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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.

1 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.

2 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.

3 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.

4 See also

• Transform boundary
• Transform fault
• New Madrid Fault Zone
• San Andreas Fault
• Seattle Fault
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• Þingvelliringvellir ( Icelandic "THing": parliament "vellir": plane is a place in the southwest of Iceland near the peninsula of Reykjanes and the Hengill volcanic area. It is famous for two reasons: As one of the most important places in Icelandic history. In the (in IcelandThe Republic of Iceland is an island nation in the northern Atlantic Ocean, located between Greenland and Scotland, northwest of the Faroe Islands. Lydveldid Island ( In Detail) (Full size) National motto: none Official languageNone. Icelandic de facto''.)
• Great Glen FaultThe Great Glen Fault is a long strike-slip fault that runs through its namesake the Great Glen (Glen Albyn) in Scotland. However, the fault is actually much longer and over 200 millions years old. Aligned northeast to southwest, the Great Glen Fault exten