Earth's crust, the surface layer of the planet, is not solid and unbroken. The forces that rage inside the planet have fractured this brittle layer. Some of these fractures, called faults, lie beneath the surface of the crust. Other faults, however, have ruptured the surface, cracking the crust into various-sized blocks of rock. These blocks dip and rise along faults in response to pressure underground. One block may move up while the other moves down. Sometimes the movement is enough to form valleys or mountains. Other times that movement is not vertical but horizontal, as one block slips along the fault relative to the block on the other side.
Movement of crustal blocks along faults may be regular and slow or sporadic and sudden. When two blocks are forced to move against each other but are locked into position, stress builds up. When that stress becomes greater than the forces holding the blocks together, the blocks are forced to move suddenly and violently. The ground vibrations accompanying that r
