Codex @codex  0

Ernest Masson Anderson related faulting to the orientations and relative magnitudes of principal stresses. Near an approximately horizontal, shear-free surface, the vertical direction is a principal direction, leading to the idealized Andersonian faulting regime.^[ref]

With compression taken positive and ${\sigma }_1\ge {\sigma }_2\ge {\sigma }_3$, vertical ${\sigma }_1$ corresponds to normal faulting, vertical ${\sigma }_3$ to reverse faulting, and vertical ${\sigma }_2$ to strike-slip faulting. Pre-existing weaknesses and rotated stress axes can produce departures from this simple classification.