An earthquake is a sudden and violent shaking of the ground, sometimes causing great destruction and human casualties. It is the direct result of movements within the tectonic plates or even the volcanic activity.
Major earthquakes are confined to particular active regions of the earth's crust corresponding to the edges of the tectonic plates. Most earthquakes are due to the release of strain energy associated with the relative motions of the plates. The intensity of earthquakes is expressed by the Richter scale, destructive earthquakes generally measuring between about 7 and 9.