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The M S 6.4 earthquake occurred in Yangbi County, Yunnan Province, China, on 21 May, 2021. The earthquake is located on the west side of the Red River fault, the western boundary of the Sichuan-Yunnan block in the SE margin of the Tibetan Plateau. The Yangbi M S 6.4 earthquake has aroused great concern due to its unique geographical location and frequent strong earthquakes in Yunnan. In this study, a total of 4695 local earthquakes of the Yangbi M S 6.4 earthquake sequence are relocated using the double-difference location method, including the mainshock source location of 99.879°E, 25.687°N, and depth of 9.67 km. The mainshock has a moment magnitude of 6.1, and one of the fault-plane solutions is 137°, 83° and − 163° in strike, dip, rake respectively. Combined with the relocation and focal mechanism solutions results, it is suggested that the seismogenic fault of the Yangbi M S 6.4 earthquake is a NW-SE trending fault on the southwest side of the Weixi-Qiaohou fault (WQF), with high dip of SW, which is a previously-unknown blind fault. The regional 2D stress field shows that the principal compressive stress around the Yangbi earthquake is NNW-SSE and nearly N-S direction, which appears to be deflected counterclockwise by a small angle on its east side and changed to N-S and NNE-SSW direction on its west side, while both the north and south sides of the Yangbi earthquake show directional coherence. Strong differential deformation distorts in the upper crust to the northeast of the seismic source as reflected by tectonic stress field and the upper crust seismic anisotropy, indicating strong influence of Lijiang-Xiaojinhe fault (LXF). The stress field in the source area is jointly controlled by southeastward movement of southeastern margin of Tibetan Plateau, the northeastward movement of Indian plate and the differential movement between the different tectonic blocks. • The Yangbi M s6.4 earthquake sequences are relocated to reveal two burst areas of small earthquakes. The seismogenic fault is a blind fault. • The new detailed 2D stress field is released, and a model of stress field is reported around the source area. • Strong differential deformation distorts in the upper crust to the northeast of the source area. [ABSTRACT FROM AUTHOR]

Continued northeastward compression owing to the colliding Indian subcontinent has led to pronounced surface uplift and significant crustal and mantle deformation in the Sichuan–Yunnan region along the southeastern margin of the Tibetan Plateau. Seismic anisotropy is an effective physical property for characterising crustal and upper-mantle deformation, as these deformation mechanisms can be investigated by analysing the splitting of compressional-to-shear-wave (P-to-S) converted phases (XKS, which includes the SKS, SKKS, PKS and PKKS phases) at the core–mantle boundary. However, we cannot distinguish any crustal and/or mantle anisotropy variations owing to the relatively low vertical resolution of XKS splitting. Here we compute the P-wave receiver functions from teleseismic events recorded at 90 permanent and 350 mobile broadband seismic stations that were established by the ChinArray project in the Sichuan–Yunnan region and analyse the P-to-S converted phases at the Moho interface (Pms) to measure the crustal anisotropic parameters. The Pms splitting results are then regarded as the crustal anisotropic parameters, and the anisotropic parameters in the upper mantle beneath 347 stations are extracted from the XKS splitting parameters by removing this crustal anisotropy. The results indicate that an approximately E–W anisotropic fast-wave polarisation direction (FPD) exists in the upper mantle beneath the southeastern margin of the Tibetan Plateau, and that the splitting times range from 0.06 to 1.39 s, with an average splitting time of 0.93 s. The approximately E–W anisotropic FPD in the upper mantle is mainly due to asthenospheric flow induced by the eastward subduction of the Indian Plate beneath Myanmar. The small splitting times within the southern Sichuan–Yunnan diamond-shaped block (SYDSB) may be attributed to vertical growth of the lithosphere induced by mantle-plume-related magmatic uplift. [Display omitted] • P-to-S converted phases at the Moho are used to estimate crustal anisotropy. • Anisotropy in the upper mantle is extracted from the XKS splitting. • This upper-mantle anisotropy is due mainly to asthenospheric flow. • There is no an obvious deformation transition zone beneath the SE Tibet near ~26°N. [ABSTRACT FROM AUTHOR]