FAULT

in geology, fracture in the rocks of the Earth's crust, where compressional or tensional forces cause the rocks on the opposite sides of the fracture, or fault, to be displaced relative to each other. Faults range in length from a few centimetres to many hundred kilometres, and displacement likewise may range from less than a centimetre to several hundred kilometres along the fracture surface (the fault plane). In some instances, the movement is distributed over a fault zone composed of countless individual faults that occupy a belt hundreds of metres wide. The distribution of faults is uneven; some large areas have almost none, others are cut by innumerable faults. Faults may be vertical, horizontal, or inclined at any angle. Although the angle of inclination of a specific fault plane tends to be relatively uniform, it may differ considerably from place to place along the fault line. When rocks slip past each other in faulting, the upper or overlying block along the fault plane is called the hanging wall, or headwall; the block below is called the footwall. The dip of a fault plane is its angle of inclination measured from the horizontal. The fault plane's angle of inclination from the vertical is called the hade. Movement of rock along a fault may occur as a continuous creep or as a series of spasmodic jumps of a few metres during a few seconds. Such jumps are separated by intervals during which stress builds up until it overcomes the frictional forces along the fault plane. Most, if not all, earthquakes are caused by rapid movement along faults. Faults are classified according to their angle of inclination and their relative and apparent movement. Normal, or gravity, faults are produced by vertical compression as the Earth's crust lengthens, or spreads. The hanging wall slides down relative to the footwall at an angle of inclination generally greater than 45. Normal faults are common throughout the world. In the Great Basin province of Utah and Nevada, such faults bound many of the mountain ranges on one or both sides; the mountains have been formed by the sliding downward of the hanging walls many thousands of metres, where they have become the valley floors. A block that has dropped relatively downward between two normal faults dipping toward each other is called a graben. The elongated troughs known as rift valleys are grabens. A block that has been relatively uplifted between two normal faults that dip away from one another is called a horst. (See horst and graben.) A tilted block that lies between two normal faults dipping in the same direction is a tilted fault block. Thrust faults result from horizontal compressional forces caused by a shortening, or contraction, of the Earth's crust. With the easiest path of relief upward, the hanging wall moves up and over the footwall, usually at a dip of less than 45; similar faults of greater than 45 are called reverse faults, and thrust faults with a very low angle of dip and a very large total displacement are often called overthrusts. Large thrust faults are characteristic of the Appalachian region in the Ridge and Valley province of Virginia and Tennessee. Strike-slip (also called transcurrent, wrench, or lateral) faults are similarly caused by horizontal compression but with the easiest relief in a horizontal direction almost parallel to the compressional force. The fault plane is essentially vertical, and the movement is lateral along it. These faults are widespread and are responsible for the repeated offsetting of mid-oceanic ridges. A well-known terrestrial example of this type is the San Andreas Fault, which, during the San Francisco earthquake of 1906, had a maximum movement of 6 m (20 feet). The total movement along this fault during the last few millions of years appears to have been several tens of kilometres. The displacement of the blocks on the opposite sides of the fault plane usually is measured in relation to sedimentary strata or to markers, such as veins and dikes. (Absolute movement relative to a plane such as sea level is generally unknown.) The movement along a fault may be rotational, the blocks rotating relative to one another. The apparent movement of a fault may be quite different from the actual movement, where erosion has removed the evidence. Faulting may smoothly polish the walls of the fault plane, marking them with scratches called slickensides, or it may crush them to a fine-grained, claylike substance known as fault gouge; when the crushed rock is relatively coarse-grained it is called fault breccia. Occasionally the beds adjacent to the fault plane fold or bend as they resist slippage because of friction. Areas of deep soil cover often show no surface indications of the faulting below.