Fault (geology) 1

Fault (geology)In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significantdisplacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from theaction of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zonesor transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes,such as occurs on the San Andreas Fault, California.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 thezone of complex deformation associated with the fault plane.The two sides of a non-vertical fault are known as the hanging wall and footwall. By definition, the hanging walloccurs above the fault plane and the footwall occurs below the fault.[2] This terminology comes from mining: whenworking a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall hangingabove him.[3]

Mechanics

Large normal fault in Triassic to Lower Jurassic Blue AnchorFormation sediments near Blue Anchor, Somerset, UK, with several

smaller normal faults in its hanging wall

The relative motion of rocks on either side of the faultsurface controls the origin and behavior of faults, inboth an individual small fault and within larger faultzones which define the tectonic plates.

Because of friction and the rigidity of the rock, therocks cannot glide or flow past each other. Rather,stress builds up in rocks and when it reaches a levelthat exceeds the strain threshold, the accumulatedpotential energy is dissipated by the release of strain,which is focused into a plane along which relativemotion is accommodated—the fault.

Strain is both accumulative and instantaneousdepending on the rheology of the rock; the ductilelower crust and mantle accumulates deformationgradually via shearing, whereas the brittle upper crust reacts by fracture - instantaneous stress release - to causemotion along the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great. Theenergy released by instantaneous strain release causes earthquakes, a common phenomenon along transformboundaries.

Fault (geology) 2

Microfracturing and Accelerating Moment Release (AMR) theory

Dextral slickenside of pyrite on a possiblemicrofault

Microfracturing, or microseismicity, is often thought of as a symptomcaused by rocks under strain, where small-scale failures, perhaps onareas the size of a dinner plate or a smaller area, release stress underhigh strain conditions. Only when sufficient microfractures link up intoa large slip surface can a large seismic event or earthquake occur.According to this theory, after a large earthquake, the majority of thestress is released and the frequency of microfracturing is exponentiallylower. A connected theory, accelerating moment release (AMR),claims that the seismicity rate accelerates in a well-behaved way priorto major earthquakes, and that it might provide a helpful tool forearthquake prediction on the scale of days to years.

AMR may be used to predict rock failures within mines, and applications are being attempted for the portions offaults within brittle rheological conditions. Researchers observe like behavior in tremors preceding volcaniceruptions.

Slip, heave, throw

A fault in the Grands Causses as seen fromBédarieux, France. The left side moves down

while the right side moves up. The warping of therock layers on the right is likely due to drag

folding.

Slip is defined as the relative movement of geological features presenton either side of a fault plane, and is a displacement vector. A fault'ssense of slip is defined as the relative motion of the rock on each sideof the fault with respect to the other side.[4] In measuring the horizontalor vertical separation, the throw of the fault is the vertical componentof the dip separation and the heave of the fault is the horizontalcomponent, as in "throw up and heave out".[5]

Fault (geology) 3

Microfault showing a piercing point

The vector of slip can be qualitatively assessed by studying the faultbend 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 onlyby finding common intersection points on either side of the fault(called a piercing point). In practice, it is usually only possible to findthe slip direction of faults, and an approximation of the heave andthrow vector.

Fault types

The Junction fault, dividing the AlleghenyPlateau and the true Appalachian Mountains in

Pennsylvania, United States

Geologists can categorize faults into three groups based on the sense ofslip:1.1. a fault where the relative movement (or slip) on the fault plane is

approximately vertical is known as a dip-slip fault2.2. where the slip is approximately horizontal, the fault is known as a

transcurrent or strike-slip fault3. an oblique-slip fault has non-zero components of both strike and

dip slip.

For all naming distinctions, it is the orientation of the net dip and senseof slip of the fault which must be considered, not the present-dayorientation, which may have been altered by local or regional foldingor tilting.

Dip-slip faults

Students look at a section of the exposed WasatchFault (normal fault), Utah

Dip-slip faults can occur either as "reverse" or as "normal" faults. Anormal fault occurs when the crust is extended. Alternatively such afault can be called an extensional fault. The hanging wall movesdownward, relative to the footwall. A downthrown block between twonormal faults dipping towards each other is called a graben. Anupthrown block between two normal faults dipping away from eachother is called a horst. Low-angle normal faults with regional tectonicsignificance may be designated detachment faults.

A reverse fault is the opposite of a normal fault—the hanging wallmoves up relative to the footwall. Reverse faults indicate shortening ofthe crust. The dip of a reverse fault is relatively steep, greater than 45°.

Fault (geology) 4

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

A thrust fault has the same sense of motionas a reverse fault, but with the dip of thefault plane at less than 45°. Thrust faultstypically form ramps, flats and fault-bend(hanging wall and foot wall) folds. Thrustfaults form nappes and klippen in the largethrust belts. Subduction zones are a specialclass of thrusts that form the largest faultson Earth and give rise to the largestearthquakes.

The fault plane is the plane that representsthe fracture surface of a fault. Flat segmentsof 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 foundassociated 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 (faultinversion). A normal fault may therefore become a reverse fault and vice versa.

Strike-slip faults

The San Andreas Fault, aright-lateral strike-slip fault, caused

the massive 1906 San Franciscoearthquake

The fault surface is usually near vertical and the footwall moves either left orright or laterally with very little vertical motion. Strike-slip faults withleft-lateral motion are also known as sinistral faults. Those with right-lateralmotion are also known as dextral faults.[6]

A special class of strike-slip faults is the transform fault, where such faults forma plate boundary. These are found related to offsets in spreading centers, such asmid-ocean ridges, and less commonly within continental lithosphere, such as theAlpine Fault, New Zealand. Transform faults are also referred to as conservativeplate boundaries, as lithosphere is neither created nor destroyed.

Fault (geology) 5

Schematic illustration of the two strike-slip faulttypes.

Oblique-slip faults

Oblique-slip fault

A fault which has a component of dip-slip and a component ofstrike-slip is termed an oblique-slip fault. Nearly all faults will havesome component of both dip-slip and strike-slip, so defining a fault asoblique requires both dip and strike components to be measurable andsignificant. Some oblique faults occur within transtensional andtranspressional regimes, others occur where the direction of extensionor shortening changes during the deformation but the earlier formedfaults remain active.

The hade angle is defined as the complement of the dip angle; it is theangle between the fault plane and a vertical plane that strikes parallel to the fault.

Listric faultA listric fault is a type of fault in which the fault plane is curved. The dip of the fault plane becomes shallower withincreased depth and may flatten into a sub-horizontal décollement.

Ring faultRing faults are faults that occur within collapsed volcanic calderas. Ring faults may be filled by ring dikes.

Fault rock

Salmon-colored fault gouge and associated faultseparates two different rock types on the left

(dark grey) and right (light grey). From the Gobiof Mongolia.

All faults have a measurable thickness, made up of deformed rockcharacteristic of the level in the crust where the faulting happened, ofthe rock types affected by the fault and of the presence and nature ofany mineralising fluids. Fault rocks are classified by their textures andthe implied mechanism of deformation. A fault that passes throughdifferent levels of the lithosphere will have many different types offault rock developed along its surface. Continued dip-slip displacementtends to juxtapose fault rocks characteristic of different crustal levels,with varying degrees of overprinting. This effect is particularly clear inthe 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 bygenerally angular clasts and rock fragments in a finer-grained matrix of similar composition.

Fault (geology) 6

Inactive fault from Sudbury to SaultSte. Marie, Northern Ontario,

Canada

• Tectonic or Fault breccia - a medium- to coarse-grained cataclasitecontaining >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

• Clay smear - clay-rich fault gouge formed in sedimentary sequencescontaining clay-rich layers which are strongly deformed and shearedinto the fault gouge.

• Mylonite - a fault rock which is cohesive and characterized by a welldeveloped planar fabric resulting from tectonic reduction of grain size, andcommonly containing rounded porphyroclasts and rock fragments of similarcomposition to minerals in the matrix

• Pseudotachylite - ultrafine-grained vitreous-looking material, usually blackand flinty in appearance, occurring as thin planar veins, injection veins or as amatrix to pseudoconglomerates or breccias, which infills dilation fractures in the host rock.

Impacts on structures and people

In geotechnical engineering a fault often forms a discontinuity that may have a large influence on the mechanicalbehavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, foundation, or slopeconstruction.

The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing theseismic shaking and tsunami hazard to infrastructure and people in the vicinity. In California, for example, newbuilding construction has been prohibited directly on or near faults that have moved within the Holocene Epoch (thelast 11,000 years) (Hart and Bryant, 1997).[7] Also, faults that have shown movement during the Holocene plusPleistocene Epochs (the last 2.6 million years) may receive consideration, especially for critical structures such aspower plants, dams, hospitals, and schools. Geologists assess a fault's age by studying soil features seen in shallowexcavations and geomorphology seen in aerial photographs. Subsurface clues include shears and their relationshipsto carbonate nodules, translocated clay, and iron oxide mineralization, in the case of older soil, and lack of such signsin the case of younger soil. Radiocarbon dating of organic material buried next to or over a fault shear is oftencritical in distinguishing active from inactive faults. From such relationships, paleoseismologists can estimate thesizes of past earthquakes over the past several hundred years, and develop rough projections of future fault activity.

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] USGS. "Hanging wall Foot wall" (http:/ / www. nature. nps. gov/ geology/ usgsnps/ deform/ ghangft. html). Visual Glossary. . Retrieved 2

April 2010.[3] Tingley, J.V.; Pizarro K.A. (2000). Traveling America's loneliest road: a geologic and natural history tour (http:/ / books. google. com/

?id=qBZo1TNsgAcC& pg=PA8& lpg=PA8& dq=fault+ footwall+ hanging+ wall+ mining+ term#v=onepage& q=fault footwall hanging wallmining term& f=false). Nevada Bureau of Mines and Geology Special Publication. 26. Nevada Bureau of Mines and Geology. pp. 132.ISBN 978-1-888035-05-6. . Retrieved 2010-04-02.

[4] Marquis, John; Hafner, Katrin; Hauksson, Egill. "The Properties of Fault Slip" (http:/ / www. data. scec. org/ Module/ sec1pg14. html).Investigating Earthquakes through Regional Seismicity. Southern California Earthquake Center. p. 14. . Retrieved 19 March 2010.

[5] "Faults: Introduction" (http:/ / ic. ucsc. edu/ ~casey/ eart150/ Lectures/ 2faultsintro. html/ 2faultsintro. htm). University of California, SantaCruz. . Retrieved 19 March 2010.

[6] Park, R.G. (2004). Foundation of Structural Geology (http:/ / books. google. com/ ?id=ycASqdxSG3YC& pg=PA11& lpg=PA11&dq=sinistral+ dextral+ structural+ geology#v=onepage& q=sinistral dextral structural geology& f=false) (3 ed.). Routledge. p. 11.ISBN 978-0-7487-5802-9. .

Fault (geology) 7

[7][7] Hart, E.W., and Bryant, W.A., 1997, Fault rupture hazard in California: Alquist-Priolo earthquake fault zoning act with index to earthquakefault zone maps: California Division of Mines and Geology Special Publication 42.

References• Brodie, Kate; Fettes, Douglas; Harte, Ben; Schmid, Rolf (29 January 2007). 3. Structural terms including fault

rock terms (http:/ / www. bgs. ac. uk/ SCMR/ docs/ papers/ paper_3. pdf). Recommendations by the IUGSSubcommission on the Systematics of Metamorphic Rocks.

• Davis, George H.; Reynolds, Stephen J. (1996). "Folds" (http:/ / knovel. com/ web/ portal/ browse/display?_EXT_KNOVEL_DISPLAY_bookid=2132& VerticalID=0). Structural Geology of Rocks and Regions(2nd ed.). New York: John Wiley & Sons. pp. 372–424. ISBN 0-471-52621-5.

• Fichter, Lynn S.; Baedke, Steve J. (13 September 2000). "A Primer on Appalachian Structural Geology" (http:/ /csmres. jmu. edu/ geollab/ vageol/ vahist/ struprimer. html). James Madison University. Retrieved 19 March2010.

• McKnight, Tom L.; Hess, Darrel (2000). "The Internal Processes: Types of Faults". Physical Geography: ALandscape Appreciation. Upper Saddle River, N.J.: Prentice Hall. pp. 416–7. ISBN 0-13-020263-0.

External links• Fault Motion Animations (http:/ / www. iris. edu/ gifs/ animations/ faults. htm) at IRIS Consortium• Aerial view of the San Andreas fault in the Carrizo Plain, Central California, from "How Earthquakes Happen"

(http:/ / pubs. usgs. gov/ gip/ earthq1/ how. html) at USGS• LANDSAT image of the San Andreas Fault in southern California, from "What is a Fault?" (http:/ / geomaps. wr.

usgs. gov/ sfgeo/ quaternary/ stories/ what_fault. html) at USGS• Photo gallery of faults (http:/ / www. photoree. com/ collections/ gallery/ 345350) at PhotoRee

Article Sources and Contributors 8

Article Sources and ContributorsFault (geology)  Source: http://en.wikipedia.org/w/index.php?oldid=539426649  Contributors: 3DBrendzz, AVand, Aalox, AdjustShift, Adpete, Afaqniazi, Aitias, Alansohn, Allens,Allmightyduck, Androstachys, Antandrus, Asterion, AtheWeatherman, Avriette, Awickert, Ballista, Bennetto, Berland, Blahthethird, Bob73456, Bobo192, Boing! said Zebedee, Borgx, Brewsohare, BrianKnez, Bryan Derksen, Budyhead, Burgundavia, Cameron Dewe, CanadianLinuxUser, Capricorn42, Carolinegrace1, Cferrero, CharlotteWebb, Chmee2, Chongkian, Chris 73,Chrislk02, Ckatz, Codking, Conartist555, ConstellationMagick, Cool Blue, Courcelles, Cureden, Curtis23, D, DARTH SIDIOUS 2, DJO CODY, DVdm, DanielCD, Dawnseeker2000, Deeb,Dentren, Dgw, Dino, Doctormatt, Dr.g.s.pujar, Dragonire, Drahcir, Dunro, E Wing, E2eamon, Earlypsychosis, Edcolins, Ekko, Elliskev, Ellywa, Elockid, Epbr123, Epolk, Erebus555,Ethanoconnor, Excirial, FCSundae, Fastilysock, Finetooth, Fossiliferous, Freakmighty, Funandtrvl, Fæ, Gaccorsi, Gaianauta, GainLine, Galactor213, GeoWriter, Geoff Plourde, Geologicharka,Geologyguy, Giftlite, Gilgamesh, Gilgamesh he, Gingavitus777, Glane23, Glenn, Graeme Bartlett, HalfShadow, Happysailor, Heracles31, Heron, Hike395, Hraefen, Husond, Hydrogen Iodide,Höyhens, Ideal gas equation, Igodard, IronGargoyle, Isdarts222, Ixfd64, J.delanoy, JDiPierro, JTB01, Jahiegel, JamesBWatson, Janos.urai, Jhbuk, Jhonnyboy99, Jhuiplk, Jim1138, JohnCub,Jsayre64, Kasjanek21, Katanada, Katxijasotzaile, Kbdank71, Killiondude, Kjkolb, KnowledgeOfSelf, Kr1st1deejay97, Krazyxkayla, Kuru, L Kensington, Last poo, Lcarscad, Leandrod, Lee,Leon II, Leonard G., Ligulem, Lindsay Prothro, Look2See1, Looxix, Loserpedia, Luckas Blade, Lugia2453, Luk, Magog the Ogre, Mani1, Marek69, Martin451, Materialscientist, Mato,MattieTK, Mattisse, Megaman en m, Mh26, MickMacNee, Mikenorton, Mikeo, Minimac, Miracle Pen, Mkantonelli1, Moeron, MorgynWT, Mwtoews, NawlinWiki, Nbgeo1, Neelix, Nfidler,Niceguyedc, Noctibus, Nuno Tavares, Oikos-team, Ojigiri, Onco p53, Opelio, Parmoon randi, Patrick, Perennialstudent66, Petrb, Phil Boswell, Pinethicket, Pingveno, Piperh, Pollinator, Possum,Prillen, Qfl247, Quadell, QueenCake, Quentin X, Qxz, R Stillwater, RainmakerUSA, Razimantv, Revenantchaos, RexNL, Reykholt, Rich Farmbrough, Rigadoun, Rjb uk, Robert Hack, Rockfang,Rolinator, Rrburke, Rubberstamp, Rudjek, Rygel, M.C., SBaker43, SJP, SWAdair, Sander123, Scarpe, Schuetzler 62, Sdornan, Sean.hoyland, Seaphoto, Secret (renamed), Sephiroth BCR, Sgt.Bacon, Siim, SkerHawx, Snowolf, Soldier.pitre, Some jerk on the Internet, Spiderbo13, Sport woman, Stausifr, Stephenb, SteveChervitzTrutane, Tai112, Tatterfly, Tbhotch, Techman224,Thatguyflint, The High Fin Sperm Whale, The Thing That Should Not Be, Theo10011, TicketMan, Tide rolls, Titoxd, Tmangray, Tnxman307, Tobias1984, Tohd8BohaithuGh1, Tom Morris,Tomdo08, Treisijs, Tursirog01, TutterMouse, Tyler, Type Something In The Box And Hope It's Not Taken, Uncle Dick, Vanished user 39948282, Vanka5, Velella, Versus22, Vibhijain,Vicpeters, Vidale, Volcanoguy, Vsmith, WadeSimMiser, Waltpohl, Watdafrenchtoast?, Wayne Slam, Whitepaw, Wikfr, WikiChanny742, WikiPuppies, Wilson44691, Wingchi, Winterst,Wjejskenewr, Woohookitty, Xhienne, ZX81, Zaheen, Zamphuor, Zaphod Beeblebrox, Zeizmic, 768 anonymous edits

Image Sources, Licenses and ContributorsFile:The Blue Anchor Fault - geograph.org.uk - 2455274.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:The_Blue_Anchor_Fault_-_geograph.org.uk_-_2455274.jpg  License:Creative Commons Attribution-Share Alike 2.0 Generic  Contributors: Ashley DaceFile:PySlick.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:PySlick.JPG  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Qfl247 (talk) (Transferred byCitypeek/Original uploaded by Qfl247)File:Faille des Causses depuis Bedarieux.dsc02071.cropped.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Faille_des_Causses_depuis_Bedarieux.dsc02071.cropped.jpg  License:Creative Commons Attribution-Sharealike 3.0,2.5,2.0,1.0  Contributors: XhienneFile:Microfault.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Microfault.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Qfl247File:Junction fault 0112.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Junction_fault_0112.jpg  License: GNU Free Documentation License  Contributors: Pollinator aten.wikipediaImage:WasatchFault.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:WasatchFault.JPG  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Qfl247 (talk)(Transferred by Citypeek/Original uploaded by Qfl247)File:Nor rev.png  Source: http://en.wikipedia.org/w/index.php?title=File:Nor_rev.png  License: GNU Free Documentation License  Contributors: Original uploader was Cferrero at en.wikipedia.Later version(s) were uploaded by Heron at en.wikipedia.File:San Andreas Fault Aerial View.gif  Source: http://en.wikipedia.org/w/index.php?title=File:San_Andreas_Fault_Aerial_View.gif  License: Public Domain  Contributors: USGSFile:strike slip fault.png  Source: http://en.wikipedia.org/w/index.php?title=File:Strike_slip_fault.png  License: GNU Free Documentation License  Contributors: Original uploader was Cferreroat en.wikipediaFile:Oblique slip fault.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Oblique_slip_fault.jpg  License: Public Domain  Contributors: CodkingImage:FaultGouge.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:FaultGouge.JPG  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Qfl247 (talk)(Transferred by Citypeek/Original uploaded by Qfl247)File:CREIGHTON.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CREIGHTON.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Soldier.pitre, 1anonymous edits

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