STUDY ON HETEROGENEITY OF THE STRESS FIELD IN THE YANGBI EARTHQUAKE FAULT ZONE

The fine crustal stress field plays an important role in geodynamics. The 2021 Yangbi earthquake sequence occurred in an area with densely deployed seismic stations. Before the mainshock, there occurred multiple 3-4 magnitude earthquakes. The mainshock was followed by strong aftershocks, MS5.0 and MS5.2, occurring 7 and 36 minutes later respectively. The earthquake sequence is a typical example of a "foreshock-mainshock-aftershock" earthquake sequence. The abundant seismic data of the 2021 Yangbi earthquake sequence provide many seismic focal mechanisms for the fine stress field analysis in the study region. To study the relationship of the stress field, fault structure, and earthquake dynamics in the Yangbi earthquake source region, the central focal mechanism solution algorithm is selected for the earthquakes with several focal mechanisms to ensure the accuracy of the focal mechanism data, and 93 precisely determined focal mechanism data are determined. The overall stress field in the source region is determined as a compressive stress axis of nearly NS direction and extensional stress axis of nearly EW direction. Then, to reveal the heterogeneity of the stress field in the source region, according to the location of the earthquake sequence, the focal mechanism solutions are divided into 6 regions by using the moving window strategy and obtain the stress field in each sub-region. To verify the inversion results are not caused by the selection of a specific partition mode, we used two different partition methods to discuss the stress field inversion experiments: 1)change the number of sub-regions from 6 to 8, the number of focal mechanisms in each subregion is still 23, and moving the 15 focal mechanisms in each iteration; 2)the number of the sub-region is still 6, change the number of focal mechanism to 28 in each subregion. It can be found that although the different partition strategies are changed, the characteristics of the obtained stress field will not change. Finally, the earthquake dynamics revealed in the heterogeneous stress area are analyzed. The results show that the compressive stress axis changed from NNW-SSE direction in the northwest of the Yangbi earthquake focal area to NNE-SSW direction in the southwest region, with the rotation angle of 23°; And the stress shape factor in the northwest part of the rupture zone is always larger than that in the southeast region. Combined with the geodynamics studies of crustal motion map, tomography from seismic data, hydrographic net distribution, and topography of the study region, it is speculated that the change of the stress field in the northwest and the southeast is caused by the combined action of the blocked southward movement of the material in the northern part of the fracture area and the NNE extension in the shallow part of the study area due to the low angle NNE subduction of the Indo-Burma arc. The horse-tail-like fault distribution in the southeast of the Yangbi earthquake fault zone and the mountain and river alignment around the Yangbi earthquake are consistent with the predicted stress deflection and stress shape factor change. These studies are of significance for understanding the characteristics of fault activity and earthquake dynamics in study regions. © 2023 State Seismology Administration. All rights reserved.