In geology, a fault or fault line is a planar rock fracture, which shows evidence of relative movement. Large faults within the Earth's crust are the result of shear motion and active fault zones are the causal locations of most earthquakes. Earthquakes are caused by energy release during rapid slippage along faults. The largest examples are at tectonic plate boundaries but many faults occur far from active plate boundaries. Since faults do not usually consist of a single, clean fracture, the term fault zone is used when referring to the zone of complex deformation that is associated with the fault plane.
The two sides of a non-vertical fault are called 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.

Microfracturing and AMR theory
The sense of slip is defined by the relative movements of geological features present on either side of the fault plane and is a vector. The sense of slip defines the type of fault. This is distinct from the throw of the fault, which is the vertical offset. Heave is the measured horizontal offset of the fault.
The vector of slip can be qualitatively measured by fault bend folding, drag folding of strata on either side of the fault (figure 2), and the direction and magitude of heave and throw can be measured only by finding common intersection points on either side of the fault. In practise it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector.

Slip, heave, throw
Faults can be categorized into three groups based on the sense of slip. A fault where the main sense of movement (or slip) on the fault plane is vertical is known as a dip-slip fault. Where the main sense of slip is horizontal the fault is known as a transcurrent or strike-slip fault. Oblique-slip faults have significant components of both strike and dip 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 or tilting.


Fault types
Dip-slip faults include both normal and reverse. 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 are indicative of 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 (hanging wall and foot wall) folds. Thrust faults are responsible for forming nappes and klippen in the large thrust belts.
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 hangingwall 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.

Dip-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. A special class of strike-slip faults is the transform faults which are a plate tectonics feature related to spreading centers such as mid-ocean ridges.

Strike-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.

Fault rock

Allegheny Plateau
Duplex fault
Fault mechanics
Mitigation of seismic motion
Seismic hazard
Shear (geology)
Transform boundary