In geology, a fault is a planar fracture or
discontinuity in a volume of rock, across
which there has been significant displacement
along the fractures as a result of earth movement.
Large faults within the Earth's crust result
from the action of plate tectonic forces,
with the largest forming the boundaries between
the plates, such as subduction zones or transform
faults.
Energy release associated with rapid movement
on active faults is the cause of most earthquakes.
A fault line is the surface trace of a fault,
the line of intersection between the fault
plane and the Earth's surface.
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
known as the hanging wall and footwall.
By definition, the hanging wall occurs above
the fault plane 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.
Mechanisms of faulting
Because of friction and the rigidity of the
rock, the rocks cannot glide or flow past
each other.
Rather, stress builds up in rocks and when
it reaches a level that exceeds the strain
threshold, the accumulated potential energy
is dissipated by the release of strain, which
is focused into a plane along which relative
motion is accommodated—the fault.
Strain is both accumulative and instantaneous
depending on the rheology of the rock; the
ductile lower crust and mantle accumulates
deformation gradually via shearing, whereas
the brittle upper crust reacts by fracture
- 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, a common phenomenon
along transform boundaries.
Slip, heave, throw
Slip is defined as the relative movement of
geological features present on either side
of a fault plane, and is a displacement vector.
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.
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".
The vector of slip can be qualitatively assessed
by studying 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
Based on direction of slip, faults can be
generally categorized as:
strike-slip, where the offset is predominately
horizontal, parallel to the fault trace.
dip-slip, offset is predominately vertical
and/or perpendicular to the fault trace.
oblique-slip, combining significant strike
and dip slip.
Strike-slip faults
The fault surface is usually near vertical
and the footwall moves either left or right
or laterally with very little vertical motion.
Strike-slip faults with left-lateral motion
are also known as sinistral faults.
Those with right-lateral motion are also known
as dextral faults.
Each is defined by the direction of movement
of the ground on the opposite side of the
fault from an observer.
A special class of strike-slip faults is the
transform fault, where such faults form a
plate boundary.
These are found related to offsets in spreading
centers, such as mid-ocean ridges, and less
commonly within continental lithosphere, such
as the San Andreas Fault in California, or
the Alpine Fault, New Zealand.
Transform faults are also referred to as conservative
plate boundaries, as lithosphere is neither
created nor destroyed.
Dip-slip faults
Dip-slip faults can occur either as "reverse"
or as "normal" faults.
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.
An upthrown block between two normal faults
dipping away from each other is called a horst.
Low-angle normal faults with regional tectonic
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 compressive shortening
of the crust.
The dip of a reverse fault is relatively steep,
greater than 45°.
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 folds.
Thrust faults form nappes and klippen 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.
The fault plane is the plane that represents
the fracture surface of a fault.
Flat segments of thrust fault planes are known
as flats, and inclined sections of the thrust
are known as ramps.
Typically, thrust faults move within formations
by forming flats, and climb up section with
ramps.
Fault-bend folds are formed by movement of
the 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.
A normal fault may therefore become a reverse
fault and vice versa.
Oblique-slip faults
A fault which has a component of dip-slip
and a component of strike-slip is termed an
oblique-slip fault.
Nearly all faults will have some component
of both dip-slip and strike-slip, so defining
a fault as oblique requires both dip and strike
components to be measurable and significant.
Some oblique faults occur within transtensional
and transpressional regimes, others occur
where the direction of extension or shortening
changes during the deformation but the earlier
formed faults remain active.
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
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 horizontal slip
on a horizontal plane.
The illustration shows slumping of the hanging
wall along a listric fault.
Where the hanging wall is absent the footwall
may slump in a manner that creates multiple
listric faults.
Ring fault
Ring faults are faults that occur within collapsed
volcanic calderas and the sites of bolide
strikes, such as the Chesapeake Bay impact
crater.
Ring faults may be filled by ring dikes.
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.
Those faults may be accompanied by rollover
anticline.
Fault rock
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 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 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.
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
porphyroclasts and rock fragments of similar
composition to minerals in the matrix
Pseudotachylite - ultrafine-grained vitreous-looking
material, usually black and flinty in appearance,
occurring as thin planar veins, injection
veins or as a matrix 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 mechanical behavior of soil
and rock masses in, for example, tunnel, foundation,
or slope construction.
The level of a fault's activity can be critical
for locating buildings, tanks, and pipelines
and 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.
Also, faults that have shown movement during
the Holocene plus Pleistocene Epochs 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, translocated
clay, and iron oxide mineralization, in the
case of older soil, and lack of such signs
in the case of younger soil.
Radiocarbon dating of organic 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.
See also
Mitigation of seismic motion
Mountain formation
Orogeny
Seismic hazard
Striation
Notes
References
External links
Fault Motion Animations at IRIS Consortium
Aerial view of the San Andreas fault in the
Carrizo Plain, Central California, from "How
Earthquakes Happen" at USGS
LANDSAT image of the San Andreas Fault in
southern California, from "What is a Fault?"
at USGS
