From the point of view of the relative displacement of the blocks involved, the faults are classified as:[1]​[2][3]​.

• - Normal, direct or gravity failure:[4]​ when the hanging or roof block moves downward with respect to the lying or wall block. The fault plane is inclined.

• - Reverse fault: when the hanging block moves upward with respect to the bed. Low dip angle reverse faults are called thrusts. The fault plane is inclined.

• - Strike, direction, directional, transcurrent or strike-slip fault: when the displacement is horizontal and parallel "Parallelism (mathematics)") to the strike of the fault. They can be, depending on the direction of movement of the blocks (referring to the position of an observer located on one of the blocks), sinistral or left directional, when the block opposite the one occupied by the observer moves to the left, and dextral or right directional, when the block moves to the right. The fault plane can be inclined or vertical. A particular type of strike-slip faults are transform faults, which displace segments of constructive plate boundaries and the fault plane is usually vertical.

• - Oblique or mixed fault: when the displacement is oblique to both the strike and the direction of dip. They are simply described as a combination of the terminology of the previous ones, resulting in four possible cases: reverse sinistral, normal sinistral, reverse dextral and normal dextral.

• - Rotational fault: when there has been a rotation component in the relative displacement between the two blocks separated by the fault. In turn, they can be divided into:[3]​

• Scissors failure: when the axis of rotation is perpendicular to the failure plane.

• Cylindrical failure: when the axis of rotation is parallel to the failure plane. The fault plane is usually curved.

• Conical failure: when the axis of rotation is oblique to the failure plane. The fault plane is usually curved.

Contenido

Las estructuras vinculadas con las fallas dependen del tipo de régimen tectónico regional en el que se han formado. Sin embargo, hay algunas formas y términos comunes a todas ellas: es frecuente que las fallas varíen de buzamiento en su recorrido, mostrando zonas relativamente horizontales, rellanos, alternando con zonas más inclinadas, rampas. Los bloques delimitados entre rampas de fallas se denominan escamas tectónicas o horses y el apilamiento de estas escamas se denomina dúplex.[1]​.

In regions of tectonic extension

In a limited extension regime and in conditions of brittle deformation, systems of stepped normal faults develop, more or less parallel, which form sunken zones, called Grabens "Graben (geology)") or tectonic grabens, which can alternate with elevated zones, called Horst or tectonic pillars.[1]​.

If the extension is wide, the faults tend to horizontalize in depth (listric faults). In the development of the extension, fault systems can form with ramps and landings that follow one another and replace each other, delimiting scales that can be grouped into extensional duplexes.[1]​.

At the cortical scale, extensional faults that develop on the surface with brittle behavior pass at depth to the ductile domain, producing mylonitic bands in the detachment zone. In cases where stretching is important, thinning of the crust can occur—a process called tectonic denudation—and deep rocks can be lifted to the surface by isostatic readjustment.[1]​.

In regions of tectonic compression

The most common forms associated with compression are produced by reverse faults: thrusts and thrust mantles, typical of the external zones of collision orogens, in what is called "thrust belt" and corresponds to the thin-skinned tectonic style.[1]​.

In some regions affected by compression, with thrusts with detachments at the base of the upper crust or deeper (thick-skinned tectonic style), elevations of the pop-up type and depressions of the pop-down type (depressions between two thrusts of opposite vergence) can occur, both limited by reverse faults — in which they differ from horst and grabens, limited by normal faults. This model of pops-up and pops-down is applied, for example, to the Spanish Central System.[5]​.

At a cortical scale, the imbrication and stacking of fragments of continental crust can occur, as in the case of the Himalayan mountain range, in which extensive cortical blocks, delimited by large faults, ride on top of each other.[2]​.

In zones of transcurrent tectonics

In large strike-slip faults, whose displacement component is mainly horizontal, areas of local compression or extension may be delimited that produce uplift or subsidence movements. The relay or bridge between two nearby faults or the local curvature of a fault in direction produces an area in which the local direction of fracturing is oblique or perpendicular to the direction of main displacement, forming flakes and associated duplexes.[1]​.

Depending on the relief or rotation of the faults, to the right or left, and depending on their horizontal displacement, dextral or sinestral, the link zone between the two will have a compressive or distensive behavior of the scales and duplex that have been formed, developing fan elevations, push-up type, or pull-apart type tectonic depressions.[1]​.

They are those that are produced exclusively by the effect of gravity, not by the action of tectonic forces. They can occur in different geological contexts:[1]​.

• - In the mid-oceanic ridges, where they delimit the mid-oceanic rift.

• - In karst terrain, due to the dissolution of the substrate or collapse of cavities.

• - In volcanic regions, due to the collapse of magmatic chambers or the sliding of unstable volcanic buildings.

• - On slopes or steep slopes.

In many cases, friction in the fault plane produces the crushing or deformation of the rocks that make it up. The deformation band can reach several tens of meters thick. Depending on the training conditions, they can be of different types, among which there is a continuous gradation:[1]​.

• - Under conditions of brittle deformation, fault breccias occur, when the fragments (clasts) are visible to the naked eye, or fault flours, when the clasts are microscopic.

• - In deeper conditions and with higher temperatures, cataclasites are formed, which are rocks with greater cementation than the previous ones. If the friction of the fault increases the temperature, up to the melting point of some of the finer components of the rock, pseudotachylites, dark rocks with a glassy texture, can be produced.

• - When the deformation occurs in the ductile or brittle-ductile domain, under conditions of metamorphism, mylonites and ultramylonites are produced, which define the shear bands, with a characteristic banding of the rock.

The following characteristics are often useful in identifying faults in the terrain:.

• - Fault scarp: is the morphological feature produced on the Earth's surface due to the displacement of a fault.[6]​ They constitute rectilinear morphologies through which the topography varies abruptly. Three main types can be distinguished:[7]​

• Primitive or original fault scarp: when the scarp is recent or has not yet been dismantled by erosion.

• Fault line scarp or derivative: when the blocks involved have been eroded and the original fault jump is not preserved or the relief has even been inverted due to differential erosion.

• Composite fault scarp when the fault has been reactivated and the blocks involved have suffered differential erosion.

• - Triangular facets or trapezoidal: are shapes associated with fault line scarps or compound scarps due to erosion, due to the intersection of gullies or valleys perpendicular to the rectilinear plane of a fault scarp and oriented towards the sunken block.[7]​.

A fault is considered active when it has moved one or more times in the last 10,000 years.[8] Active faults are recognized by associated earthquakes, and in some cases they become evident by manifesting themselves with surface ruptures. Active faults can be seismic or seismic. In the first case, the displacement along segments of the fault plane occurs sporadically, due to the application of tectonic stresses in the vicinity of the fault, which produces the elastic deformation of the rocks in that environment. When the shear resistance of the rocks is exceeded by the magnitude of the stresses, rupture and displacement occur along the fault. The sudden displacement gives rise to an earthquake. After an earthquake, there are periods of less or no activity, in which the rocks begin to accumulate stress again.

Seismic faults, on the other hand, occur when stresses are permanently released by processes such as creep (creep")), or by successive small ruptures that cause earthquakes of very low magnitude and closely spaced in time.

When fault displacement is analyzed over geologic time (thousands to millions of years), regardless of whether the faults are seismic or aseismic, both types move at average speeds of a few millimeters to a few centimeters per year.

An example is the San Andreas fault system in southern and central California in the USA, which has generated the earthquakes of San Francisco (M=7.8, on the Richter scale) in 1906, Los Angeles (M=6.7) in 1994 and recently Hector Mine (M=7.1) in 1999 and San Luis Obispo "San Luis Obispo (California)") (M=6.6) in 2003. The faults of the central part of the San Andrés system, on the other hand, slip aseismically.

Inactive faults are those that originated in the geological past, and that have not shown recent activity. They do not represent any seismic danger to nearby populations.

• - Altyn Tagh Fault.

• - San Andreas Fault.

• - San Ramón Fault.

• - North Anatolian Fault").

• - Motagua Fault.

• - Fagnano-Magallanes Fault.

• - Albertino Fault.

• - Diaclase.

• - Reef (geology) "Reef (geology)").

• - Hydrogeology of geological faults.

• - Resistance of materials.

• - Plate tectonics.

• - Breaking stress.

• - Wikimedia Commons hosts a multimedia category on Failure.

From the point of view of the relative displacement of the blocks involved, the faults are classified as:[1]​[2][3]​.

• - Normal, direct or gravity failure:[4]​ when the hanging or roof block moves downward with respect to the lying or wall block. The fault plane is inclined.

• - Reverse fault: when the hanging block moves upward with respect to the bed. Low dip angle reverse faults are called thrusts. The fault plane is inclined.

• - Strike, direction, directional, transcurrent or strike-slip fault: when the displacement is horizontal and parallel "Parallelism (mathematics)") to the strike of the fault. They can be, depending on the direction of movement of the blocks (referring to the position of an observer located on one of the blocks), sinistral or left directional, when the block opposite the one occupied by the observer moves to the left, and dextral or right directional, when the block moves to the right. The fault plane can be inclined or vertical. A particular type of strike-slip faults are transform faults, which displace segments of constructive plate boundaries and the fault plane is usually vertical.

• - Oblique or mixed fault: when the displacement is oblique to both the strike and the direction of dip. They are simply described as a combination of the terminology of the previous ones, resulting in four possible cases: reverse sinistral, normal sinistral, reverse dextral and normal dextral.

• - Rotational fault: when there has been a rotation component in the relative displacement between the two blocks separated by the fault. In turn, they can be divided into:[3]​

• Scissors failure: when the axis of rotation is perpendicular to the failure plane.

• Cylindrical failure: when the axis of rotation is parallel to the failure plane. The fault plane is usually curved.

• Conical failure: when the axis of rotation is oblique to the failure plane. The fault plane is usually curved.

Contenido

Las estructuras vinculadas con las fallas dependen del tipo de régimen tectónico regional en el que se han formado. Sin embargo, hay algunas formas y términos comunes a todas ellas: es frecuente que las fallas varíen de buzamiento en su recorrido, mostrando zonas relativamente horizontales, rellanos, alternando con zonas más inclinadas, rampas. Los bloques delimitados entre rampas de fallas se denominan escamas tectónicas o horses y el apilamiento de estas escamas se denomina dúplex.[1]​.

In regions of tectonic extension

In a limited extension regime and in conditions of brittle deformation, systems of stepped normal faults develop, more or less parallel, which form sunken zones, called Grabens "Graben (geology)") or tectonic grabens, which can alternate with elevated zones, called Horst or tectonic pillars.[1]​.

If the extension is wide, the faults tend to horizontalize in depth (listric faults). In the development of the extension, fault systems can form with ramps and landings that follow one another and replace each other, delimiting scales that can be grouped into extensional duplexes.[1]​.

At the cortical scale, extensional faults that develop on the surface with brittle behavior pass at depth to the ductile domain, producing mylonitic bands in the detachment zone. In cases where stretching is important, thinning of the crust can occur—a process called tectonic denudation—and deep rocks can be lifted to the surface by isostatic readjustment.[1]​.

In regions of tectonic compression

The most common forms associated with compression are produced by reverse faults: thrusts and thrust mantles, typical of the external zones of collision orogens, in what is called "thrust belt" and corresponds to the thin-skinned tectonic style.[1]​.

In some regions affected by compression, with thrusts with detachments at the base of the upper crust or deeper (thick-skinned tectonic style), elevations of the pop-up type and depressions of the pop-down type (depressions between two thrusts of opposite vergence) can occur, both limited by reverse faults — in which they differ from horst and grabens, limited by normal faults. This model of pops-up and pops-down is applied, for example, to the Spanish Central System.[5]​.

At a cortical scale, the imbrication and stacking of fragments of continental crust can occur, as in the case of the Himalayan mountain range, in which extensive cortical blocks, delimited by large faults, ride on top of each other.[2]​.

In zones of transcurrent tectonics

In large strike-slip faults, whose displacement component is mainly horizontal, areas of local compression or extension may be delimited that produce uplift or subsidence movements. The relay or bridge between two nearby faults or the local curvature of a fault in direction produces an area in which the local direction of fracturing is oblique or perpendicular to the direction of main displacement, forming flakes and associated duplexes.[1]​.

Depending on the relief or rotation of the faults, to the right or left, and depending on their horizontal displacement, dextral or sinestral, the link zone between the two will have a compressive or distensive behavior of the scales and duplex that have been formed, developing fan elevations, push-up type, or pull-apart type tectonic depressions.[1]​.

They are those that are produced exclusively by the effect of gravity, not by the action of tectonic forces. They can occur in different geological contexts:[1]​.

• - In the mid-oceanic ridges, where they delimit the mid-oceanic rift.

• - In karst terrain, due to the dissolution of the substrate or collapse of cavities.

• - In volcanic regions, due to the collapse of magmatic chambers or the sliding of unstable volcanic buildings.

• - On slopes or steep slopes.

In many cases, friction in the fault plane produces the crushing or deformation of the rocks that make it up. The deformation band can reach several tens of meters thick. Depending on the training conditions, they can be of different types, among which there is a continuous gradation:[1]​.

• - Under conditions of brittle deformation, fault breccias occur, when the fragments (clasts) are visible to the naked eye, or fault flours, when the clasts are microscopic.

• - In deeper conditions and with higher temperatures, cataclasites are formed, which are rocks with greater cementation than the previous ones. If the friction of the fault increases the temperature, up to the melting point of some of the finer components of the rock, pseudotachylites, dark rocks with a glassy texture, can be produced.

• - When the deformation occurs in the ductile or brittle-ductile domain, under conditions of metamorphism, mylonites and ultramylonites are produced, which define the shear bands, with a characteristic banding of the rock.

The following characteristics are often useful in identifying faults in the terrain:.

• - Fault scarp: is the morphological feature produced on the Earth's surface due to the displacement of a fault.[6]​ They constitute rectilinear morphologies through which the topography varies abruptly. Three main types can be distinguished:[7]​

• Primitive or original fault scarp: when the scarp is recent or has not yet been dismantled by erosion.

• Fault line scarp or derivative: when the blocks involved have been eroded and the original fault jump is not preserved or the relief has even been inverted due to differential erosion.

• Composite fault scarp when the fault has been reactivated and the blocks involved have suffered differential erosion.

• - Triangular facets or trapezoidal: are shapes associated with fault line scarps or compound scarps due to erosion, due to the intersection of gullies or valleys perpendicular to the rectilinear plane of a fault scarp and oriented towards the sunken block.[7]​.

A fault is considered active when it has moved one or more times in the last 10,000 years.[8] Active faults are recognized by associated earthquakes, and in some cases they become evident by manifesting themselves with surface ruptures. Active faults can be seismic or seismic. In the first case, the displacement along segments of the fault plane occurs sporadically, due to the application of tectonic stresses in the vicinity of the fault, which produces the elastic deformation of the rocks in that environment. When the shear resistance of the rocks is exceeded by the magnitude of the stresses, rupture and displacement occur along the fault. The sudden displacement gives rise to an earthquake. After an earthquake, there are periods of less or no activity, in which the rocks begin to accumulate stress again.

Seismic faults, on the other hand, occur when stresses are permanently released by processes such as creep (creep")), or by successive small ruptures that cause earthquakes of very low magnitude and closely spaced in time.

When fault displacement is analyzed over geologic time (thousands to millions of years), regardless of whether the faults are seismic or aseismic, both types move at average speeds of a few millimeters to a few centimeters per year.

An example is the San Andreas fault system in southern and central California in the USA, which has generated the earthquakes of San Francisco (M=7.8, on the Richter scale) in 1906, Los Angeles (M=6.7) in 1994 and recently Hector Mine (M=7.1) in 1999 and San Luis Obispo "San Luis Obispo (California)") (M=6.6) in 2003. The faults of the central part of the San Andrés system, on the other hand, slip aseismically.

Inactive faults are those that originated in the geological past, and that have not shown recent activity. They do not represent any seismic danger to nearby populations.

• - Altyn Tagh Fault.

• - San Andreas Fault.

• - San Ramón Fault.

• - North Anatolian Fault").

• - Motagua Fault.

• - Fagnano-Magallanes Fault.

• - Albertino Fault.

• - Diaclase.

• - Reef (geology) "Reef (geology)").

• - Hydrogeology of geological faults.

• - Resistance of materials.

• - Plate tectonics.

• - Breaking stress.

• - Wikimedia Commons hosts a multimedia category on Failure.