## 4 Discussion
### Asymmetry of the Damage Zone  
#### 4.3.1 Differences in Damage Zones Due to Stress Distribution  
Mechanical test results have shown that the size and angle of the pre-existing fractures in rock samples significantly control the development of microfractures [68; 69], because the size and angle of the mentioned fractures in the rock samples greatly affect the stress distribution pattern around the specimen during testing [70; 71; 72]. Similar variations in stress distribution can also occur around active fault zones due to differences in the geometric spreading of faults. It is well known that when all other factors are kept constant and only different stress states are considered, the deformation characteristics of the rocks in the damage zones remain constant [73; 74]. Therefore, to understand the phenomena behind the development of the rock damage pattern in different zones of the fault, we analyzed the stress field distribution characteristics in different parts of the fault using simple numerical models. In these models, we restrict the displacement of the model in all directions except the applied load direction and kept the mechanical parameters, fault lengths, load magnitudes, etc., the same in all models (Table 2). The simulation results revealed that the angle (\(\alpha\)) between the regional far-field principal stress orientation and the fault strike significantly controls the stress distribution characteristics around the fault (Figure 6e). Stress concentration zones form at the tips of isolated faults and at fault junctions, whereas stress relaxation occurs at the fault walls (Figure 6). When the \(\alpha\) increases from 0 to 90\({}^{\circ}\) for an isolated fault, both the magnitude and stress disturbance area increase sharply at the fault tips. The stress disturbance area increases while the magnitude decreases on the two sides of the fault wall (Figure 6(a1-d1)). We performed similar numerical simulations for two neighboring faults. The stress magnitude in the linking damage zone is higher than that at the tip, regardless of if faults are under-lapping or adjacent (Figure 6(a2-d2), Figure 6(a3-d3)). However, this relationship becomes reversed when the two faults partially overlap, with a stronger stress concentration at the fault tip damage zone compared to the linking damage zone formed by the fault overlap (Figure 6(a4-d4)). This explains why the width of the TDZ of the XSHF is larger than that of the LDZ. The clockwise rotation of the NNW-NWW in the Sichuan-Yunnan block maintains a small angle with the strike of the XSHF (Figure 6e), resulting in the spatial spreading of the left echelon pattern, with slight overlaps of the XSHF in the region where the regional stress field is similar to that in the generalized Figure 6(b4). However, the WDZ is always in a low-stress zone and is lower than the stress zone in the TDZ or LDZ. We believe that the damage driven by the wide-area stress concentration in the TDZ or LDZ is more intense than that caused by fault slip in the WDZ, which may be an important reason for the difference in damage widths in different damage zones along the XSHF, i.e., the width of the damage in the TDZ or LDZ is wider than that in the WDZ. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ \end{tabular} & \begin{tabular}{c} **Elasticity** \\ **Modulus** \\ **(GPa)** \\ \end{tabular} & \begin{tabular}{c} **Shear** \\ **Stiffness** \\ **(GPa)** \\ \end{tabular} & \begin{tabular}{c} **Normal** \\ **Stiffness** \\ **(GPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Cohesion** \\ **(kPa)** \\ \end{tabular} &
\begin{tabular}{c} **Friction** \\ **Angle (\({}^{\circ}\))** \\ \end{tabular} \\ \hline Rock & 0.25 & 25.0 & — & — & — & — \\ \hline Fault zone & — & — & 2 & 2 & 100 & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical parameters of the model.