Earthquakes are caused by tectonic movements in the Earth's crust. The main cause is that when tectonic plates collide, one rides over the other, causing orogeny (mountain building), earthquakes and volcanoes. The boundaries between moving plates form the largest fault surfaces on Earth.
Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. When the rocks break, the earthquake occurs.
Earthquakes can be extremely violent or so slight that they only register on instruments. They are caused by sudden movements of the Earth's tectonic plates. The study of earthquakes is called seismology. ... The magnitude of an earthquake, and the intensity of shaking, is usually reported on the Richter scale.