The zones are defined by turning points connected by straight lines. Most of the turning points are identified by roads, drainages, and other features on the ground. Earthquake Fault Zones are plotted on topographic maps at a scale of 1 inch equals 2,000 feet. 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.