## 3 Results
### Inversion of the tectonic Stress Field  
When the damping parameter is small, the data misfit has a relatively larger effect on the inversion results, while the model length has a smaller effect. The selection of an appropriate damping parameter can reduce the complexity of the stress model and make the model smoother [41]. Based on the trade-off curve of data misfit and model length, this study determined the appropriate damping parameter as 1.4, the confidence region of the stress model was set to 95%, and stress field inversion with 1000 bootstrap resamplings was conducted. The results are shown in Figure 4.  
Figure 4a shows the probability density function of the stress shape ratio \(\varphi=(\sigma_{2}-\sigma_{3})/(\sigma_{1}-\sigma_{3})\) of the three principal stress axes, and the stress shape ratio equals 0.62 \(\pm\) 0.05. Figure 4b shows the azimuth distribution results of the three principal stress axes. The azimuth of the maximum principal stress \(\sigma_{1}\) is 97.2 \(\pm\) 86.9\({}^{\circ}\) in the NNE direction, the azimuth of the intermediate principal stress \(\sigma_{2}\) is 276.7 \(\pm\) 59.6\({}^{\circ}\), and the azimuth of the minimum principal stress \(\sigma_{3}\) is 124.8 \(\pm\) 22.7\({}^{\circ}\). Figure 4c-e are the dip distribution functions of the three principal stresses. The maximum principal stress \(\sigma_{1}\) is approximately horizontal, and the dip angle is 3.1 \(\pm\) 5.0\({}^{\circ}\). The intermediate principal stress is nearly vertical, and the dip angle is 84.5 \(\pm\) 2.1\({}^{\circ}\). The minimum principal stress is nearly horizontal, and the dip angle is 4.5 \(\pm\) 2.3\({}^{\circ}\). The maximum and minimum principal stresses are close to horizontal, while the intermediate principal stress is close to vertical, indicating that the Yingjiang area has a strike-slip stress system, which is basically consistent with previous research results (e.g., [50]). The stress field inversion results show that the maximum principal stress \(\sigma_{1}\) in the Yingjiang Mg 5.0 earthquake area in 2021 has an NNE orientation, which is consistent with the geological structure of the Yingjiang area and the velocity field direction based on GPS measurements [15] (Figure 1). Related studies have proposed that the Nabang fault and Sudian fault on the east and west sides are both dextral strike-slip faults, and their inner regions easily form conjugate shear planes with NE and NW orientations under the action of force coupling [4; 8; 51; 52]. Coupled with the east-west extensional action of the Dayingjiang fault at the southern tip, the overall stress field in the Yingjiang area is oriented NNE. In addition, the epicenter of the studied earthquake was close to the intersection of the NE-SW-striking Xima-Panlongshan fault and the N-S-striking Sudian fault. Researchers [11; 53] believe that under the influence of the principal stress structure in the NNE direction, tectonic stress is easily concentrated at the intersection, and the earthquake might have been triggered by the right-lateral strike-slip movement of the Sudian fault and the pushing effect of the Xima-Panlongshan fault. The lower the value of the stress shape factor R (\(R=1-\varphi\)), the closer the intermediate stress axis is to the principal compressive stress axis, and the compressive stress regime becomes more dominant; when the R value is near 0.5, it indicates that the maximum, intermediate, and minimum principal compressive stress axes remain relatively stable (e.g., [54]). The R value calculated in this study is 0.38 (close to 0.5), indicating that the  
Figure 4: Map of the stress field inversion results of the Mg 5.0 Yingjiang earthquake in 2021. (**a**) Probability density function of the principal stress shape ratio; (**b**) azimuth distribution of the principal stress axis; (**c–e**) probability density function of the plunge angles of principal stresses \(\sigma_{1}\), \(\sigma_{2}\), and \(\sigma_{3}\).