ABSTRACT
The three principal values of a stress tensor are theoretically independent, and so, in general, there is no reason to expect any particular relation between them. However, the analyses of some sets of in situ principal stress magnitudes suggest the existence of specific ratios. For example, the ratios σ1/σ3 = 2.3 and σ2/σ3 = 1.5 have been experienced (Fig. 1). Given that such relations exist, what is the reason for them? Does it mean that the principal stress magnitude ratios are those which would be expected simply from the process of ordering the three principal stresses obtained from stress measurement and taking the ratios, i.e. as a mathematical artifact of ordering the three numbers, or are they linked by a mechanical process, for example through a failure criterion?
