Combining GNSS And Seismic Anisotropy Data To Constrain Upper Crustal Deformation In The Northeastern Tibetan Plateau

The northeastern Tibetan Plateau is a confluence of several blocks,including Qiangtang,Bayan Har,Qaidam,and Qilian.It serves as the leading edge where the Tibetan Plateau interacts with surrounding Tarim,Alxa,Ordos,and South China Blocks.The complex crustal structure records the remote effects of land-land collision since the Cenozoic era,showing complex and strong medium deformation,which makes the plateau a key area to understand the plateau uplift and dynamics evolution.The study of crustal deformation provides crucial insights into deep structure of crust and the interactions between stress and media.In this thesis,GNSS velocity field data alongside seismic anisotropy data are utilized to jointly constrain shallow and deep deformation characteristics within the upper crust of northeastern Tibetan Plateau.Over the past 30 years,space geodetic measurement methods,represented by GNSS,have provided robust data support for crustal deformation studies.However,GNSS observation sites are usually deployed at the surface,and the crustal deformation obtained by inversion is based on the assumption of uniform and continuous elasticity of the medium,which ignores the difference of deep and shallow deformation caused by the local structure of the crust.Seismic anisotropy,as a vital research method for deformation,is a direct reflection of the change of medium structure.The direct S-wave seismic phase used for near-field S-wave splitting propagates in the upper crust and can constrain the depth range of crustal deformation based on the source location.Combining these two methods can obtain the deep and shallow deformation characteristics from the surface to the upper crust and deepen the understanding of the subsurface medium structure and deformation patterns.In this study,we use the newly estimated GNSS velocity field data observed between 1999 and 2022,employing multiscale spherical wavelet methods to compute strain rate.Utilizing the small-earthquake waveform data from 2010 to 2021,shear-wave splitting systematic analysis method is applied to calculate anisotropic parameters.By comparing principal compressive strain rate directions with fast-wave polarization orientations of near-field S-wave splitting,we analyze surface and upper crustal deformation along with their discrepancies in the northeastern Tibetan Plateau.GNSS-derived strain rates reveal regional characteristics in surface deformation across the northeastern Tibetan Plateau.The surface deformation of Qilian and Qaidam blocks in the northern part of the study area is dominated by NE-SW extrusion and NW-SE tension,and the principal strain is greater in compression than in tension.The principal compressive strain direction of Bayan Har block in the central part of the study area change from NE-oriented towards EW at around 34°N.The compressive deformation of Qiangtang block is weaker,and the principal strain is greater in tension than in extrusion.High-strain-rate values are distributed in strips.The principal strain rates are larger along Haiyuan,Kunlun as well as Ganzi-Yushu-Xianshuihe Faults.The maximum shear strain rates exhibit higher values in several large strike-slip faults(Kunlun and Haiyuan Faults),reaching peak levels near Xianshuihe Fault,reflecting strong tectonic activity.The negative values of surface strain rate are consistent with the distribution of reverse faults.The Qiangtang and Qaidam blocks are dominated by clockwise motion,while the Bayan Har block and the Qilian orogenic belt in the north are dominated by counterclockwise motion.The near-field S-wave splitting parameters can characterize the deformation of upper crustal media.In this thesis,the anisotropy parameters obtained from seismic stations are collected.Parameters for 27 permanent seismic stations are calculated and analyzed using the S-wave splitting analysis method.The fast S-wave polarization direction(i.e.,fast-wave polarization)is generally parallel to the principal compressive stress direction,and the slow S-wave time delay(i.e.,time delay)can reflect the degree of medium deformation.These two parameters can reveal the deformation state of the medium.The results show that the fast-wave polarization direction is dominated by NE direction,which is consistent with the regional principal stress direction,and the weaker NW direction of the fast-wave polarization exists in the northern and southeastern part of Qilian block and the northern part of Qiangtang,which is approximately parallel to the widely distributed NW trending fractures in the region.The discrete fast-wave polarization directions in the northeast margin of Qaidam are influenced by a combination of stress,fracture,and rock properties.The time delay of the northern Qilian fault is larger than the one in the west,and the time delay of the eastern South Qilian and the northeastern Qaidam margin are similar,which may reflect the differences in stress environment and rock type distributions.The occurrence of WNW-oriented fast-wave dominant polarization direction and relatively more consistent time delays at the northern edge of the Qaidam Basin may be a joint response to high-pressure metamorphism of the fracture at depth.The time delay in the northern part of the Qiangtang block is smaller than that in the northeastern margin of the Tibetan Plateau(southeastern part of the Qilian block and northeastern margin of the Qaidam block),which may be related to the difference in rock types and properties.In order to explore the characteristics of deep and shallow deformation on the upper crust of the study area,we collected near-field S-wave splitting results in recent year to supplement our analysis.We compared principal compressive strain directions with fast wave polarization orientations that were computed for 96 stations,revealing quantitative angular differences.The areas with large differences are mainly concentrated in the southeastern part of the Qilian block near 102°E and the block junction area east of it(including southeastern edge of the Qilian-Qaidam block,the eastern part of the Bayan Har block and the western edge of the Ordos)along with the inner part of the Tibetan Plateau west of 98°E(including the central part of the Qaidam block and the northern part of the Qiangtang block),revealing the local tectonic features and regional deformation mechanisms are different.At the junction of the blocks,the difference in the deep and shallow deformation of the upper crust may be influenced by a combination of tectonic factors such as fluids,weak rock layers,fractures,and principal compressive stresses,which are related to the presence of oil and gas and rock formations at the top of the brittle crust.Inside the plateau,the principal compressive strain and the fast wave polarization direction of the fracture strike differ greatly,and the deep and shallow deformation of the surface and upper crust may be influenced by the fracture.By combining GNSS data with seismic anisotropy information,this thesis illustrates the characteristics of shallow and deep deformation in the upper crust of northeastern Tibetan Plateau and reveals deformation discrepancies influenced by local structures.It deepens the understanding of the crustal structure and the deep dynamic mechanism it implies.