Earthquake waves are basically of two types — body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.  

• The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.  
• The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity.  
• Their direction also changes as they reflect or refract when coming across materials with different densities. 
• There are two types of body waves. They are called P and S-waves. P-waves move faster and are the first to arrive at the surface. These are also called ‘primary waves’.  
• The P-waves are similar to sound waves. They travel through gaseous, liquid and solid materials.  

• S-waves arrive at the surface with some time lag. These are called secondary waves. An important fact about S-waves is that they can travel only through solid materials.  

Source: NCERT – XI Fundamentals of Physical Geography