## 4 Discussion  
### Analysis of Geomagnetic Field Changes Based on the Meta-Instability Theory  
Figure 4 gives the phase-by-phase variations in the horizontal vector of the LGF in the western Yunnan region before and after the Yangbi earthquake. The first three panels are all drawn based upon the observational data before the earthquake, while the last panel is the observational data before (one month) and after (one month) the earthquake. Figure 4a shows the changes from 2018 to 2019. There are obvious low values in most areas from west to east of the Yongsheng-Binchuan fault and around the connection between the Weixi-Qiaohou-Weishan fault and the Honghe fault. The horizontal vector direction is divided into two different trends from west to east and from south to north at the intersection of the Longling-Lancang fault and the Wanding-Anding fault. With the direction of the trend distribution, the change in its value gradually decreases. When the fault is in the meta-instability stage after the peak stress intensity, it has entered an irreversible deformation stage, which indicates that the occurrence of earthquakes is inevitable [34]. According to the theory of meta-instability, there is a peak stress point around the Yongsheng-Binchuan fault zone before the meta-instability stage begins, and the magnitude in the surrounding area is relatively small. This indicates that the surrounding area is subjected to regional stress loading, which is a metastable state. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Components**} & \multicolumn{4}{c}{**Different Annual Variation Cycles**} \\ \cline{2-5}  & **May 2018 to May 2019** & **May 2019 to May 2020** & **May 2020 to May 2021** & **May–June 2021** \\ \hline Declination & 0.86 & 0.91 & \(-\)0.20 & 0.07 \\ (\(\prime\)) & 0.18–0.96 & 0.65–1.31 & \(-\)0.67–0.51 & \(-\)0.37–0.78 \\ \cline{2-5} Inclination & 0.54 & \(-\)1.73 & 0.29 & \(-\)0.12 \\ (\(\prime\)) & \(-\)0.25–0.65 & \(-\)2.43–1.03 & \(-\)0.26–0.67 & \(-\)0.18–0.27 \\ \cline{2-5} Total intensity & 3.8 & \(-\)7.3 & \(-\)1.7 & \(-\)1.9 \\ (nT) & 1.9–5.0 & \(-\)11.2–4.7 & \(-\)7.6–1.1 & \(-\)3.0–1.5 \\ \cline{2-5} North component & \(-\)1.7 & 10.1 & \(-\)3.9 & \(-\)0.4 \\ (nT) & \(-\)2.4–4.7 & 4.3–14.2 & \(-\)5.8—1.2 & \(-\)1.4–1.7 \\ \cline{2-5} East component & 9.2 & 9.6 & \(-\)2.0 & 0.7 \\ (nT) & 1.9–10.1 & 6.9–13.8 & \(-\)7.1–5.6 & \(-\)4.0–8.5 \\ \cline{2-5} Horizontal intensity & \(-\)1.9 & 9.9 & \(-\)3.8 & \(-\)0.4 \\ (nT) & \(-\)2.6–4.7 & 4.1–14.0 & \(-\)6.5—0.5 & \(-\)1.5–1.8 \\ \cline{2-5} Vertical intensity & 8.3 & \(-\)23.2 & 2.0 & \(-\)2.5 \\ (nT) & \(-\)1.4–9.6 & \(-\)15.5—33.1 & \(-\)7.9–7.0 & \(-\)3.9–3.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The magnitude of the annual variation in each component of the magnetic field at the epicenter of the Yangbi earthquake and within a radius of 50 km. The upper row corresponding to each component is the amplitude of the epicenter position, and the lower row is the variation amplitude within a radius of 50 km around the epicenter. Figure 4b shows the variations from 2019 to 2020. The local area enclosed by the arc section of the Lancangjiang fault, the western branch of the Nanninghe fault, the Longling-Lancang fault and the Kejie fault is an area where the value of the horizontal vector is low. The horizontal vector direction is generally oriented from west to east from the Nujiang fault and Wanding-Anding fault and gradually becomes south to north after passing through the Honghe fault and Yongsheng-Binchuan fault. Compared with the vector in the previous period (2018 to 2019), the vector is roughly oriented in the same direction, but its magnitude is greater than that of the previous period. Compared with the change in Figure 4a, the \"weakly variable area\" migrates from northeast to southwest. According to the determination of the vector direction and magnitude, this area is still undergoing a stress loading process in the region, or it is in a metastable state. Figure 4c shows the changes from 2020 to 2021.