A fault is a fracture along which the blocks of crust (the outermost major layer of the Earth) on either side have moved relative to one another parallel to the fracture.
Strike-slip faults are vertical (or nearly vertical) fractures where the blocks have mostly moved horizontally. If the block opposite an observer looking across the fault moves to the right, the slip style is termed right lateral; if the block moves to the left, the motion is termed left lateral. A transform fault is a special variety of strike-slip fault that accommodates relative horizontal slip between other tectonic elements, such as oceanic crustal plates.
Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically.
If the rock mass above an inclined fault moves down, the fault is termed normal (normal fault), whereas if the rock above the fault moves up, the fault is termed reverse (reverse fault).
Oblique-slip faults have components of both strike-slip and dip-slip faults. Most faults are of this type.
A thrust fault is a reverse fault with a dip of 45° or less. A blind thrust fault is one that does not rupture all the way up to the surface so there is no evidence of it on the ground. It is "buried" under the uppermost layers of rock in the crust.
Surface faulting is displacement that reaches the Earth's surface during slip along a fault. This commonly occurs with shallow earthquakes, those with an epicenter less than 20 km. Surface faulting also may accompany aseismic creep or natural or man-induced subsidence.
A fault trace is the intersection of a fault with the ground surface. These features are commonly plotted on geologic maps to show the surface location of a fault.
A fault scarp is the feature on the surface of the Earth that looks like a step caused by slip on the fault.
Fault creep is the slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Faults that are creeping do not tend to have large earthquakes.
A locked fault is a fault that is not slipping because frictional resistance on the fault is greater than the shear stress across the fault (it is stuck). Such faults may store strain for extended periods that is eventually released in an earthquake when frictional resistance is overcome.
Fault gouge is crushed and ground-up rock produced by friction between the two sides when a fault moves.
A fault plane is the planar (flat) surface along which there is slip during an earthquake.
An active fault is a fault that is likely to have another earthquake sometime in the future. Faults are commonly considered to be active if they have moved one or more times in the last 10,000 years.