Mechanics
See also: Fault mechanicsThe relative motion of rocks on either side of the fault surface controls the origin and behavior of faults, in both an individual small fault and within larger fault zones which define the tectonic plates.
Because of friction and the rigidity of the rock, the rocks cannot glide or flow past each other. Rather, 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 is both accumulative and instantaneous 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.
Read more about this topic: Fault (geology)
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