Fault

Fault is a term used in geology to describe a fracture or discontinuity in a volume of rock, across which there has been significant displacement due to rock-mass 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.

Types of Faults

There are three main types of faults: Normal, Reverse (or thrust), and Strike-slip. Normal and reverse faults form in areas undergoing extension or shortening, such as divergent and convergent plate boundaries. Strike-slip faults occur in areas of lateral shear, such as transform plate boundaries.

Normal Faults

Normal faults occur mainly in areas where the crust is being extended. The hanging wall moves down relative to the footwall. Normal faults occur where two blocks of rock are pulled apart, as by tension.

Reverse Faults

Reverse faults, also called thrust faults, slide one block of crust on top of another. These faults are commonly found in collision zones, where tectonic forces push upwards.

Strike-slip Faults

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.

Fault Zones

A fault zone is a cluster of interconnected faults. Fault zones can be tens to hundreds of kilometers long, such as the San Andreas Fault Zone, and they can accommodate large cumulative displacements over geologic time.

Effects of Faults

Faults can have significant effects on the Earth's landscape and can cause earthquakes. The energy released in an earthquake is transmitted to the surface of the Earth by seismic waves, and the study of these waves provides information about the Earth's interior.

See Also

• Earthquake
• Plate tectonics
• Seismic wave

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