When an earthquake occurs, three types of waves are generated from the epicenter.
Called earthquake waves.
$(1)$ Primary or longitudinal waves: These waves are short wavelength and high wave waves. So the earthquake has reached the registration center first.
These waves pass through both solid and liquid media. In the central part they pass at a speed of $8$ to $14 \ km$ per second. These waves refract as the density of the material changes.
$(2)$ Secondary waves / sub waves or S waves: These waves have lower velocity than primary waves.
They only pass through a solid medium.
In this the vibrations of the atoms occur up and down at right angles to the direction of the wave.
These waves are similar to light waves.
It calms down in the sea or liquid medium. These waves refract as the density of the material changes.
$(3)$ Surface waves / Surface waves: These waves move perpendicular to the circumference of the earth's surface.
Its speed is $3 \ km$ per second.
These gloves are the most destructive.
The devastation caused by the earthquake is due to these waves.
When these waves enter the ocean, very high giant and destructive waves bounce.
This is called a tsunami.
Since these waves do not pass underground, it is not useful to know the internal structure of the earth.