Due to catastrophic (sudden) movements like collision of plates in the interior of the Earth or heavy pressure, the weak upper crust of Earth experiences sudden tremors. This is called ‘earthquake’.

The place from where these movements start is called ‘Focus’. The focus of the earthquake is in the interior of the earth.

The seismic waves caused due to an earthquake ; travel at different velocities in different directions. The nearest place on the surface of Earth, perpendicular to the focus of the earthquake is called its ‘Epicentre’. The area around the epicentre suffers maximum damage. The impact of the earthquake becomes less as we go away from the epicentre. The effect of an earthquake depends on its intensity.

Seismographs placed at various centres on the surface? record the earthquake very minutely. The locations o. the focus and the intensity of the earthquake can be ascertained with the help of a seismograph. 

There are ‘ three types of earthquakes :

1. Volcanic,

2. Tectonic and

3. Plutonic.