| With the improvement of geophysical geodetic observation technology,the accumulation of observation material and the advancement of inversion methods,the study of the internal structure of the Earth by combining various observational material has become an important research direction of geophysical geodesy.In this thesis,we use the gravity anomaly data of different scales,taking the density structute of Tibetan Plateau and its adjacent areas as the main target,and combine the new achievements and new understandings of solid geophysics and tectonics to study the current tectonic framework,formation and evolution,crust-mantle interaction,earthquake gestation and deep dynamic process.This thesis systematically discussed the theories and methods of heavy mechanics in the study on density structure and seismogenic environment in Tibetan Plateau and its adjacent areas,which fully embodies the characteristics of the combination and mutual penetration of geodesy and geophysics,geology.First of all,this paper introduces the basic theories and methods of gravity mechanics,and determines the relevant processes and methods of gravity data processing.Then,we used residual gravity anomaly after separation to invert the three-dimensional density structure of the lithosphere of Tibetan Plateau and its adjacent areas,completed the imaging work.By combining with other geophysical and geological research results,geophysical qualitative analysis and quantitative interpretation of the inversion results were carried out.Finally,from the perspective of heavy mechanics,the related issues are discussed about crustal movement and deformation characteristics,seismogenic structure of Tibetan Plateau and its adjacent areas.The main achievements of this thesis are as follows:(1)The research of gravity data processing and interpretation.Geophysical data has high spatial resolution,and geological results are more realistic and reliable.This thesis used seismic,geophysical and geological research results as priori constraints,which provided important basic data support for the subsequent processing of gravity data.By processing the gravity data and analyzing the gravity data of different frequencies,the gravity response segments caused by the density changes of different layers were extracted,and the changes of the Moho surface of the study area were obtained.The three-dimensional inversion of gravity data is a very important quantitative interpretation method.The two-dimensional observation of gravity data can obtain the three-dimensional density distribution characteristics of underground space,which provides important information for studying regional structural deformation,kinematics and dynamics research.At the same time,in order to improve the practical application ability of gravity data,this thesis proposed a fast gravity inversion method to overcome the problems caused by the low computational efficiency caused by the excessive conditional maturity matrix,which effectively improves the computational efficiency of gravity three-dimensional inversion.This method is applied to obtain the three-dimensional density structure od the underground space,and density change in the underground space of the study area.(2)The characteristics and significance of gravity field in Tibetan Plateau and its adjacent areas.The basic structure of the study area is obtained by using the obtained regional gravity field model.The structural form of the region is quantitatively explained by combining the geological tectonic background and tectonic block activity of Tibetan Plateau and its adjacent areas,as well as geodetic and seismic observation data.Based on the existing models of solid geophysics,rheology and tectonic geodesy,quantitative analysis was carried out about the tectonic framework caused by gravity in Tibetan Plateau and its adjacent areas,tectonic driving forces,tectonic environment,and explained sit distribution characteristics and its tectonic grid and evolution information.The regional gravity field better reflects the deep gravitational field background of the lithosphere of the main tectonic units in this area.The local gravity anomaly above the regional background depicts the lithospheric structural features at the intersection of each structural unit in more detail.The possible reason isthe self-weight drag effect of high-density continental crust lithosphere inserted into low-density hot mantle,which leads to the decoupling of crust-mantle in the southern margin of the Tibetan Plateau.(3)The relationship between density structure and tectonic movement of Tibetan Plateau and its adjacent areas.Data processing of high-precision static and dynamic gravitational fields in the region is carried out,three-dimensional and four-dimensional density structure distribution are obtained by density inversion.Combined with regional geological and geophysical observational data,qualitative analysis,semi-quantitative and quantitative interpretation are carried out about the structural deformation and motion characteristics in this region.Study on the density structure of Tibetan Plateau and its adjacent areas:The density of the crust in the Tibetan Plateau appears stratified longitudinally,and there is obvious unevenness in the lateral direction.The layered density structure clearly delineates the distribution of different blocks and deep and large faults.In the earth's crust,the contours of the density anomalies are basically consistent with the surface faults.Among them,in the depth of the upper crust,the variation of regional density corresponds very well to the surface structure,showing a complex tectonic framework.The crustal density structure of Tibetan Plateau and its adjacent areas provides constraints for earthquake preparation,occurrence and lower crustal material flow in this area.The density contour has a clockwise deflection at the upper mantle depth,which indicates that the deep matter of Tibetan Plateau is hindered by the Alashan block and the Ordos block when moving to the northeast.It also implies that there may be a large degree of decoupling in the crust of the area.At the depth of 80 km?90 km,the study area presents high-density and low-velocity anomaly distribution.This may be due to the continuous subduction of the Indian Plate to the Eurasian Plate,resulting in the continuous deformation and movement of the Tibetan Plateau.Finally,the strong tectonic compression and high-temperature background in the region have changed the rock structure and properties.Study on the density structure of local areas:Firstly,for the 2017 Mainling Mw6.5 earthquake event,the medium environment and seismogenic mechanism of the eastern Himalayan syntaxis tectonic region were studied,and the Mainling earthquake event detailed analysis was carried out through joint inversion and density structure,and the evolution model of the eastern tectonic knot and the trend of the current tectonic movement are explained.It is believed that the deformation characteristics and tectonic setting of the region satisfy the evolution model dominated by "Tectonic Aneurysm"and "Extensional Extrusion",and the Mainling earthquake event is not a product of tectonic activity in the region,and the seismic hazard in the region still needs continuous attention.Secondly,the dynamic gravity field information acquired by the seismic gravity observation network in the eastern margin of Tibetan Plateau is studied,and the cause of the time-varying gravity anomaly is modeled and analyzed.After various corrections,the residual gravity anomaly was obtained,and it was found that it still contained abundant anomalous signals,and the cause could not be explained.Then the residual gravity anomaly is used to invert the gravity anomaly,and the four-dimensional density structure is obtained.Through the theory of boundary analysis,fluid mechanics and rheology,the causes are discussed.According to the simulation analysis,the four-dimensional density structure of the region is likely to reflect the change of fluid in the rock,which needs further study. |
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