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In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults.  Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep. 
A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault.  
A fault zone is a cluster of parallel faults.   However, the term is also used for the zone of crushed rock along a single fault.  Prolonged motion along closely spaced faults can blur the distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed. 
Owing to friction and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. The regions of higher friction along a fault plane, where it becomes locked, are called asperities. Stress builds up when a fault is locked, and when it reaches a level that exceeds the strength threshold, the fault ruptures and the accumulated strain energy is released in part as seismic waves, forming an earthquake. 
Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock; the ductile lower crust and mantle accumulate deformation gradually via shearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault.  A fault in ductile rocks can also release instantaneously when the strain rate is too great.
Slip is defined as the relative movement of geological features present on either side of a fault plane. A fault's sense of slip is defined as the relative motion of the rock on each side of the fault concerning the other side.  In measuring the horizontal or vertical separation, the throw of the fault is the vertical component of the separation and the heave of the fault is the horizontal component, as in "Throw up and heave out". 
The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault.  Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault.  The direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of the fault (called a piercing point). In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector.
The two sides of a non-vertical fault are known as the hanging wall and footwall. The hanging wall occurs above the fault plane and the footwall occurs below it.  This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall above him.  These terms are important for distinguishing different dip-slip fault types: reverse faults and normal faults. In a reverse fault, the hanging wall displaces upward, while in a normal fault the hanging wall displaces downward. Distinguishing between these two fault types is important for determining the stress regime of the fault movement.
Faults are mainly classified in terms of the angle that the fault plane makes with the earth's surface, known as the dip, and the direction of slip along the fault plane.  Based on the direction of slip, faults can be categorized as:
- strike-slip, where the offset is predominantly horizontal, parallel to the fault trace;
- dip-slip, offset is predominantly vertical and/or perpendicular to the fault trace; or
- oblique-slip, combining strike-slip and dip-slip.
In a strike-slip fault (also known as a wrench fault, tear fault or transcurrent fault),  the fault surface (plane) is usually near vertical, and the footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known as sinistral faults and those with right-lateral motion as dextral faults.  Each is defined by the direction of movement of the ground as would be seen by an observer on the opposite side of the fault.
A special class of strike-slip fault is the transform fault when it forms a plate boundary. This class is related to an offset in a spreading center, such as a mid-ocean ridge, or, less common, within continental lithosphere, such as the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed.
Dip-slip faults can be either normal (" extensional") or reverse.
In a normal fault, the hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is a graben. An upthrown block between two normal faults dipping away from each other is a horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults.
A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°. The terminology of "normal" and "reverse" comes from coal mining in England, where normal faults are the most common. 
A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°.   Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds.
Flat segments of thrust fault planes are known as flats, and inclined sections of the thrust are known as ramps. Typically, thrust faults move within formations by forming flats and climb up sections with ramps.
Fault-bend folds are formed by the movement of the hanging wall over a non-planar fault surface and are found associated with both extensional and thrust faults.
Faults may be reactivated at a later time with the movement in the opposite direction to the original movement (fault inversion). A normal fault may therefore become a reverse fault and vice versa.
A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault. Nearly all faults have some component of both dip-slip and strike-slip; hence, defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within transtensional and transpressional regimes, and others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.
The hade angle is defined as the complement of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.
Listric faults are similar to normal faults but the fault plane curves, the dip being steeper near the surface, then shallower with increased depth. The dip may flatten into a sub-horizontal décollement, resulting in a horizontal slip on a horizontal plane. The illustration shows slumping of the hanging wall along a listric fault. Where the hanging wall is absent (such as on a cliff) the footwall may slump in a manner that creates multiple listric faults.
Ring faults, also known as caldera faults, are faults that occur within collapsed volcanic calderas  and the sites of bolide strikes, such as the Chesapeake Bay impact crater. Ring faults are the result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled by ring dikes. 
Synthetic and antithetic faults are terms used to describe minor faults associated with a major fault. Synthetic faults dip in the same direction as the major fault while the antithetic faults dip in the opposite direction. These faults may be accompanied by rollover anticlines (e.g. the Niger Delta Structural Style).
All faults have a measurable thickness, made up of deformed rock characteristic of the level in the crust where the faulting happened, of the rock types affected by the fault and of the presence and nature of any mineralising fluids. Fault rocks are classified by their textures and the implied mechanism of deformation. A fault that passes through different levels of the lithosphere will have many different types of fault rock developed along its surface. Continued dip-slip displacement tends to juxtapose fault rocks characteristic of different crustal levels, with varying degrees of overprinting. This effect is particularly clear in the case of detachment faults and major thrust faults.
The main types of fault rock include:
Cataclasite – a fault rock which is cohesive with a poorly developed or absent planar
fabric, or which is incohesive, characterised by generally angular
clasts and rock fragments in a finer-grained
matrix of similar composition.
- Tectonic or fault breccia – a medium- to coarse-grained cataclasite containing >30% visible fragments.
- Fault gouge – an incohesive, clay-rich fine- to ultrafine-grained cataclasite, which may possess a planar fabric and containing <30% visible fragments. Rock clasts may be present
- Mylonite – a fault rock which is cohesive and characterized by a well-developed planar fabric resulting from tectonic reduction of grain size, and commonly containing rounded porphyroclasts and rock fragments of similar composition to minerals in the matrix
- Pseudotachylyte – ultrafine-grained glassy-looking material, usually black and flinty in appearance, occurring as thin planar veins, injection veins or as a matrix to pseudoconglomerates or breccias, which infills dilation fractures in the host rock. Pseudotachylyte likely only forms as the result of seismic slip rates and can act as a fault rate indicator on inactive faults. 
In geotechnical engineering, a fault often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, foundation, or slope construction.
The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the seismic shaking and tsunami hazard to infrastructure and people in the vicinity. In California, for example, new building construction has been prohibited directly on or near faults that have moved within the Holocene Epoch (the last 11,700 years) of the Earth's geological history.  Also, faults that have shown movement during the Holocene plus Pleistocene Epochs (the last 2.6 million years) may receive consideration, especially for critical structures such as power plants, dams, hospitals, and schools. Geologists assess a fault's age by studying soil features seen in shallow excavations and geomorphology seen in aerial photographs. Subsurface clues include shears and their relationships to carbonate nodules, eroded clay, and iron oxide mineralization, in the case of older soil, and lack of such signs in the case of younger soil. Radiocarbon dating of organic material buried next to or over a fault shear is often critical in distinguishing active from inactive faults. From such relationships, paleoseismologists can estimate the sizes of past earthquakes over the past several hundred years, and develop rough projections of future fault activity.
Many ore deposits lie on or are associated with faults. This is because the fractured rock associated with fault zones allow for magma ascent  or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits. 
An example of a fault hosting valuable porphyry copper deposits is northern Chile's Domeyko Fault with deposits at Chuquicamata, Collahuasi, El Abra, El Salvador, La Escondida and Potrerillos.  Further south in Chile Los Bronces and El Teniente porphyry copper deposit lie each at the intersection of two fault systems. 
- Aseismic creep
- Fault scarp – Small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other
- Fault block
- Mitigation of seismic motion
- Mountain formation – Geological processes that underlie the formation of mountains
- Orogeny – The formation of mountain ranges
- Paleostress inversion
- Seismic hazard – Probability that an earthquake will occur in a given geographic area, within a given window of time
- Striation – Groove, created by a geological process, on the surface of a rock or a mineral
- Vertical displacement - Vertical movement of Earth's crust
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