Geologists still describe faults using words borrowed from old mines
The two sides of a sloping fault are called the hanging wall and the footwall because miners working a tilted ore seam stood on one and had the other hanging overhead. Even the labels normal and reverse come from English coal mining, where normal faults were the everyday kind.
A fault is a flat fracture in rock across which the two sides have shifted significantly. The biggest ones result from plate tectonics and form the boundaries between plates. The fracture surface is the fault plane; where it meets the ground and can be mapped is the fault trace. A fault zone may mean a cluster of parallel faults or the band of crushed rock along a single one, and long movement can blur the two as rock caught between faults is ground into lenses.
Friction explains why faults are dangerous. The sides cannot always slide freely, so movement sometimes stops at rough, high-friction patches called asperities. Stress builds while a fault is locked until it suddenly gives. The brittle upper crust responds by fracturing, releasing strain instantly, while the ductile lower crust and mantle deform gradually by shearing. Slip, the relative movement across the fault, can be estimated from drag folds, zones where layers bent as friction resisted the motion.
Faults are sorted by the angle of the plane, called the dip, and by the direction of slip. In strike-slip faults the plane is near vertical and the blocks slide sideways, left or right. Those forming plate boundaries are transform faults, such as the Dead Sea Transform and New Zealand's Alpine Fault. In dip-slip faults the motion is mainly vertical; oblique-slip faults mix both.
In a normal fault the hanging wall drops relative to the footwall, usually on a plane steeper than 60 degrees. A block dropped between two such faults is a graben, and a block left standing between two grabens is a horst; repeated, they create basin and range scenery. Listric faults curve, steep near the surface and flattening at depth, sometimes into vast low-angle detachment faults. In a reverse fault the hanging wall rises instead. When regional stress flips over time, old faults can reactivate and run backwards, a process called fault inversion.
Source: Fault (geology)
