An earthquake is sudden motion or trembling of the ground produced by the abrupt displacement of rock masses. The earth's outer shell is divided into seven major and some smaller plates which are constantly in a dynamic state, pushing against, pulling away from, or grinding past one another.
Forces build up as the plates attempt to move in relation to each other.
When the adhesions along the fault give way, stored energy is released in the form of earth tremors, volcanic activity etc. Oceanic plates pulling away from each other leads to hot volcanic material being expelled from cracks to form mid-ocean ridges. Oceanic plates colliding with and forced under continental plates leads to mountain ranges being pushed up, accompanied by earthquakes and volcanic eruptions. This sudden movement produces seismic waves which travel from the source through the crust to the earth's surface. When they arrive, the vibratory motion we feel is called an earthquake.