According to the Unified Earthquake Catalogue of China Earthquake Networks, using the seismic phase data compiled by the Seismic Data Center, the observations of 101 fixed and mobile seismic stations in the Yunnan region and its surrounding seismic network from May 18 to 28, 2021, we conducted precise positioning research on the foreshock-mainshock-aftershock sequence of the Yangbi earthquake using the double-difference positioning method, and obtained the precise locations of 2 144 earthquakes. It is found that the distribution of the main aftershocks and the long axis orientation of the intensity isoseismal are not consistent with the image position of the Weixi-Qiaohou Fault, and the strike intersects with a small angle. The seismogenic fault of this earthquake may be a secondary fault of the Weixi-Qiaohou Fault. On the basis of the precision positioning results, the Bayesian Bootstrap Optimization(BABO)algorithm is used to perform a moment tensor inversion on the M6.4 earthquake and the M3.6 and above earthquake sequences in Yangbi, Yunnan from May 18 to 28, 2021. The results show that the sequence of Yangbi earthquake in Yunnan has obvious segmentation. The M6.4 Yangbi main shock is of right-handed strike-slip type with a small amount of normal dip-slip component, and the centroid depth is 5.9km. Most aftershocks have the same focal mechanism as the main shock, mainly right-handed strike-slip, except for the earthquakes in the west branch of the southeast section of the aftershock area, where the source property is obviously different, showing a normal strike-slip motion. The centroid depth of the entire earthquake sequence is 3.5~8.2km. The inversion results show that the principal compressive stress field of the earthquake area is in the near NS direction and the strike-slip dislocation is associated with a slight normal dip-slip component.
The spatial distribution of earthquakes shows that this earthquake sequence gradually developed from NW to SE, and the seismic density gradually dispersed from NW to SE. Therefore, it can be considered that the stress was mainly concentrated in the NW direction before the earthquake, and then gradually spread to the SE. Therefore, the main power source of this earthquake may be the southeastward extrusion of the Sichuan-Yunnan block. The rupture process and rupture pattern of this earthquake represents the “relaxation” process of the Sichuan-Yunnan rhombic block after being extruded.
The southern part of the Sichuan-Yunnan block is the material diffusion zone resulting from the eastward extrusion of Qinghai-Tibet Plateau. In this area, the subduction and westward retreat of the Indian plate led to the absence of lateral restraint on the Sichuan-Yunnan block, which may be the main reason causing the earthquake sequences in this area changing from convergence to dispersion, from strike-slip to normal fault type.
The Sichuan-Yunnan block is one of the most insensive areas where the Qinghai-Tibet Plateau squeezes out and escapes southeastward. Regarding the regional dynamic mode, we believe that under the background of continuous eastward extrusion of the material in the eastern part of the Qinghai-Tibet Plateau, and due to the lack of rigid blocks in the horizontal direction, it is more prone to velocity migration in the horizontal direction when the Sichuan-Yunnan block extrudes in the direction of SE and crosses the eastern structural junction and Longmen Mountains. The velocity migration in the study area may be caused by plate subduction or mantle underplating. The study of the lithospheric structure in the Sichuan-Yunnan area found that the crustal thickness of the sub-blocks in central Yunnan gradually thinned from north to south, and the lithospheric thickness in the area west of the Honghe fault zone shows a gradual thinning trend from east to west. It may be related to the intrusion of hot mantle material caused by the subduction and westward retreat of the Indian plate. Tomography results show that in the Ailaoshan-Honghe fault zone, the Yangtze block subducted downwards accompanied by mantle disturbance and asthenosphere upwelling, which led to magmatic activity and intrusion in the Cenozoic. Both of the above two effects can make the western boundary of the Sichuan-Yunnan block have a tensile stress background, and make the focal mechanism of major earthquakes on the nearby tectonic belt possible to appear normal. In summary, the dynamic source of earthquakes in the study area mainly comes from the escape of the Sichuan-Yunnan block to the southeast, and plate subduction or mantle underplating is possibly the deep-seated dynamic background for the lateral velocity migration of the study area.
