All natural earthquakes take place in the lithosphere.
It is sufficient to note here that the lithosphere refers to the portion of depth up to 200 km from the surface of the earth.
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 a 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.
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’ or ‘longitudinal waves’.
The P-waves are similar to sound waves.
They travel through gaseous, liquid and solid materials.
P-waves vibrate parallel to the direction of the wave. In case of these waves, the direction of wave propagation and the displacement of particles is the same.
Their velocity increases with increasing density of rocks.
This exerts pressure on the material in the direction of the propagation. P waves are also called Pressure waves.
As a result, it creates density differences in the material leading to stretching and squeezing of the material.
Certain animals, such as dogs, can feel the P waves much before an earthquake hits the crust.
S-waves arrive at the surface with some time lag.
These are called secondary waves.
They are also called transverse waves. These are like light waves.
An important fact about S-waves is that they can travel only through solid materials.
The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.
The surface waves are the last to report on seismograph.
These waves are more destructive.
They cause displacement of rocks, and hence, the collapse of structures occurs.
Propagation of Earthquake Waves causes vibration in the body of the rocks through which they pass.
Surface waves are considered to be the most damaging waves.