Search

The distribution of block boundary faults, both horizontally and vertically, has significant implications for crustal movements and structural variations. In the SE margin of the Tibetan Plateau, the south part of the Sichuan–Yunnan block is surrounded by the first-order block boundaries of the Honghe faults (HHF) trending NW–SE and the Xiaojiang faults (XJF) trending N–S. Crustal anisotropy provides valuable insights into the ongoing deformation and evolution. This study investigates the upper-crustal anisotropy at 26 stations, using local seismic waveforms from the temporary linear Honghe–Xiaojiang seismic broad-band Array (i.e. the HX Array) in 2017–2019 and regional permanent stations in 2012–2019. The PFS (i.e. polarization of fast S wave) shows an abrupt change in the Yimen region, which is the middle of the HX Array survey line between two large faults. The NW–SE dominant PFS orientations across the HHF agree well with the strike of faults and the direction of regional principal compressive stress, illustrating cracks or minerals aligned with the HHF, influenced by regional stress. The NE–SW dominant PFS orientations crossing the XJF differ from the faults strike or regional stress orientation, suggesting alignment with microcracks caused by the in situ principal compressive stress. The comparison between the upper-crustal and whole-crustal anisotropy demonstrates a noticeable PFS consistency across the two large faults, the HHF and the XJF. This observation implies that these two large faults may tectonically control a wide zone, with wide rigid widths of approximately 150 and 200 km, respectively. This study presents a new pattern indicating the control of deep physical properties on the boundary faults and uncovering the layered deformation in the upper and lower crust. [ABSTRACT FROM AUTHOR]

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]

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]