Answer:
A fault is formed in the Earth's crust as a brittle response to stress. Generally, the movement of the tectonic plates provides the stress, and rocks at the surface break in response to this. ... At the other end of the spectrum, some plate-boundary faults are thousands of kilometers in length.
Another answer
Earthquakes occur on faults. A fault is a thin zone of crushed rock separating blocks of the earth's crust. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. ... Faults can extend deep into the earth and may or may not extend up to the earth's surface.
