Although the earth feels solid, it is really only partly so. The earth's crust is made up of giant plates of solid material that is riding on top of a more liquid portion of the earth's mantle. The plates have been moving slowly for millions of years. A fault is a fracture in the crust of the earth along which rocks on one side have moved relative to those on the other side. It is a thin zone of crushed rock between two blocks of rock, and can be any length, from centimeters to thousands of kilometers.
Most faults are the result of repeated displacements over a long period of time.
They are areas of stress that cause a crack in the rock or soil along the earth's surface. Some may extend deep underground, towards the earth's interior. Many earthquakes occur where the earth's crustal plates grind and shift along one another. Other earthquakes occur in the middle of the plates and are caused by the geophysical factors. An epicenter is the point on the earth's surface directly above the point inside the earth's mantle that is the focus or the source of the earthquake.