## 4 Discussion
### Analysis of Geomagnetic Field Changes Based on the Meta-Instability Theory
Compared with Figure 4b, the horizontal vector in the variation diagram in Figure 4c is generally small, and a \"weak variation area\" crosses the epicenter and appears on the northern and southern sides of the epicenter. Among them, the \"weakly variable areas\" on the northern side are distributed from west to east on the Weixi-Qiaohou-Weishan fault, Lancangjiang fault, and Nujiang fault. The magnitudes of local areas reach zero. In addition, the weak variation areas on the southern side are along the Lancangjiang fault and Kejie fault. The magnitude of the local area near the intersection also reaches zero. The horizontal vector directions spreading to the southeast and southwest from the epicenter show divergent trends from northwest to southeast and northeast to southwest, respectively, and the corresponding magnitudes also increase from small to large. The scope of the \"weak variation area\" is seen to have expanded compared with those of the previous two periods, and the  
Figure 4: Annual variations in the horizontal vector of the magnetic field in the epicentral and surrounding areas of the Yangbi earthquake. (**a**) The annual change from May 2018 to May 2019, (**b**) the change from May 2019 to May 2020, (**c**) the change from May 2020 to May 2021, and (**d**) the change from May–June 2021. The direction of the arrow represents the horizontal vector direction, and the size of the arrow is 1\({}^{\circ}\) = 50 nT. northern and southern sides are around the epicenter. At the same time, the horizontal vector direction is completely reversed in the southwestern part of the Weixi-Qiaohou-Weishan fault compared with those of the previous two periods, and there is a clockwise rotation of nearly \(90^{\circ}\) in the vector direction around the Honghe fault. From the analysis of the above characteristics, a large-scale \"weak variation region\" appears in this period, accompanied by the change in vector direction and a gradual increase in magnitude in two local regions; these features indicate that the region with the change in vector direction has completely entered the metastable state, which implies that an earthquake will inevitably occur. The observation in western Yunnan was completed approximately 15 days before the earthquake, which shows that the observed anomalous characteristics of the metastable state occurred 15 days after the Yangbi MS 6.4 earthquake, and that its epicenter was located at the edge of the \"weak variation area\". Figure 4d shows the variation from May-June 2021. For the coseismic effect of the horizontal vector of the LGF associated with the Yangbi earthquake, the vector magnitude in the region from the Weixi-Qiaohou-Weishan fault to the Yongsheng-Binchuan fault reached a value of zero, which indicates that the stress loading in this area was still at the peak stage. From the Weixi-Qiaohou-Weishan fault to the west along the Lancangjiang fault and then to the Nujiang fault, the vector direction spread from west to east, and the magnitude gradually increased. There are two situations in this area: stress remains loaded, or strain is released. These results also show that the seismomagnetic effect does not subside rapidly in a short time after the earthquake. The results of the TSF show that the azimuth of \(\sigma_{1}\) in the epicentral area was NNW, close to positive N, before the Yangbi earthquake, and changed to SSE, close to the positive S direction, after the earthquake [33]. The seismogenic structure is subjected to dextral strike-slip movement under the regional stress of near north-south compression and east-west tension. The formation of the structure may be associated with the SE slip of the Sichuan-Yunnan block and the clockwise rotation of the Southwest Yunnan block [35].