Natural earthquakes are classified into tectonic and volcanic earthquakes. They occur in the seismic belts or seismic zones of the Earth, which are areas with special geologic characteristics, such as areas with weak crusts, low thicknesses, or with abundant fractures or fissures. These earthquakes are caused by the relative movements of the tectonic plates forming the surface of the earth. As their internal stresses accumulate in the rocks along the boundaries of the moving plates, deformations result from the bending, tension, and pressure, as well as the frictional forces, across the interfaces between tectonic plates. This process contributes to the increasingly large accumulation of stresses in the rock mass. When the accumulated stress exceeds the maximum amount of stress the rock mass can bear, rupture and the sudden movement of the rocks happen for them to release this accumulated stress. This stress is a form of stored energy, which is released by the rocks in the form of seismic waves. This is how earthquakes or seismic tremors are generated. Volcanic earthquakes, on the other hand, occur as a result of the movements of magma under the surface of the earth or through volcanic vents, which also releases seismic waves and are felt as ground tremors or earthquakes. The magnitudes of the volcanic earthquakes are relatively lower and are seldom high and destructive.
Man-made earthquakes are effective seismic waves that emanate from human activities, such as through artificial and nuclear explosions, the construction of giant dams, the formation of artificial lakes, the withdrawal of water and oil, etc.