Earthquakes can be categorized into naturally occurring earthquakes and man-made earthquakes (such as those caused by nuclear explosion). In general, the earthquakes that we experience are called naturally occurring earthquakes and the causes for these earthquakes can also be divided into: 1) tectonic earthquakes, 2) volcanic earthquakes, and 3) impacting earthquakes (such as those caused by the impact of meteorites). Among all these causes, earthquakes are predominantly produced by crustal deformation (tectonic earthquakes), which is caused by plate movement.
Rock layers are stressed by pressures in the Earth. When the stress is stronger than the strength that the rock layer can withstand, the rock layer will move outward (dislocation). Such dislocations release a great deal of energy, producing elastic waves called seismic waves. When seismic waves reach the surface of the Earth, they cause shaking, also known as earthquake.