An earthquake is the vibration, sometimes violent, of the Earth's surface that follows a release of energy in the Earth's crust. This energy can be generated by a sudden dislocation of segments of the crust, by a volcanic eruption, or even by manmade explosions. These are known as tectonic earthquakes . Faults A fault is a fracture within some particular rocky mass within the earth's crust. The depth and length of faults vary greatly.
Some faults can be many miles long.
So, elaborating somewhat on what we said above, an earthquake is caused by the sudden movement of the two sides of a fault with respect to another. Faults are divided into three main groups, depending on how they move. The occurrence of tectonic earthquakes can be explained by the theory of elastic rebound , which was first advanced by H. B. Reid following his observations of ground displacement both before and after the great San Francsico earthquake of 1906. Over the course of time, one can observe that the two sides of an active fault are in slow but continous movement relative to one another. The rate of this movement may be as little as a few inches or so per year.