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.