Analysis of geologic strain data in strain-magnitude space

Strain-magnitude space is defined by an orthogonal coordinate system using the natural principal strains Ex, Ey, Ez. Distance from the origin provides an invariant measure of total-strain magnitude Et, which can be decomposed into two orthogonal components: volume Ev and deviatoric Ed strain. The volume-strain axis Ev is equidistant from the Ex, Ey, Ez axes, and the Ed component lies in an orthogonal section called the deviatoric section. Ev and Ed are independent of the rotational component of the deformation and are shown to be fundamental measures of strain magnitude. viatoric section contains the Flinn and Nadai diagrams, which are commonly used to identify strain symmetry (oblate vs prolate). The Nadai diagram is preferred because it provides an undistorted representation of the deviatoric section. Other sections that include the volume-strain axis are better suited for distinguishing strain type (flattening vs constriction). -magnitude space ignores the directional information contained in strain-tensor data. This leads to a significant bias which is illustrated by comparing a strain-magnitude distribution with its average, as determined by a tensor-averaging method. The mode of the distribution will generally give an unbiased estimate of the average volume strain and an estimate of the average deviatoric strain on the high side.