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Resolving the detail of crustal deformation of the seismically and technically active Sichuan-Yunnan block located in the southeast margin of the Tibetan Plateau is crucial for understanding the lateral extrusion of the Tibetan Plateau. Using seismic data from two regional seismograph stations of the China Earthquake Networks Center and twenty recently deployed broad-band temporary stations from the MCD Array (Middle ChuanDian block seismic array), we constrained crustal thickness and seismic anisotropy at the west boundary of the Sichuan-Yunnan block, near the east Himalayan syntaxis by receiver function techniques including CCP stacking and sinusoidal moveout of PmS conversions. Crustal thickness varies significantly across major faults inside the block which increases by >20 km from west to east. East of the Chenghai Fault, a clear positive phase lies above the Moho and is interpreted to be caused by the magma intrusion related to a late Permian Emeishan large igneous province (ELIP). The crustal anisotropy shows strong zoning features and clockwise rotation with respect to due north. To the west of the Lijiang-Xiaojinhe and the Honghe Faults, anisotropic fast orientations are near NS, indicating strong crustal deformation and movement of southward crustal flow along the Nujiang-Lancangjiang Fault; to the east of this fault, anisotropic fast orientations are near NE. The variation of the anisotropic fast orientations indicates that the southeastward crustal flow from the Tibetan Plateau may be prevented by fast velocity anomalies beneath the ELIP. These observations provided new evidence for strong crustal deformation along the tectonic boundary in the southeastern Tibetan Plateau with the expansion of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

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]