= Conjugate fault
One of a pair of <fault> orientations that can form under a common <principal stress> field with opposite senses of resolved <shear stress>. For newly formed, optimally oriented <faults> obeying the same <Mohr-Coulomb failure criterion>, their acute intersection angle is $90^\circ-\phi$, where $\phi$ is the <friction angle>. The maximum compressive <principal stress> bisects that acute angle. This permits an estimate $\mu=\tan\phi$ only under the common-formation and optimal-orientation assumptions, rather than for arbitrary inherited <faults>.
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