A normal fault is a type of dip slip fault in which the hanging wall moves downward relative to the footwall. It generally forms when the crust is subjected to extension, meaning that tectonic forces tend to pull the rocks apart. As the crust stretches, blocks of rock can move downward along inclined fault surfaces, producing relative subsidence of the hanging wall and relative uplift of the footwall.
Normal faults are characteristic of extensional tectonic environments such as continental rift zones, mid ocean ridges, and regions where the crust is being stretched or gravitationally extended. The movement can be gradual over geological time or occur episodically through earthquakes. Normal faulting is therefore both a structural geological process and an important earthquake mechanism.
The terms hanging wall and footwall describe the positions of blocks relative to an inclined fault plane. If a person could stand on the inclined fault surface, the hanging wall would be the block above the surface and the footwall would be the block below it. In a normal fault, the hanging wall moves downward relative to the footwall. This terminology is especially useful because it describes relative motion rather than requiring a particular compass direction. The fault can dip in different directions and can be oriented at many angles, so simply saying that one side moves southward or eastward would not define the fault type. The defining feature is the relative downward movement of the hanging wall.
Normal faults are associated with extensional stress and commonly contribute to crustal thinning. In continental regions undergoing rifting, numerous normal faults can form parallel or branching fault systems. Blocks between faults may move downward to form grabens or remain relatively elevated to form horsts. A graben is a depressed block bounded by normal faults, while a horst is an elevated block between faults. Large scale extensional systems can therefore create alternating basins and uplifted blocks. The East African Rift provides a major modern example of continental extension involving normal fault systems, while the Basin and Range Province of the western United States contains extensive normal faulting associated with crustal extension.
Normal faulting is also important at divergent plate boundaries. At mid ocean ridges, tectonic plates move apart and the newly formed oceanic crust undergoes extension and fracturing. Normal faults can accommodate this extension and contribute to the characteristic topography of spreading centres. On continents, rifting can eventually develop into a new ocean basin if extension continues sufficiently for continental crust to break apart and new oceanic crust to form. Normal faults therefore participate in the long term evolution of continents and oceans. The amount of extension, crustal thickness, thermal structure, rock strength, fault geometry, and rate of deformation all influence the resulting geological architecture.
Normal faults can produce earthquakes when accumulated elastic strain is released through sudden slip. The earthquake mechanism typically involves the hanging wall moving downward relative to the footwall during rupture. The amount of movement can range from very small displacements associated with minor earthquakes to substantial offsets during large earthquakes. Surface rupture may produce a fault scarp in which one side of the ground is permanently displaced vertically relative to the other. Repeated earthquakes on the same fault system can build significant topographic relief, although erosion, sedimentation, and subsequent faulting modify the landscape. Earthquake hazard in extensional regions therefore depends on both individual faults and interconnected fault systems.
Normal faults can have different geometries. Some are relatively steep, while others flatten with depth and form listric geometries. A listric normal fault curves from a relatively steep surface expression to a shallower dip at depth. Such faults can be associated with rotational movement of fault blocks and sedimentary basin development. Normal faults can also interact with folds, transfer zones, accommodation zones, and other faults. In large rift systems, extension may be distributed across many faults rather than concentrated on one continuous structure. This distributed deformation can make it difficult to identify the exact fault responsible for a particular earthquake without detailed geological and geophysical evidence.
Normal faulting can create distinctive geological relationships. As the hanging wall moves downward, sedimentary basins may develop adjacent to the fault and accumulate thick sequences of sediment. Older rock layers can be vertically offset, while growth strata may record fault movement occurring during deposition. Fault scarps and tilted blocks can preserve evidence of repeated movement. Geologists can determine the history of normal faults by measuring offsets, dating sediments, examining stratigraphic relationships, and analysing seismic reflection data. These methods can reveal both long term tectonic extension and individual prehistoric earthquakes, helping scientists estimate fault slip rates and reconstruct the evolution of rift systems.
A normal fault should be distinguished from a reverse fault primarily by the direction of hanging wall movement. In a normal fault, the hanging wall moves down relative to the footwall and the crust generally undergoes extension. In a reverse fault, the hanging wall moves up relative to the footwall and the crust generally undergoes shortening. The terms normal and reverse describe relative displacement and should not be interpreted as referring to whether a fault is safe or dangerous. Normal faults can generate destructive earthquakes, and reverse faults can also be active without producing obvious surface relief in every event. Normal faulting is thus a specific structural and seismological category defined by movement geometry and associated stress regime.
