that the seismic waves pass through the earth is mainly because of its being an elastic body- i.e. when a stress is applied it becomes deformed more and more, but when the stress is removed it returns to its original shape. If the stress applied is too great the elastic body will yield permanently.
The seismic wave have been considered to be the outcome of the elastic deformation of rocks. Since, all elastic bodies can be subjected to two types of deformation, compression and shear, the seismic waves are also related to these.
On the basis of their amplitude, wave length and nature of vibration the seismic waves are classified into three main types namely-P-wave, S- wave, and L-wave.
(a) Primary or P-waves These are longitudinal or compres- sional waves, similar to the sound waves. In this case, rocks vibrate parallel to the direction of wave propagation i.e. in the same direction as the waves are moving.
This kind of wave motion leads to the longitudinal compression and rarefaction of the substance. These waves travel in all the media i.e. in solid, liquid as well as gaseous, even though their propagation is inconsistent as far as the velocity is concerned.
Some authors consider them to be true sound waves because they are often audible as deep rumblings or even as loud reports in the early stages of earthquakes. In general the P-waves travel at 1.7 times the speed of the S-waves. They have short wave length and high frequency.
Due to their superior speed the P-wave are the first waves to arrive at points distant from the place of earthquake origin and are therefore called Primary-waves. These are also known as Pm-SA waves, Longitudinal waves etc. The velocity of P-wave depends on density and compressibility of the medium.