A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers.
Most faults produce repeated displacements over geologic time.
During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse , depending on their motion. Great animations of these types of fault motion are available from the IRIS Consortium . The following definitions are adapted from The Earth by Press and Siever.