*3.2. Angles*

For the analysis of the simulation results, we introduced several angles; see Figure 1. The shear angle *φ* is the angle between the PSZ and the original surface plane. Analogously, we define *ψ* as the angle between the rake face and the PSZ; it is:

$$
\psi = \frac{\pi}{2} + a - \phi. \tag{1}
$$

The force angle *χ* is the angle between the total force and the original surface plane. It satisfies the relation:

$$
\chi = \beta - \mathfrak{a} \tag{2}
$$

with the friction angle *β*. This friction angle is given by the components of the force parallel (*R*) and perpendicular (*N*) to the rake face by:

$$
\tan \beta = \frac{R}{N}.\tag{3}
$$
