Normal fault is a geological term, used to describe a type of fault in which the hanging wall has moved down relative to the footwall. The spelling of "normal fault" in IPA phonetic transcription is /ˈnɔːməl ˈfɔːlt/. The word "normal" is pronounced with the stressed syllable on the first syllable, followed by the schwa sound in the second syllable, and a long "o" sound in the final syllable. "Fault" is pronounced with the stressed syllable on the first syllable and a long "o" sound in the final syllable.
A normal fault is a geological feature that occurs when the Earth's crust experiences tensional stress, causing rock layers to move vertically relative to one another. This fault is characterized by the hanging wall, which lies above the fault plane and moves downward in relation to the footwall, located below the fault plane. The displacement along a normal fault is commonly caused by the stretching and thinning of the Earth's crust, resulting in the elongation and subsidence of the rock layers.
The fault plane of a normal fault is inclined, dipping away from the vertical at an angle less than 90 degrees. This dipping angle, known as the fault dip, often influences the shape and dimensions of the resulting fault scarp – a steep slope or cliff formed due to the differential vertical movement of the rock layers. In some cases, normal faults create long, linear valleys or troughs called grabens, where the hanging wall has moved downward and the footwall is relatively uplifted.
Normal faults are a common feature in regions undergoing extensional tectonic forces, such as divergent plate boundaries, where lithospheric plates are moving apart. They are essential components of the crustal deformation that contributes to the formation of mountain ranges, basins, and other geological structures. The activity along normal faults can lead to the displacement of groundwater, triggering earthquakes, and exposing previously buried rock layers, providing geologists with insights into the geological history of a region.
The word "normal fault" originates from the field of geology and has an etymology connected to the behavior of rocks under tectonic stress.
The term "fault" comes from the Old English word "fealte", which means "a defect" or "a flaw". It entered the geologic context in the late 17th century as a technical term for fractures or surfaces along which rocks on either side have moved relative to each other.
The term "normal" in "normal fault" refers to the relative movement of rocks on either side of the fault plane. In this case, the hanging wall moves downward relative to the footwall, creating a steeper slope. The term "normal" is derived from Latin "norma", meaning "rule" or "standard", as this type of fault displays the expected or common behavior associated with faulting.