The zones vary in width, but average about one-quarter mile wide. 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. Most faults are the result of repeated displacements over a long period of time. A fault trace is the line on the earth's surface defining the fault. For the purposes of the Act, an active fault is one that has ruptured in the last 11,000 years.
Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface.
Surface ruptures associated with the 1992 Landers Earthquake, in San Bernardino County, extended for 50 miles with displacements of an inch to 20 feet. Not all earthquakes result in surface rupture. The Loma Prieta Earthquake of 1989 caused major damage in the San Francisco Bay Area but the movement deep in the earth did not break through to the surface. Fault rupture almost always follows preexisting faults, which are zones of weakness. Rupture may occur suddenly during an earthquake or slowly in the form of fault creep.