Earthquakes are tremors which are produced by the passage of vibratory waves through the rocks of the earth. If you throw a stone in a pound of still water, a series of concentric waves are produced on the surface of water. These waves spread out in all directions from the point where the stone strikes the water. Similarly, any sudden disturbances in the earth’s crust may produce vibrations in the crust which travels in all directions from the point of disturbance.

Earthquakes and volcanoes are endogenic forces while weathering and agents of sculpture are exogenic forces. Some are sudden forces and bring about sudden changes. Earthquakes and volcanoes are examples of sudden forces. There are other forces which work slowly and take longer duration of time to show their effects.

FOCUS AND EPICENTRE

The place of origin of an earthquake inside the earth is called its focus. The shock waves travel in all directions from the focus. On the earth’s surface, the shaking is strongest near the epicentre.

There are several types of earthquakes which affect different parts of the world at different times. Some of the common types of earthquakes are briefly described.

Tectonic Earthquakes- There are the most common earthquakes and generated by sliding of rocks along a fault plane.

Volcanic Earthquakes- As the name indicates these earthquakes are caused by volcanic eruptions and are quite devastating. However, these are confined to areas of active volcanoes.

Collapse Earthquakes- In areas of intense mining activity, often the roofs of underground mines collapse and minor tremors take place. These are called collapse earthquakes.

Explosion Earthquakes- When a chemical or nuclear explosion takes place, ground shaking occurs and an earthquake is known as explosion earthquakes.

Reservoir induced Earthquakes- Such earthquakes are caused by the pressure of water collection in large reservoir. The koyna earthquake was cause by koyna reservoir.