View source for Fault (geology)

{{Short description|Fracture or discontinuity in displaced rock}}
{{Redirect|Fault line}}{{Redirect|Shatter belt|the concept in geopolitics|Shatter belt (geopolitics)}}
{{Use dmy dates|date=July 2022}}
[[File:Piqiang Fault, China detail.jpg|thumb|upright=1.3|Satellite image of a fault in the [[Taklamakan Desert]]. The two colorful ridges (at bottom left and top right) used to form a single continuous line, but have been split apart by movement along the fault.]]
In [[geology]], a '''fault''' is a [[Fracture (geology)|planar fracture]] or discontinuity in a volume of [[Rock (geology)|rock]] across which there has been significant displacement as a result of rock-mass movements. Large faults within [[Earth]]'s [[crust (geology)|crust]] result from the action of [[Plate tectonics|plate tectonic]] forces, with the largest forming the boundaries between the plates, such as the megathrust faults of [[subduction|subduction zones]] or [[transform fault]]s.<ref>{{cite book |last1=Lutgens |first1=Frederick K. |last2=Tarbuck |first2=E.J. |last3=Tasa |first3=D. (illustrator) |title=Essentials of geology |date=2012 |publisher=Prentice Hall |location=Boston |isbn=978-0321714725 |page=32 |edition=11th}}</ref> Energy release associated with rapid movement on [[active fault]]s is the cause of most [[earthquake]]s. Faults may also displace slowly, by [[aseismic creep]].<ref name="Ohnaka">{{cite book |url=https://books.google.com/books?id=Bp0gAwAAQBAJ&pg=PA234|title=The Physics of Rock Failure and Earthquakes|publisher=Cambridge University Press|author=Ohnaka, M. |year=2013 |isbn=978-1-107-35533-0}}</ref>

A ''fault plane'' is the [[Plane (geometry)|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 [[geological map]]s to represent a fault.<ref>{{citation |ref=CITEREFUSGSFault Traces|title=fault trace |work=Earthquake Glossary |publisher=United States Geological Survey|access-date=10 April 2015|url=https://earthquake.usgs.gov/learn/glossary/?term=fault%20trace.php}}</ref><ref>{{citation |ref=CITEREFUSGSFault Lines |first=Robert |last=Tristram |title=Where are the Fault Lines in the United States East of the Rocky Mountains?|publisher=United States Geological Survey |date=30 April 2003|access-date=6 March 2010 |url=https://earthquake.usgs.gov/learn/topics/faults_east.php |archive-date=18 November 2009|archive-url=https://web.archive.org/web/20091118043816/http://earthquake.usgs.gov/learn/topics/faults_east.php}}</ref>

A '''''fault zone''''' is a cluster of parallel faults.<ref>[https://web.archive.org/web/20111119125927/http://www.merriam-webster.com/dictionary/fault%20zone “Fault zone.”] Merriam-Webster.com Dictionary, Merriam-Webster. Retrieved 8 Oct. 2020.</ref><ref>{{cite book |last1=Fillmore|first1=Robert|title=Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs|date=2010|publisher=University of Utah Press|location=Salt Lake City |isbn=9781607810049|page=337}}</ref> However, the term is also used for the zone of crushed rock along a single fault.<ref>{{cite journal |last1=Caine|first1=Jonathan Saul|last2=Evans|first2=James P.|last3=Forster|first3=Craig B. |title=Fault zone architecture and permeability structure |journal=Geology|date=1 November 1996|volume=24|issue=11|pages=1025–1028|doi=10.1130/0091-7613(1996)024<1025:FZAAPS>2.3.CO;2 |bibcode=1996Geo....24.1025S}}</ref> 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.<ref>{{cite journal |last1=Childs |first1=Conrad |last2=Manzocchi |first2=Tom |last3=Walsh |first3=John J. |last4=Bonson |first4=Christopher G. |last5=Nicol |first5=Andrew |last6=Schöpfer |first6=Martin P.J. |title=A geometric model of fault zone and fault rock thickness variations |journal=Journal of Structural Geology |date=February 2009 |volume=31 |issue=2 |pages=117–127 |doi=10.1016/j.jsg.2008.08.009 |bibcode=2009JSG....31..117C}}</ref>

==Mechanisms of faulting==
{{See also|Fault mechanics}}Due 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 ''[[Asperity (faults)|asperities]]''. [[Stress (mechanics)|Stress]] builds up when a fault is locked, and when it reaches a level that exceeds the [[Shear strength|strength]] threshold, the fault ruptures and the accumulated [[strain energy]] is released in part as [[seismic wave]]s, forming an [[earthquake]].<ref name="Ohnaka"/>

Strain occurs accumulatively or instantaneously, depending on the [[rheology|liquid state]] of the rock; the [[Ductility (Earth science)|ductile]] lower crust and [[Mantle (geology)|mantle]] accumulate deformation gradually via [[Shear (geology)|shearing]], whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault.<ref>{{cite book |last1=Fossen |first1=Haakon |title=Structural geology |date=2016 |location=Cambridge, UK |publisher=Cambridge University Press |isbn=9781107057647 |pages=117, 178 |edition=2nd}}</ref> A fault in ductile rocks can also release instantaneously when the strain rate is too great.

==Slip, heave, throw==
''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.<ref>{{Harvnb|SCEC|Education Module|p= 14}}.</ref> 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".<ref>{{cite web |url=http://ic.ucsc.edu/~casey/eart150/Lectures/2faultsintro.html/2faultsintro.htm|archive-url=https://web.archive.org/web/20110927152011/http://ic.ucsc.edu/~casey/eart150/Lectures/2faultsintro.html/2faultsintro.htm|archive-date=2011-09-27 |title=Faults: Introduction|publisher=[[University of California, Santa Cruz]]|access-date=19 March 2010}}</ref>
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.<ref>{{cite journal |last1=Choi |first1=Pom-yong |last2=Lee |first2=Seung Ryeol |last3=Choi |first3=Hyen-Il |last4=Hwang |first4=Jae-ha |last5=Kwon |first5=Seok-ki |last6=Ko |first6=In-sae |last7=An |first7=Gi-o |title=Movement history of the Andong Fault System: Geometric and tectonic approaches |journal=Geosciences Journal |date=June 2002 |volume=6 |issue=2 |pages=91–102 |doi=10.1007/BF03028280 |bibcode=2002GescJ...6...91C |s2cid=206832817}}</ref> Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault.{{sfn|Fossen|2016|p=479}} 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.

==Hanging wall and footwall==
[[File:Hanging & footwall.jpg|thumb|Hanging & footwall]]
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.<ref>{{citation|ref=CITEREFUSGSHanging Wall|title=Hanging wall Foot wall|publisher=United States Geological Survey|access-date=2 April 2010|url=http://www.nature.nps.gov/geology/usgsnps/deform/ghangft.html|archive-date=8 May 2009|archive-url=https://web.archive.org/web/20090508011706/http://www.nature.nps.gov/geology/usgsnps/deform/ghangft.html|url-status=dead}}</ref> 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.<ref>{{citation|last1=Tingley|first1=J.V.|last2=Pizarro|first2=K.A.|title=Traveling America's loneliest road: a geologic and natural history tour|publisher=Nevada Bureau of Mines and Geology|year=2000|series=Nevada Bureau of Mines and Geology Special Publication|volume=26|isbn=978-1-888035-05-6|access-date=2010-04-02|url=https://books.google.com/books?id=qBZo1TNsgAcC&pg=PA8|page=132}}</ref> 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.

The problem of the hanging wall can lead to severe stresses and [[rock bursts]], for example at [[Frood Mine]].<ref name=thompson60/>

==Fault types==

Faults are mainly classified in terms of the angle that the fault plane makes with the Earth's surface, known as the [[Strike and dip|dip]], and the direction of slip along the fault plane.<ref>{{cite web|title=What is a fault and what are the different types?|url=https://www.usgs.gov/faqs/what-a-fault-and-what-are-different-types?qt-news_science_products=0#qt-news_science_products|website=USGS: Science for a Changing World|access-date=13 October 2021}}</ref>
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.

===Strike-slip faults===
{{further|Strike-slip tectonics}}
[[File:strike slip fault.png|thumb|Schematic illustration of the two strike-slip fault types, as seen from above]]
In a '''strike-slip fault''' (also known as a ''wrench fault'', ''tear fault'' or ''transcurrent fault''),<ref>{{cite dictionary|editor-last=Allaby|editor-first=Michael|entry=Strike-Slip Fault|dictionary=A Dictionary of Geology and Earth Sciences|edition=4th|publisher=Oxford University Press|year=2015|isbn=978-0-19-965306-5|chapter-url=http://www.oxfordreference.com/view/10.1093/acref/9780199653065.001.0001/acref-9780199653065-e-8171}}</ref> 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.<ref>{{cite book|last=Park|first=R.G.|title=Foundation of Structural Geology|edition=3|page=11|year=2004|publisher=Routledge|isbn=978-0-7487-5802-9|url=https://books.google.com/books?id=ycASqdxSG3YC&pg=PA11}}</ref> 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 tectonics|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===
[[File:Nor rev.png|thumb|Vertical [[Cross section (geology)|cross-sectional]] view, along a plane perpendicular to the ''fault plane'', illustrating normal and reverse dip-slip faults]]

'''Dip-slip faults''' can be either ''normal'' ("[[extensional fault|extensional]]") or ''reverse''. The terminology of "normal" and "reverse" comes from [[coal mining]] in England, where normal faults are the most common.<ref name="Peacock_etal_2000">{{cite journal|last1=Peacock |first1=D. C. P.|last2=Knipe|first2=R. J. |last3=Sanderson|first3=D. J.|year=2000|title=Glossary of normal faults|journal=Journal of Structural Geology|volume=22|issue=3 |page=298|doi=10.1016/S0191-8141(00)80102-9 |bibcode=2000JSG....22..291P}}</ref>

With the passage of time, a regional reversal between [[Tension (geology)|tensional]] and [[Compression (geology)|compressional]] [[Stress (mechanics)|stresses]] (or vice-versa) might occur, and faults may be reactivated with their relative block movement inverted in opposite directions to the original movement (fault inversion). In such a way, a normal fault may therefore become a reverse fault and vice versa.

{{clear}}

====Normal faults====
[[File:Normal fault diagram.gif|thumb|upright=0.6|left|Normal fault diagram]]
In a normal fault, the hanging wall moves downward, relative to the footwall. The [[Strike and dip|dip]] of most normal faults is at least 60 degrees but some normal faults dip at less than 45 degrees.<ref name="Oskin_2019">{{cite web|author=Oskin, Michael E. |url=https://geo.libretexts.org/Courses/University_of_California_Davis/GEL_101%3A_Structural_Geology_(Oskin)/Part_I%3A_Brittle_Deformation_and_Mechanics/2A%3A_Normal_Faults|title=Normal Faults |work=LibreTexts|date=3 June 2019|access-date=6 April 2022}}</ref>
{{clear}}

=====Basin and range topography=====
[[File:Fault-Horst-Graben.svg|thumb|upright=1.35|Diagram illustrating the structural relationship between grabens and horsts.]]
A downthrown block between two normal faults dipping towards each other is a [[graben]]. A block stranded between two grabens, and therefore two normal faults dipping away from each other, is a [[Horst (geology)|horst]]. A sequence of grabens and horsts on the surface of the Earth produces a characteristic ''[[basin and range topography]]''.
{{clear}}

=====Listric faults=====
A listric fault is a type of normal fault that has a concave-upward shape with the upper section near Earth's surface being steeper, becoming more horizontal with increased depth.  Normal faults can evolve into listric faults with the fault plane curving into the Earth.  They can also form where the hanging wall is absent (such as on a cliff), where the footwall may slump in a manner that creates multiple listric faults.
<gallery>
File:Listric fault diagram.jpg|Cross-section diagram of a listric fault
File:Listric walls in a cliff wall.jpg|Cross-section diagram of multiple listric faults in a cliff wall
</gallery>

=====Detachment faults=====
The fault planes of listric faults can further flatten and evolve into a horizontal or near-horizontal plane, where slip progresses horizontally along a [[decollement]]. [[Extensional tectonics|Extensional]] decollements can grow to great dimensions and form [[detachment fault]]s, which are low-angle normal faults with regional [[tectonics|tectonic]] significance.

Due to the curvature of the fault plane, the horizontal extensional displacement on a listric fault implies a geometric "gap" between the hanging and footwalls of the fault forms when the slip motion occurs. To accommodate into the geometric gap, and depending on its [[rheology]], the hanging wall might fold and slide downwards into the gap and produce [[Rollover anticlines|rollover folding]], or break into further faults and blocks which fill in the gap.  If faults form, '''imbrication fans''' or '''domino faulting''' may form.
<gallery>
File:Rollover.png|Cross-section diagram of a listric fault (red line), with a resulting rollover fold
File:Listric faults and imbrication fan.jpg|Cross-section diagram showing how an imbrication fan forms
The fault planes of multiple listric faults can flatten and connect at depth into a common [[décollement]], .
</gallery>

====Reverse faults====
[[File:Reverse fault.gif|thumb|upright=0.6|left|Reverse fault]]
A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall.<br>
Reverse faults indicate compressive shortening of the crust.
{{clear}}

=====Thrust faults=====
[[File:Thrust with fault bend fold.svg|thumb|Cross-section diagram of a thrust fault with a fault-bend fold]]
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°.<ref name="USGSGloss">{{cite web|url=https://earthquake.usgs.gov/learn/glossary/?term=dip%20slip|title=dip slip|work=Earthquake Glossary|publisher=United States Geological Survey|access-date=13 December 2017|archive-url=https://web.archive.org/web/20171123090300/https://earthquake.usgs.gov/learn/glossary/?term=dip%20slip|archive-date=23 November 2017|url-status=live}}</ref><ref name="UCSB">{{cite web|url=http://scienceline.ucsb.edu/getkey.php?key=2845|title=How are reverse faults different than thrust faults? In what way are they similar?|work=UCSB Science Line|publisher=[[University of California, Santa Barbara]]|date=13 February 2012|access-date=13 December 2017|archive-url=https://web.archive.org/web/20171027080203/http://scienceline.ucsb.edu/getkey.php?key=2845|archive-date=27 October 2017|url-status=live}}</ref> Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds.

A section of a hanging wall or foot wall where a thrust fault formed along a relatively weak bedding plane is known as a ''flat'' and a section where the thrust fault cut upward through the stratigraphic sequence is known as a ''ramp''.<ref>{{cite book|last=Park|first=R.G.|title=Foundation of Structural Geology|edition=3|page=15|year=2004|publisher=Routledge|isbn=978-0-7487-5802-9|url=https://books.google.com/books?id=ycASqdxSG3YC&pg=PA11}}</ref> Typically, thrust faults move ''within'' formations by forming flats and climbing up sections with ramps. This results in the hanging wall flat (or a portion thereof) lying atop the foot wall ramp as shown in the fault-bend fold diagram.

Thrust faults form [[nappe]]s and [[klippe]]n in the large thrust belts. [[Subduction]] zones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes.

===Oblique-slip faults===
[[File:Oblique slip fault.svg|thumb|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 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 [[Shear (geology)#Transtension|transtensional]] and [[Shear (geology)#Transpression|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 [[Angle#complementary angle|complement]] of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.

===Ring fault===
'''Ring faults''', also known as '''caldera faults''', are faults that occur within collapsed volcanic [[caldera]]s<ref name=":0">{{cite web|url=http://maps.unomaha.edu/Maher/geo330/melissa1.html|title=Structural Geology Notebook – Caldera Faults.|website=maps.unomaha.edu|access-date=2018-04-06|archive-url=https://web.archive.org/web/20181119172858/http://maps.unomaha.edu/Maher/geo330/melissa1.html|archive-date=2018-11-19|url-status=live}}</ref> 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 dike]]s.<ref name=":0" />

===Synthetic and antithetic faults===
'''''Synthetic''''' and '''''antithetic''''' 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).

==Fault rock==
[[File:FaultZone.jpg|thumb|Structure of a fault<ref>{{cite journal |last1=Jin-Hyuck |first1=Choi |last2=Paul |first2=Edwards |last3=Kyoungtae |first3=Ko |last4=Kim |first4=Young-Seog |title=Definition and classification of fault damage zones: A review and a new methodological approach |journal=Earth-Science Reviews|date=January 2016|volume=152|pages=70–87 |doi=10.1016/j.earscirev.2015.11.006 |bibcode=2016ESRv..152...70C |url=https://www.sciencedirect.com/science/article/pii/S0012825215300672|url-access=subscription }}</ref>]]
[[File:FaultGouge.JPG|thumb|Salmon-colored [[fault gouge]] and associated fault separates two different rock types on the left (dark gray) and right (light gray). From the [[Gobi]] of [[Mongolia]].]]
[[File:CREIGHTON-fault-sudbury-basin-science-north.jpg|thumb|upright|Inactive fault from [[Greater Sudbury|Sudbury]] to [[Sault Ste. Marie, Ontario|Sault Ste. Marie]], Northern Ontario, Canada]]

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 [[Hydrothermal circulation|mineralising fluids]]. Fault rocks are classified by their [[Rock microstructure|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 fault]]s and major [[thrust fault]]s.

The main types of fault rock include:
* [[Cataclasite]] – a fault rock which is cohesive with a poorly developed or absent planar [[Fabric (geology)|fabric]], or which is incohesive, characterised by generally angular [[Clastic rock|clasts]] and rock fragments in a finer-grained [[Matrix (geology)|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 particles|ultrafine]]-grained cataclasite, which may possess a planar fabric and containing <30% visible fragments. Rock clasts may be present
*** [[Clay smear]] – clay-rich fault gouge formed in [[Sedimentary rock|sedimentary]] sequences containing clay-rich layers which are strongly deformed and sheared into the fault gouge.
* [[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 [[porphyroclast]]s and rock fragments of similar composition to [[mineral]]s in the matrix
* [[Pseudotachylyte]] – ultrafine-grained glassy-looking material, usually black and [[flint]]y in appearance, occurring as thin planar [[Vein (geology)|veins]], injection veins or as a matrix to [[Conglomerate (geology)|pseudoconglomerates]] or [[breccia]]s, 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.<ref>{{cite journal|last1=Rowe|first1=Christie|last2=Griffith|first2=Ashley|year=2015|title=Do faults preserve a record of seismic slip: A second opinion|url=https://par.nsf.gov/servlets/purl/10041361|journal=Journal of Structural Geology|volume=78|pages=1–26 |doi=10.1016/j.jsg.2015.06.006|bibcode=2015JSG....78....1R}}</ref>

==Impacts on structures and people==
In [[geotechnical engineering]], a fault often forms a [[Discontinuity (Geotechnical engineering)|discontinuity]] that may have a large influence on the mechanical behavior (strength, deformation, etc.) of [[soil]] and rock masses in, for example, [[tunnel]], [[Foundation (engineering)|foundation]], or [[Slope stability analysis|slope]] construction.

The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the [[Seismic wave|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 [[Geologic time scale|Holocene]] Epoch (the last 11,700 years) of the Earth's geological history.<ref>{{citation|last1=Brodie|first1=Kate|last2=Fettes|first2=Douglas|last3=Harte|first3=Ben|last4=Schmid|first4=Rolf|date=29 January 2007|publisher=International Union of Geological Sciences|title=Structural terms including fault rock terms|url=http://www.bgs.ac.uk/SCMR/products.html}}</ref> 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 rock|carbonate]] [[Nodule (geology)|nodules]], [[Erosion|eroded]] clay, and [[Iron ore#Sources|iron]] [[Oxide minerals|oxide]] mineralization, in the case of older soil, and lack of such signs in the case of younger soil. [[Radiocarbon dating]] of [[Organic compound|organic]] material buried next to or over a fault shear is often critical in distinguishing active from inactive faults. From such relationships, [[paleoseismology|paleoseismologists]] can estimate the sizes of past [[earthquakes]] over the past several hundred years, and develop rough projections of future fault activity.

==Faults and ore deposits==
Many ore deposits lie on or are associated with faults. This is because the fractured rock associated with fault zones allow for magma ascent<ref>{{Cite journal|last1=Troll|first1=V R|last2=Mattsson|first2=T|last3=Upton|first3=B G J|last4=Emeleus|first4=C H |last5=Donaldson |first5=C H|last6=Meyer|first6=R|last7=Weis|first7=F|last8=Dahrén|first8=B|last9=Heimdal|first9=T H|date=2020-10-09|title=Fault-Controlled Magma Ascent Recorded in the Central Series of the Rum Layered Intrusion, NW Scotland|journal=Journal of Petrology |volume=61 |issue=10 |doi=10.1093/petrology/egaa093|issn=0022-3530|doi-access=free|hdl=10023/23208|hdl-access=free}}</ref> or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits.<ref name=Piqueretal1019>{{cite journal |last1=Piquer Romo|first1=José Meulen |last2=Yáñez |first2=Gonzálo |last3=Rivera |first3=Orlando |last4=Cooke |first4=David |year=2019 |title=Long-lived crustal damage zones associated with fault intersections in the high Andes of Central Chile |doi=10.5027/andgeoV46n2-3108 |url=http://www.andeangeology.cl/index.php/revista1/article/view/V46n2-3106/pdf|journal=[[Andean Geology]] |volume=46 |issue=2 |pages=223–239 |access-date=June 9, 2019|archive-url=https://web.archive.org/web/20190808191345/http://www.andeangeology.cl/index.php/revista1/article/view/V46n2-3106/pdf|archive-date=August 8, 2019 |url-status=live|doi-access=free}}</ref>

An example of a fault hosting valuable [[porphyry copper deposit]]s is northern Chile's [[Domeyko Fault]] with deposits at [[Chuquicamata]], [[Collahuasi]], [[El Abra, Chile|El Abra]], [[El Salvador mine|El Salvador]], [[Escondida|La Escondida]] and [[Potrerillos Mine|Potrerillos]].<ref>{{cite book|last=Robb|first=Laurence|title=Introduction to Ore-Forming Processes|edition=4th|year=2007|publisher=[[Blackwell Science]] Ltd|location=[[Malden, MA]], United States|isbn=978-0-632-06378-9|page=104}}</ref> Further south in Chile [[Los Bronces mine|Los Bronces]] and [[El Teniente]] porphyry copper deposit lie each at the intersection of two fault systems.<ref name=Piqueretal1019/>

Faults may not always act as conduits to surface. It has been proposed that deep-seated "misoriented" faults may instead be zones where magmas forming porphyry copper stagnate achieving the right time for—and type of—[[igneous differentiation]].<ref name=piqueetal2021>{{Cite journal |last1=Piquer |first1=José |last2=Sanchez-Alfaro |first2=Pablo |last3=Pérez-Flores |first3=Pamela |title=A new model for the optimal structural context for giant porphyry copper deposit formation|journal=[[Geology (journal)|Geology]] |volume=49 |pages=597–601 |issue=5 |year=2021 |doi=10.1130/G48287.1 |bibcode=2021Geo....49..597P |s2cid=234008062 |doi-access=free}}</ref> At a given time differentiated magmas would burst violently out of the fault-traps and head to shallower places in the crust where porphyry copper deposits would be formed.<ref name=piqueetal2021/>

== Groundwater ==
As faults are zones of weakness, they facilitate the interaction of water with the surrounding rock and enhance chemical [[weathering]]. The enhanced chemical weathering increases the size of the weathered zone and hence creates more space for [[groundwater]].<ref>{{Cite journal |last1=Pradhan|first1=Rudra Mohan|last2=Singh|first2=Anand|last3=Ojha|first3=Arun Kumar|last4=Biswal|first4=Tapas Kumar|date=2022-07-12 |title=Structural controls on bedrock weathering in crystalline basement terranes and its implications on groundwater resources |journal=Scientific Reports|volume=12|issue=1|pages=11815|doi=10.1038/s41598-022-15889-x|issn=2045-2322|pmc=9276672|pmid=35821387 |bibcode=2022NatSR..1211815P}}</ref> Fault zones act as [[aquifer]]s and also assist groundwater transport.

== Gallery ==
<gallery>
File:Microfault.jpg|Microfault showing a [[piercing point]] (the coin's diameter is {{Convert|18|mm|abbr=on}})
File:Fault in Seppap Gorge Morocco.jpg|A normal fault in [[Morocco]]. The fault plane is the steeply leftward-dipping line in the centre of the photo, which is the plane along which the rock layers to the left have slipped downwards, relative to the layers to the right of the fault.
File:Falla normal Morro Solar Peru.jpg|Normal fault with displacement motion from top left to bottom right. [[La Herradura Formation]], [[Morro Solar]], [[Peru]].
File:Fault Breccia from Galicia Spain 1.jpg|Fault [[breccia]] from Spain
</gallery>

==See also==
{{Portal|Earth sciences}}
* [[List of fault zones]]
* [[Anderson's theory of faulting]]
* [[Aseismic creep]]
* {{annotated link|Fault block}}
* {{annotated link|Fault scarp}}
* {{annotated link|Joint (geology)|Joint}}
* {{annotated link|Mitigation of seismic motion}}
* {{annotated link|Mountain formation}}
* {{annotated link|Orogeny}}
*[[Paleostress inversion]]
* {{annotated link|Seismic hazard}}
* {{annotated link|Striation (geology)|Striation}}
* [[Vertical displacement]] – Vertical movement of Earth's crust

==References==
{{Reflist}}

==Other reading==
<!-- {{Harvnb|Davis|Reynolds|1996|p= }} -->
*{{cite book|last1=Davis|first1=George H.|last2=Reynolds|first2=Stephen J.|year=1996|chapter=Folds|title=Structural Geology of Rocks and Regions|edition=2nd|pages=372–424|publisher=John Wiley & Sons|isbn=0-471-52621-5|chapter-url=http://knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=2132&VerticalID=0}}

<!-- {{Harvnb|Fichter|Baedke|2000|p= }} -->
*{{cite web|title=A Primer on Appalachian Structural Geology|last1=Fichter|first1=Lynn S.|last2=Baedke|first2=Steve J.|date=13 September 2000|publisher=James Madison University|access-date=19 March 2010|url=http://csmres.jmu.edu/geollab/vageol/vahist/struprimer.html|url-status=dead|archive-date=12 January 2010|archive-url=https://web.archive.org/web/20100112111556/http://csmres.jmu.edu/geollab/vageol/vahist/struprimer.html}}

<!-- {{Harvnb|Hart|Bryant|1997|p= }} -->
*{{cite report|last1=Hart|first1=E.W.|last2=Bryant|first2=W.A.|date=1997|title=Fault rupture hazard in California: Alquist-Priolo earthquake fault zoning act with index to earthquake fault zone maps|publisher=California Division of Mines and Geology|series=Special Publication 42}}

<!-- {{Harvnb|SCEC|Education Module|p= 14}} -->
*{{citation|ref=CITEREFSCECEducation Module|last1=Marquis|first1=John|last2=Hafner|first2=Katrin|last3=Hauksson|first3=Egill|title=The Properties of Fault Slip|work=Investigating Earthquakes through Regional Seismicity|publisher=Southern California Earthquake Center|access-date=19 March 2010|url=http://www.data.scec.org/Module/sec1pg14.html|archive-url=https://web.archive.org/web/20100625133126/http://www.data.scec.org/Module/sec1pg14.html|archive-date=25 June 2010|url-status=dead}}

<!-- {{Harvnb|McKnight|Hess|2000 |p= }} -->
*{{cite book|last1=McKnight|first1=Tom L.|last2=Hess|first2=Darrel|year=2000|chapter=The Internal Processes: Types of Faults|title=Physical Geography: A Landscape Appreciation|pages=[https://archive.org/details/physicalgeographmckn/page/416 416–7]|publisher=Prentice Hall|isbn=0-13-020263-0|chapter-url=https://archive.org/details/physicalgeographmckn/page/416}}

==External links==
{{Wikibooks|Historical Geology|Faults}}
{{Commons category|Faults}}
* [https://web.archive.org/web/20050217095327/http://www.iris.edu/gifs/animations/faults.htm Fault Motion Animations] at [[IRIS Consortium]] (archived 17 February 2005)
* [https://pubs.usgs.gov/gip/earthq1/how.html Aerial view of the San Andreas fault in the Carrizo Plain, Central California, from "How Earthquakes Happen"] at [[USGS]]
* [https://web.archive.org/web/20080404030212/http://geomaps.wr.usgs.gov/sfgeo/quaternary/stories/what_fault.html LANDSAT image of the San Andreas Fault in southern California, from "What is a Fault?"] at [[USGS]] (archived 4 April 2008)

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