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Encyclopedia > Geological fault
Old fault exposed by roadcut near . Such faults are common in the folded .
Old fault exposed by roadcut near Harrisburg, Pennsylvania. Such faults are common in the folded Appalachians.

In geology, faults are discontinuities (cracks) in the Earth's crust that are the result of differential motion within the crust. Faults are the source of many earthquakes that are caused by slippage vertically or laterally along the fault. The largest examples are at tectonic plate boundaries, but many small faults are known to exist that are far from active plate boundaries.


The two sides of a fault are called the hangingwall and footwall. By definition, the fault always dips away from the footwall. Faults can be categorized into three groups: normal faults, transform (or strike-slip) faults and reverse (or thrust) faults.

Contents

Normal (or detachment) fault

This occurs when the crust is in tension. The hangingwall moves downwards (i.e. towards the centre of the Earth) relative to the footwall. The depressed ground between two parallel normal faults is called a graben. A ridge between two parallel normal faults is called a horst.


Reverse (or thrust) fault

This occurs when the crust is in compression. The hangingwall moves upwards (i.e. away from the centre of the Earth) relative to the footwall.

Schematic illustration of normal and reverse faults. Note that the view is a cross-section through the Earth, such that the up-direction on the page is away from the centre of the Earth.
Schematic illustration of normal and reverse faults. Note that the view is a cross-section through the Earth, such that the up-direction on the page is away from the centre of the Earth.

Strike-slip faults

The fault surface is vertical and the footwall moves either left or right (with respect to the plane perpendicular to the fault and to the Earth's surface). Strike-slip faults with left-lateral motion are also known as sinistral faults Those with right-lateral motion are also known as dextral faults.

Schematic illustration of the two strike-slip fault types. The view is of the Earth's surface as from space.
Schematic illustration of the two strike-slip fault types. The view is of the Earth's surface as from space.

See also


  Results from FactBites:
 
Geologic fault - Wikipedia, the free encyclopedia (761 words)
A fault where the main sense of movement (or slip) on the fault plane is vertical is known as a dip-slip fault.
Reverse faults are indicative of shortening of the crust.
The fault plane is the plane that represents the fracture surface of a fault.
Geologic fault (206 words)
Faults are discontinuities in the earth which are the result of differential motion within the crust.
The two sides of a fault are defined the hangingwall and footwall, such that the fault always dips away from the hangingwall.
Such faults are the source of many earthquakes that are caused by slippage vertically or laterally at the fault.
  More results at FactBites »

 
 

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