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"Fault line" redirects here. For other uses, see Fault line (disambiguation).

In geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement. Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit A fracture is any local separation or discontinuity plane in a Geologic formation, such as a joint or a fault that divides the rock into two or more Large faults within the Earth's crust are the result of differential or shear motion and active fault zones are the causal locations of most earthquakes. In Geology, a crust is the outermost solid shell of a planet or moon An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer Earthquakes are caused by energy release during rapid slippage along a fault. A fault which runs along the boundary between two tectonic plates is called a transform fault. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere A transform fault is a fault which runs along the boundary of a Tectonic plate.

Since faults do not usually consist of a single, clean fracture, the term fault zone is used when referring to the zone of complex deformation that is associated with the fault plane. The two sides of a non-vertical fault are called the hanging wall and footwall. By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. 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 hanging above him.

Figure 1. Fault in shales near Adelaide, Australia
Figure 1. Fault in shales near Adelaide, Australia

Contents

Mechanics

Figure 2. The Junction fault, dividing the Allegheny Plateau and the true Appalachian Mountains in Pennsylvania.
Figure 2. Adelaide is the capital and most populous city of the Australian state of South Australia, and is the fifth largest city in Australia with a For a topic outline on this subject see List of basic Australia topics. The Junction fault, dividing the Allegheny Plateau and the true Appalachian Mountains in Pennsylvania. The Appalachian Mountains ( often called the Appalachians, are a vast system of mountains in eastern North America. The Commonwealth of Pennsylvania ( often colloquially referred to as PA (its abbreviation by natives and Northeasterners is a state located in the Northeastern

The creation and behaviour of faults, in both an individual small fault and within the greater fault zones which define the tectonic plates, is controlled by the relative motion of rocks on either side of the fault surface.

Because of friction and the rigidity of the rock, the rocks cannot simply glide or flow past each other. Friction is the Force resisting the relative motion of two Surfaces in contact or a surface in contact with a fluid (e Rather, stress builds up in rocks and when it reaches a level that exceeds the strain threshold, the accumulated potential energy is released as strain, which is focused into a plane along which relative motion is accommodated — the fault. Stress is a measure of the average amount of Force exerted per unit Area. Potential energy can be thought of as Energy stored within a physical system

Strain is both accumulative and instantaneous depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture, or instantaneous stress release to cause motion along the fault. Rheology is the study of the flow of matter mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics. A fault in ductile rocks can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release is the cause of earthquakes, a common phenomenon along transform boundaries. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer

Microfracturing and AMR theory

Microfracturing, or microseismicity, is sometimes thought of as a symptom caused by rocks under strain, where small-scale failures, perhaps on areas the size of a dinner plate or a small area, release stress under high strain conditions. It is only when sufficient microfractures link up into a large slip surface that a large seismic event or earthquake can occur.

According to this theory, after a large earthquake, the majority of the stress is released and the frequency of microfracturing is exponentially lower. A related theory, accelerating moment release (AMR), hypothesizes that the seismicity rate accelerates in a well-behaved way prior to large earthquakes, and may be a promising tool for earthquake prediction on the scale of days to years.

This is being increasingly used to predict rock failures within mines and applications are being attempted for the portions of faults within brittle rheological conditions. Rheology is the study of the flow of matter mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically Similar behaviour is observed in the tremors preceding volcanic eruptions.

Slip, heave, throw

A fault in Bédarieux, France. The left part moves down while the right part moves up.
A fault in Bédarieux, France. Bédarieux, (Bedarius is a commune in the Hérault department in the region of Languedoc-Roussillon in southern France The left part moves down while the right part moves up.

The sense of slip is defined by the relative movements of geological features present on either side of the fault plane and is a vector. The sense of slip defines the type of fault. This is distinct from the throw of the fault, which is the vertical offset. Heave is the measured horizontal offset of the fault.

The vector of slip can be qualitatively measured by fault bend folding, drag folding of strata on either side of the fault (figure 2), and the direction and magitude of heave and throw can be measured only by finding common intersection points on either side of the fault. In practise it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector.

Fault types

Faults can be categorized into three groups based on the sense of slip. A fault where the main sense of movement (or slip) on the fault plane is vertical is known as a dip-slip fault. Where the main sense of slip is horizontal the fault is known as a transcurrent or strike-slip fault. Oblique-slip faults have significant components of both strike and dip slip. Strike and dip refer to the orientation or attitude of a Geologic feature

For all naming distinctions, it is the orientation of the net dip and sense of slip of the fault which must be considered, not the present-day orientation, which may have been altered by local or regional folding or tilting. See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary

Dip-slip faults

Figure 3. Fault types. USGS image
Figure 3. Fault types. USGS image

Dip-slip faults can be again classified into the types "reverse" and "normal". The United States Geological Survey ( USGS) is a scientific agency of the United States government. A normal fault occurs when the crust is extended. Alternatively such a fault can be called an extensional fault. An Extensional Fault is a fault that extends the pre-existing geological layering The hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is called a graben. A graben is a depressed block of land bordered by parallel faults Graben is German for ditch. An upthrown block between two normal faults dipping away from each other is called a horst. This article refers to the geological form For other uses see Horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults. This article discusses the geologic usage for the philosophical or architectural usage see Architectonics ' Or see Plate tectonics. Detachment faulting is associated with large-scale lithospheric Extensional tectonics.

A reverse fault is the opposite of a normal fault — the hanging wall moves up relative to the footwall. Reverse faults are indicative of shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°. Strike and dip refer to the orientation or attitude of a Geologic feature

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°. A thrust fault is a type of fault, or break in the Earth's crust with resulting movement of each side against the other in which a lower stratigraphic position is pushed up Thrust faults typically form ramps, flats and fault-bend (hanging wall and foot wall) folds. See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary Thrust faults are responsible for forming nappes and klippen in the large thrust belts. In Geology, a nappe is a large sheetlike body of rock that has been moved more than 2 km (1 A klippe (from German = Cliff) is a Geological feature of Thrust fault terranes

The fault plane is the plane that represents the fracture surface of a fault. 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 section with ramps.

Fault-bend folds are formed by movement of the hangingwall 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.

Figure 4. Cross-sectional illustration of normal and reverse dip-slip faults.
Figure 4. Cross-sectional illustration of normal and reverse dip-slip faults.

Strike-slip faults

One of the United States's most famous, the San Andreas Fault, a right-lateral strike-slip fault; it caused the massive 1906 San Francisco Earthquake.
One of the United States's most famous, the San Andreas Fault, a right-lateral strike-slip fault; it caused the massive 1906 San Francisco Earthquake. The United States of America —commonly referred to as the The San Andreas Fault is a geologic Transform fault that runs a length of roughly 800 miles (1300 km through California in the United States.

The fault surface is usually near vertical and the footwall moves either left or right or laterally with very little vertical motion. Strike-slip faults with left-lateral motion are also known as sinistral faults. This article is about the geologic usage for human physiology usage see the articles Laterality. Those with right-lateral motion are also known as dextral faults. This article is about the geologic usage for human physiology usage see the articles Laterality. A special class of strike-slip faults is the transform faults which are a plate tectonics feature related to spreading centers such as mid-ocean ridges. A transform fault is a fault which runs along the boundary of a Tectonic plate. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by

Figure 5. Schematic illustration of the two strike-slip fault types.
Figure 5. Schematic illustration of the two strike-slip fault types.

Oblique-slip faults

Figure 6. Fault along the southern boundary of Makhtesh Ramon, Negev, southern Israel.
Figure 6. Fault along the southern boundary of Makhtesh Ramon, Negev, southern Israel. A makhtesh (מכתש plural מכתשים - "makhteshim" is a geological landform regarded to be unique to the Negev desert of Israel and the The Negev (נֶגֶב Tiberian vocalization: Néḡeḇ) is the Desert region of southern Israel. For a topic outline on this subject see List of basic Israel topics.

A fault which has a component of dip-slip and a component of strike-slip is termed an 'oblique-slip fault'. Nearly all faults will have some component of both dip-slip and strike-slip, so 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, others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active. Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics. Study of geological shear is related to the study of Structural geology, Rock microstructure or rock texture and fault mechanics.

Fault rock

All faults have a measurable thickness, made up of deformed rock that is characteristic of the level in the crust where the faulting happened, the rock types affected by the fault and 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 will tend 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. Detachment faulting is associated with large-scale lithospheric Extensional tectonics. A thrust fault is a type of fault, or break in the Earth's crust with resulting movement of each side against the other in which a lower stratigraphic position is pushed up

The main types of fault rock are:

See also

References

External links

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