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In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of 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. Energy release associated with rapid movement on active faults is the cause of most earthquakes. A ''fault plane'' is the plane that represents the fracture surface of a fault. A ''fault trace '' or ''fault line'' is the intersection of a fault plane with the ground surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault.〔.〕 Since faults do not usually consist of a single, clean fracture, geologists use the term ''fault zone'' when referring to the zone of complex deformation associated with the fault plane. The two sides of a non-vertical fault are known as the ''hanging wall'' and ''footwall''. By definition, the hanging wall occurs above the fault plane and the footwall occurs below the fault.〔.〕 This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall hanging above him.〔.〕 ==Mechanisms of faulting== Because of friction and the rigidity of rocks, they cannot glide or flow past each other easily and occasionally all movement stops. When this happens, stress builds up in rocks and when it reaches a level that exceeds the strain threshold, the accumulated potential energy is dissipated by the release of strain, which is focused into a plane along which relative motion is accommodated — the fault. Strain occurs accumulatively or instantaneously, depending on the rheology of the rock; the ductile lower crust and mantle accumulates deformation gradually via shearing, whereas the brittle upper crust reacts by fracture - instantaneous stress release - to cause motion along the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great. The energy released by instantaneous strain-release causes earthquakes, a common phenomenon along transform boundaries. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fault (geology)」の詳細全文を読む スポンサード リンク
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