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Strain, in any branch of science dealling with materials and their behaviour, is the geometrical expression of deformation caused by the action of stress on a physical body. Strain therefore expresses itself as a change in size and/or shape. In the case of geological action of the earth, if the release of stress through strain in rocks is sufficiently large, earthquakes may occur.

If strain is equal over all parts of the body, it is referred to as homogenous strain; otherwise, it is inhomogenous strain.

Strain in the Earth resulting from stresses across faults results in motion over the fault surface and a combination of brittle and ductile deformation of the surrounding rockss. Brittle strain is exhibited as fractures, faults and other discontinuous breaks in the fabric of the rock. Ductile strain occurs as shear zones, flow bands and folding.

Quantifying strain

Given that strain results in the deformation of a body, it can be measured by calculating the change in length of a line or by the change in angle between two lines (where these lines are theoretical constructs within the deformed body). The change in length of a line is termed the stretch and may be given by

where l is the change in length and l0 is the original undeformed length. If e is positive, the body has been lengthened; if it is negative, it has been compressed.

This equation is commonly used to calculate the beta factor for lithospheric extension during the formation of sedimentary basins.

See also: plate tectonics, geology, engineering.