, a fault is a planar fracture
or discontinuity in a volume of rock
across which there has been significant displacement as a result of rock-mass movements. Large faults within the Earth
result from the action of plate tectonic
forces, with the largest forming the boundaries between the plates, such as subduction zones
or transform fault
s. 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
A ''fault plane'' is the 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 geologic map
s to represent a fault.
A ''fault zone'' is a cluster of parallel faults. However, the term is also used for the zone of crushed rock along a single fault. 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.
Mechanisms of faulting
Owing 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 ''asperities
builds up when a fault is locked, and when it reaches a level that exceeds the strength
threshold, the fault ruptures and the accumulated strain energy
is released in part as seismic wave
s, forming an earthquake
Strain occurs accumulatively or instantaneously, depending on the liquid state
of the rock; the ductile
lower crust and mantle
accumulate deformation gradually via shearing
, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault. 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. 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".
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. Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on 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 (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
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. 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. 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.
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 and dip
In a strike-slip fault (also known as a ''wrench fault'', ''tear fault'' or ''transcurrent fault''), 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. 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
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 can be either normal ("extensional
") or reverse.
In a normal fault, the hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is a graben
. An upthrown block between two normal faults dipping away from each other is a horst
. Low-angle normal faults with regional tectonic
significance may be designated detachment fault
A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of the crust. The dip
of a reverse fault is relatively steep, greater than 45°. The terminology of "normal" and "reverse" comes from coal-mining
in England, where normal faults are the most common.
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°.
Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds.
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 sections with ramps.
Fault-bend folds are formed by the 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.
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.
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 transtensional
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 complement
of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.
Listric faults are similar to normal faults but the fault plane curves, the dip being steeper near the surface, then shallower with increased depth. The dip may flatten into a sub-horizontal décollement
, resulting in a horizontal slip on a horizontal plane. The illustration shows slumping of the hanging wall along a listric fault. Where the hanging wall is absent (such as on a cliff) the footwall may slump in a manner that creates multiple listric faults.
Ring faults, also known as caldera faults, are faults that occur within collapsed volcanic caldera
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
Synthetic and antithetic faults
Synthetic and antithetic faults 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
upInactive fault from Sudbury
to 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 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 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
The main types of fault rock include:
– a fault rock which is cohesive with a poorly developed or absent planar fabric
, or which is incohesive, characterised by generally angular clasts
and rock fragments in a finer-grained 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
-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
sequences containing clay-rich layers which are strongly deformed and sheared into the fault gouge.
– 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
– ultrafine-grained glassy-looking material, usually black and flint
y in appearance, occurring as thin planar veins
, injection veins or as a matrix to pseudoconglomerates
s, which infills dilation fractures in the host rock. Pseudotachylyte likey only forms as the result of seismic slip rates and can act as a fault rate indicator on inactive faults.
Impacts on structures and people
In geotechnical engineering
, a fault often forms a discontinuity
that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil
and rock masses in, for example, tunnel
, or slope
The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the 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 Holocene
Epoch (the last 11,700 years) of the Earth's geological history. 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 nodules
clay, and iron oxide
mineralization, in the case of older soil, and lack of such signs in the case of younger soil. Radiocarbon dating
material buried next to or over a fault shear is often critical in distinguishing active from inactive faults. From such relationships, 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 or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits.
An example of a fault hosting valuable porphyry copper deposit
s is northern Chile's Domeyko Fault
with deposits at Chuquicamata
, El Abra
, El Salvador
, La Escondida
Further south in Chile Los Bronces
and El Teniente
porphyry copper deposit lie each at the intersection of two fault systems.
* Aseismic creep
* Vertical displacement - Vertical movement of Earth's crust
Fault Motion Animations
at IRIS Consortium
Aerial view of the San Andreas fault in the Carrizo Plain, Central California, from "How Earthquakes Happen"
LANDSAT image of the San Andreas Fault in southern California, from "What is a Fault?"