Fault (geology)

A fault is a fracture, or break, in the Earth's crust (lithosphere). Some faults are active. Here, sections of rock move past each other, which sometimes causes earthquakes.

Normal fault on the coast of Somerset, England. On the left, the younger blue-grey rock is from the late Triassic to the early Jurassic. On the right, the red rock is older Triassic mudstone. The strata of the red rock deformed when it moved upwards. The line of the fault runs along the beach and is shown by the rock/sand divide.

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

Schematic illustration of the two strike-slip fault types

Three types of faults:
A. Strike-slip
B. Normal
C. Reverse

Oblique-slip fault

USGS terminology of normal (basic) faults

Faulting occurs when shear stress on a rock overcomes the forces that hold it together. The fracture itself is called a fault plane. When it is exposed at the Earth's surface, it may form a cliff or steep slope, called a fault scarp.

The angle between the fault plane and an imaginary horizontal plane is called the dip angle of the fault. Faults may dip shallowly or steeply.

Faults are categorized into three types:

1. A normal fault is one for which the fault dips toward the downthrown block. Normal faults occur in rifted terranes, such as Mid-Ocean Ridges, the African Rift, and the Basin and Range Province of western North America. It is shown on geologic maps as a black line with either a block pattern on the downthrown side, or the letters U/D showing the upthrown and downthrown sides.
2. A reverse fault (if steeply dipping) or thrust fault (if shallowly dipping)^[1] is a fault for which the fault plane dips toward the upthrown block. It is shown on the geologic map with triangular teeth pointing toward the upthrown side of the fault. Reverse and thrust faults shorten (horizontally) and thicken the crust. They are characteristic of collision origins.
3. A transform (oceanic) or strike-slip (continental) fault is one in which the relative motion is horizontal. The fault plane is usually vertical and can be horizontal. Famous examples of these include the San Andreas Fault in California, the Alpine Fault of New Zealand's South Island, and the North Anatolian Fault in Turkey.

Related pages

• Fold (geology)