When the intension is released at the focus, the released energy travels in all directions. This energy comes towards the Earth’s surface in the form of waves. The seismic waves can be divided into primary, secondary and surface waves.
(i) Primary or ‘P’ waves:
- These waves are first to reach the surface of the Earth, after the energy is emitted in the Earth’s interior. They travel at a very fast speed from the focus of the earthquake in radial direction.
- The waves moves to and fro, therefore these waves are called forward-backward waves.
- These waves can travel through all the three states – liquid, solid and gaseous.
While travelling through liquid medium, their direction gets changed. It is because of the primary waves that the buildings on the Earth’s surface move back and forth.
(ii) Secondary or ‘S’ waves :
- The waves which reach the Earth’s surface after I the primary waves are called secondary waves. :
- These waves also scatter in all directions from the focus of the earthquake.
- Their velocity is less than ’P’ waves.
- The particles lying in the way of these waves j move up and down in the direction of energy transfer.
- These waves can travel only through the solid medium and they get absorbed as they enter the liquid medium.
- The buildings on the Earth’s surface move up and down because of these waves. These are more destructive than ‘P waves.
(iii) Surface or ‘L’ waves :
- These waves are generated after the main ’P’ and ‘S’ waves reach the epicenter.
- They travel in the direction of the circumference of the Earth, along the crust and are highly destructive.