A sudden motion or trembling in the earth caused by the abrupt release of slowly accumulated strain is called an earthquake.
Our earth is made up of various layers consisting many gaps. Due to the gaps between the layers, the earth moves slowly. Energy is stored up as the plate pushes against each other. When this energy increases more than the upper layer of the earth, it tears the upper layer. The strength of the earth is exceeded and suddenly an earthquake is released.
The point on the earth's surface directly above the point at which the sudden release takes place, is known as the epicentre. The seismic waves radiate outwards from the epicentre exactly like the waves that spread when a pebble drops into a calm pond