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fault, in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized. Evidence of faults are found either at the surface (fault surface) or underground (fault plane). Faults are most evident in outcrops of sedimentary formations where they conspicuously offset previously continuous strata. Movement along a fault plane may be vertical, horizontal, or oblique in direction, or it may consist in the rotation of one or both of the fault blocks, with most movements associated with mountain building and plate tectonics. The two classes of faults include the dip-slip (up and down movement), which is further divided into normal and thrust (reverse) faults; and strike-slip (movement parallel to the fault plane). The San Andreas fault of California is of this type. In dip-slip faults the term “hanging wall” is used for the side that lies vertically above the other, called the “footwall.” A fault in which the hanging wall moves down and the footwall is stationary is called a normal fault. Normal faults are formed by tensional, or pull-apart, forces. A fault in which the hanging wall is the upthrown side is called a thrust fault because the hanging wall appears to have been pushed up over the footwall. Such faults are formed by compressional forces that push rock together and are by far the most common of the dip-slip faults. All types of faults have been recognized on the ocean floor: normal faults occur in the rift valleys associated with mid ocean ridges spreading at slow rates; strike-slip faults appear between the offset portions of mid-ocean ridges; and thrust faults occur at subducting plate boundaries. Active faults, though they may not move for decades, can move many feet in a matter of seconds, producing an earthquake. The largest earthquakes occur along thrust faults. Some faults creep from a half inch to as much as 4 in. (1 to 10 cm) per year. Fault movements are measured using laser and other devices. Faults create interpretation problems for geologists by altering the relations of strata (see stratification), such as making the same rock layer offset in two vertical cross sections of a formation or making layers disappear altogether. Faults are often seen on the surface as topographical features, including offset streams, linear lakes, and fault scarps.
faultA vertical or near vertical crustal displacement on the Earth, Moon, Mars, or some other solid body. Lunar faults are usually double but single-sided faults also occur. See also graben; Mars, surface features; rille; wrinkle ridges.
a rupture in the earth’s crust that occurs during tectonic movements and rock deformation. The walls of faults are formed by the offset rock masses. When a fault is inclined, a distinction is made between the footwall, which underlies the fault, and the hanging wall, which overlies the fault.
Some faults are marked by insignificant relative displacement of the walls (tectonic fractures), and others by significant displacement (fault displacements). Among the latter several different kinds of faults are identified. A strike-slip fault is formed by the horizontal displacement of the walls along a vertical or inclined rupture. An extension fault results from the movement of walls away from one another, and in a normal fault the hanging wall shifts downward. Reverse faults and thrust faults are formed by a displacement of the hanging wall upward; the difference between the two types of faults lies in the size of the angle of dip. An overthrust nappe is formed when a hanging wall is overthrust with great amplitude along a very gentle, a horizontal, or a wavy fault. Combined displacements (for example, combined normal and strike-slip faults) are common.
The size of faults and the amplitude of displacement along them vary. In most cases, tectonic fractures without displacement are not more than a few meters long. Faults with displacement vary from small fractures a few decimeters long to abyssal fractures that pierce the earth’s crust and part of the upper mantle. The amplitude of normal faults reaches several kilometers, whereas strike-slip faults and overthrust nappes have amplitudes of dozens of kilometers (even several hundred, according to some investigators). Different stresses cause the formation of the various types of faults. Reverse faults, thrust faults, and overthrust nappes, which are usually combined with rock folding, form in zones of crustal contractions. In zones of crustal extension normal and extension faults occur. Zones with large numbers of normal faults are called rifts.
Movement along faults may be brief or may continue for prolonged periods of geological time. In the latter case it takes the form of distinct jolts accompanied by earthquakes. The cavities of faults often serve as routes for ascending hydrothermal solutions that form vein rocks.
REFERENCEBelousov, V. V. Strukturnaia geologiia, 2nd ed. Moscow, 1971.
V. V. BELOUSOV
in geology, a tectonic fracture of the earth’s crust in which the walls of the fracture are shifted horizontally along its strike; the term “fault” may also refer to the displacement process. A distinction is made between a right fault, in which, when viewed in a plane, the relative displacement of the walls is clockwise, and a left fault, in which the movement of the walls is counterclockwise.
Faults range in length from several meters to many hundreds of kilometers, and the amplitude of displacement may vary from a few centimeters to many dozens, and probably hundreds, of kilometers. When the strike changes significantly, a fault may become a normal fault, a thrust fault, or a strike-slip fault. Characteristically pulsational, faulting only involves certain areas of the walls at one time. Displacement may occur along the fault line or in an adjacent zone as wide as several hundred kilometers, causing buckling and the formation of many fractural elements.
Faults are most characteristic of an area of folds. Large faults start forming during the orogenic period and develop as long as several tens of millions of years. The largest and most extensively studied faults are the San Andreas in California, the Talass-Fergana in the Tien-Shan, and the Great Glen of Scotland.