During fault ruptures which cause earthquakes, the sudden breakage and movement along the fault can release tremendous amount of energy. Some of this energy is used up in cracking and pulverizing the rock as the two blocks of rock separated by the fault grind past each other. Part of the energy, however, speeds through the rock as seismic waves. This waves can travel for and cause damage at great distances. Once they start, these waves continue through the earth until their energy is used up.
There are two basic types of seismic waves, and they travel at different speeds through earth. The faster p waves and the slower s waves.
Primary or push waves or P waves
These are longitudinal in nature like sound waves. The velocity of P waves is highest about 5.4 km/s and depends on the density of the rock and resistance to compression. P waves can pass through liquids also.
Secondary or shake waves or S waves
These are transverse in nature like light waves. The velocity of S waves is about 3.3 km/s. The velocity of S waves depends upon density of the rock and resistance to distortion. The S waves cannot pass through liquids.
When the P-waves and S-waves travel on the surface, they are known as surface waves
L waves and Rayleig Waves
These are also transverse in nature like S waves. The velocity of S waves is about 3.0 km/s. L waves are formed due to dashing of P and S waves against the solid crust of the earth. These are the waves, which we feel in the form of earthquake. These waves are responsible for the destruction of the life and property. Height of L waves is about 30cm and distance between two successive crests is about 10 m. and increase in their amplitude beyond only 1/16 of an inch is capable of causing lot of destruction. Due to high velocity of these waves, the civil engineering structures vibrate and a typical sound due to passage of energy is heard.