In geology Geology is the science and study of the physical matter and energy that constitute the Earth. The field of geology encompasses the study of the composition, structure, properties, and history of the planet's physical material, the processes by which it is formed, moved, and changed, the history of life on Earth, and human interactions with the, a fault is a planar fracture 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 pieces. Fractures are commonly caused by stress exceeding the rock strength. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good in rock in which the rock on one side of the fracture has moved with respect to the rock on the other side. Large faults within the Earth's crust In geology, a crust is the outermost solid shell of a rocky planet or moon, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantles result from differential or shear motion, and active fault zones are the causal locations of most earthquakes An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or. Energy release during rapid slippage along faults causes earthquakes.

A fault line is the surface trace of a fault, the line of intersection between the fault plane and the Earth's surface.[1]

Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane.

The two sides of a non-vertical fault are called[by whom?] 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.

Contents

Mechanics

See also: Fault mechanics The Junction fault, dividing the Allegheny Plateau The Allegheny Plateau is a large dissected plateau area in western and central New York, northern and western Pennsylvania, northern and western West Virginia, and eastern Ohio. It is divided into the unglaciated Allegheny Plateau and the glaciated Allegheny Plateau and the true Appalachian Mountains Saint Pierre and Miquelon, Quebec, Nova Scotia, New Brunswick, Maine, New Hampshire, Vermont, Massachusetts, Connecticut, New York, Pennsylvania, Maryland, Virginia, West Virginia, Ohio, Kentucky, Tennessee, New Jersey, North Carolina, South Carolina, Georgia, and Alabama in Pennsylvania Pennsylvania has 51 miles of coastline along Lake Erie and 57 miles (92 km) of shoreline along the Delaware Estuary, United States ^ b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language

The relative motion of rocks on either side of the fault surface controls the origin and behavior of faults, in both an individual small fault and within the greater fault zones which define the tectonic plates Plate tectonics is a scientific theory which describes the large scale motions of Earth's lithosphere. It is vital for the existence of life on earth because of the role that it plays in the global cycle that maintains the balance of carbon between the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere.[citation needed] The theory.

Because of friction Friction is the force resisting the relative lateral motion of solid surfaces, fluid layers, or material elements in contact. Its colloquial opposite is slipperiness. Friction is usually subdivided into several varieties: and the rigidity of the rock, the rocks cannot simply glide or flow past each other. Rather, stress In continuum mechanics, stress is a measure of the average force per unit area of a surface within a deformable body on which internal forces act. In other words, it is a measure of the intensity of the internal forces acting between particles of a deformable body across imaginary internal surfaces . These internal forces are produced between the builds up in rocks and when it reaches a level that exceeds the strain threshold, the accumulated potential energy In physics, Potential energy is energy stored within a physical system as a result of the position or configuration of the different parts of that system. It has the potential to be converted into other forms of energy, such as kinetic energy, and to do work in the process. The SI unit of measure for energy and work is the joule (symbol J) is released[by whom?] as strain In continuum mechanics, deformation or strain is the change in the metric properties of a continuous body B in the displacement from an initial placement κ0 to a final placement κ(B). A change in the metric properties means that a curve drawn in the initial body placement changes its length when displaced to a curve in the final placement. If, which is focused into a plane along which relative motion is accommodated — the fault.

Strain is both accumulative and instantaneous depending on the rheology Rheology is the study of the flow of matter: primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. . It applies to substances which have a complex molecular structure, such as muds, sludges, suspensions, polymers of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing Shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics, whereas the brittle upper crust reacts by fracture - instantaneous stress release - to cause motion along the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain release causes earthquakes An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are measured with a seismometer; a device which also records is known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or, a common phenomenon along transform boundaries.

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. Only when sufficient microfractures link up into a large slip surface can a large seismic event or earthquake 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 that it may provide a promising tool for earthquake prediction on the scale of days to years.

AMR is being increasingly used{[whom}} to predict rock failures within mines, and applications are being attempted for the portions of faults within brittle rheological Rheology is the study of the flow of matter: primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. . It applies to substances which have a complex molecular structure, such as muds, sludges, suspensions, polymers conditions. researchers observe similar behaviour in the tremors preceding volcanic eruptions.

Slip, heave, throw

A fault in the Grands Causses as seen from Bédarieux, France France (pronounced /ˈfrænts/ frantss or /ˈfrɑːnts/ frahnts; French pronunciation (help·info): [fʁɑ̃s]), officially the French Republic (French: République française, pronounced: [ʁepyblik fʁɑ̃sɛz]), is a state in Western Europe with several of its overseas territories and islands located on other continents and in the Indian,. The left side moves down while the right side moves up. The warping of the rock layers on the right is likely due to drag folding.

Slip is defined as the relative movement of geological features present on either side of a fault plane, and is a displacement vector In physics, displacement is the vector that specifies the change in position of a point or a particle in reference to a previous position. When the previous point is the origin, this is better referred to as a position. A fault's sense of slip is defined as the relative motion of the rock on each side of the fault with respect to the other side.[2] In measuring the horizontal or vertical separation, the throw of the fault is the vertical component of the dip separation and the heave of the fault is the horizontal component, as in "throw up and heave out".[3]

The vector of slip can be qualitatively assessed by studying the fault bend folding, i.e., the drag folding of strata on either side of 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. In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector.

Fault types

Geologists can categorize faults into three groups based on the sense of slip:

  1. a fault where the relative movement (or slip) on the fault plane is approximately vertical is known as a dip-slip fault
  2. where the slip is approximately horizontal, the fault is known as a transcurrent or strike-slip fault
  3. an oblique-slip fault has non-zero components of both strike and dip Strike and dip refer to the orientation or attitude of a geologic feature. The strike of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map this is represented with a short straight line segment oriented parallel to the compass direction of the strike. Strike can slip

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 The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds or tilting.

Dip-slip faults

Students look at a section of the exposed Wasatch Fault The Wasatch Fault is an earthquake fault located primarily on the western edge of the Wasatch Mountains in the U.S. state of Utah. The fault is 240 miles long stretching from southern Idaho, through northern Utah, before terminating in central Utah near the town of Fayette. It is made up of several segments, on average 25 miles long, each of which (Normal fault), Utah Utah is one of the most religiously homogeneous states in the Union. Between 41% and 60% of Utahns are reported to be members of The Church of Jesus Christ of Latter-day Saints , which greatly influences Utah culture and daily life

Dip-slip faults can be sub-classified into the types "reverse" and "normal". A normal fault occurs when the crust is extended. Alternatively such a fault can be called an extensional fault. The hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is called a graben In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural. An upthrown block between two normal faults dipping away from each other is called a horst In physical geography and geology, a horst is the raised fault block bounded by normal faults or graben. A horst is formed from extension of the Earth's crust. The raised block is a portion of the crust that generally remains stationary or is uplifted while the land has dropped on either side. Low-angle normal faults with regional tectonic Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth (or other planets) and particularly with the forces and movements that have operated in a region to create these structures 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 shortening of the crust. The dip Strike and dip refer to the orientation or attitude of a geologic feature. The strike of a bed, fault, or other planar feature is a line representing the intersection of that feature with a horizontal plane. On a geologic map this is represented with a short straight line segment oriented parallel to the compass direction of the strike. Strike can of a reverse fault is relatively steep, greater than 45°.

Cross-sectional illustration of normal and reverse dip-slip faults

A thrust fault A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger. Thrust faults are the result of compressional forces 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 foot wall) folds The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. Thrust faults form nappes and klippen in the large thrust belts.

The fault plane is the plane In mathematics, a plane is a flat surface Chyea. Planes can arise as subspaces of some higher dimensional space, as with the walls of a room, or they may enjoy an independent existence in their own right, as in the setting of Euclidean geometry 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 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.

Strike-slip faults

The San Andreas Fault The San Andreas Fault is a continental transform fault that runs a length of roughly 810 miles through California in the United States. The fault's motion is right-lateral strike-slip (horizontal motion). It forms the tectonic boundary between the Pacific Plate and the North American Plate, a right-lateral strike-slip fault caused the massive 1906 San Francisco earthquake The San Francisco earthquake of 1906 was a major earthquake that struck San Francisco, California, and the coast of Northern California at 5:12 a.m. on Wednesday, April 18, 1906. The most widely accepted estimate for the magnitude of the earthquake is a moment magnitude of 7.9; however, other values have been proposed, from 7.7 to as high as 8.25 Schematic illustration of the two strike-slip fault types.

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 The terms sinistral and dextral refer to the horizontal movement of blocks on either side of a fault or the sense of movement within a shear faults. Those with right-lateral motion are also known as dextral The terms sinistral and dextral refer to the horizontal movement of blocks on either side of a fault or the sense of movement within a shear faults.

A special class of strike-slip faults is the transform fault A transform fault or transform boundary, also known as conservative plate boundary, is a fault which runs along the boundary of a tectonic plate. The relative motion of such plates is horizontal in either sinistral or dextral direction. Typically, some vertical motion may also exist, but the principal vectors in a transform fault are oriented, which is a plate tectonics Plate tectonics is a scientific theory which describes the large scale motions of Earth's lithosphere. The theory builds on the older concepts of continental drift, developed during the first decades of the 20th century by Alfred Wegener, and seafloor spreading, developed in the 1960s feature related to spreading centers such as mid-ocean ridges A mid-ocean ridge is an underwater mountain range, typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor spreading. The uplifted seafloor results from convection currents which rise. Transform faults are often referred to[by whom?] as transform plate boundaries.

Oblique-slip faults

Oblique-slip fault

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 Shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics and transpressional Shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics regimes, 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 fault

A listric fault is a type of normal fault in which fault plane is curved. The dip of the fault plane becomes shallower with depth.

Ring fault

Ring faults are faults that occur within collapsed volcanic calderas A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption such as the ones at Yellowstone National Park in the US and Glen Coe in Scotland. They are sometimes confused with volcanic craters. The word comes from Spanish caldera, and this from Latin CALDARIA, meaning "cooking pot". Ring faults may be filled by ring dikes.

Fault rock

Inactive fault from Sudbury Greater Sudbury is a city in Ontario, Canada. Greater Sudbury was created in 2001 by merging the cities and towns of the former Regional Municipality of Sudbury, along with several previously unincorporated geographic townships to Sault Ste. Marie Sault Ste. Marie (nicknamed "the Sault" or "the Soo") is a city on the St. Marys River in Ontario, Canada. It is the third largest city in Northern Ontario, after Sudbury and Thunder Bay, with a population of 74,948. Residents of the city are called Saultites. With a mission established by French Jesuits in 1668, claiming of, Northern Ontario, Canada The land occupied by Canada was inhabited for millennia by various groups of Aboriginal peoples. Beginning in the late 15th century, British and French expeditions explored, and later settled, along the Atlantic coast. France ceded nearly all of its colonies in North America in 1763 after the Seven Years' War. In 1867, with the union of three

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 The lithosphere is the rigid outermost shell of a rocky planet. It comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater 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 A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger. Thrust faults are the result of compressional forces.

The main types of fault rock include:

See also

Earthquakes portal

Notes

  1. ^ USGS (30 April 2003). "Where are the Fault Lines in the United States East of the Rocky Mountains?". http://earthquake.usgs.gov/learn/topics/faults_east.php. Retrieved 6 March 2010.
  2. ^ Marquis, John; Hafner, Katrin; Hauksson, Egill. "The Properties of Fault Slip". Investigating Earthquakes through Regional Seismicity. Southern California Earthquake Center. p. 14. http://www.data.scec.org/Module/sec1pg14.html. Retrieved 19 March 2010.
  3. ^ "Faults: Introduction". University of California, Santa Cruz. http://ic.ucsc.edu/~casey/eart150/Lectures/2faultsintro.html/2faultsintro.htm. Retrieved 19 March 2010.

References

External links

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Categories: Seismic faults | Seismology and earthquake terminology | Structural geology

 

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