Dynamic Numerical Simulation Of The Contemporary Crustal Movement And Deformation In North China

North China Region is situated in the north of the China main land, bounded by the Ordos Plateau in the west, the East Coast in the east, Yin Shan Mountain and Yan Shan Mouthtain in the north, and Qin Ling Mountain and Dabie Shan Mountain in the south. Its longitude and latitude range is 112° -124° E, 31°～42° N. This region is the economical, cultural and political center of China. In addition, located among Pacific Plate, Philippine Plate and Indian Plate, the tectonic activity of this region is remarkable during Cenozoic. It's one. of the areas where strong earthquakes frequently occur in China. Studying the crustal movement and deformation pattern, the feature of the constructional stress field, and its dynamic mechanics of this region is of great theoretical and practical importance.Based on Finite element method and ANSYS software, the thesis establishes several lithosphere modeling frames (crust-up mental) of North China region by incorporating existing geological and geophysical information of this region. Using digital simulation, and taking GPS observation, crustal deformation observation and focal mechanism results as basic boundary condition and testing standard, the thesis studies the present-day crustal movement, the strain and stress field of this region. And by connecting the surface movement with its deep tectonic movement, the thesis discusses the dynamic mechanics of the region, focusing on its dynamic setting and the driving force.According to the Cenozoic tectonic feature and deep geophysical observation, the author choose a rectangular region of 112° -124° E and 31° -42° N to establish the geometrical frame of the finite element models. In addition, present-day active faults distributing, topography and crust-up mantle structure of this region is included. The author creates and tests three finite element models, 3D model, 3DCM_I,3DCM_II, and 2D model, 2DDM_I. Ignoring the real crust-up mantle stratum structure of this region, Model 3DCM_I contains 7 layers with each layer being of equal thickness, including upper crust, mid-crust, lower crust, and up mantle(4 layers). The layering of Model 3DCM_II is consistent with the deep seismic sounding results. In 3DCM_I / 3DCM_II , faults are treated as weak zones of 5-6 km wide and 10-15km deep. Each model contains 14736 nodes and 24829 3D solid elements. In Model 2DDM_I faults are treated as discontinuous contact boundary. And it contains 1843 nodes and 3547 2D solid elements.Using static structural finite element analysis method, taking GPS observations around the model area as boundary condition, the horizontal velocity field of lithosphere deformation, stress increment field, and strain increment field are calculated. Using GPS observation within the model area as standard, the optimum solution which fits the GPS observation best is searched by adjusting the parameter of fault elements in those models. The results show that the average discreteness between the optimum calculations and GPS observations is 1.171mm²/year (3D model) and 1.176 mm²/year (2D model). In addition, the predicted slip features and slip velocities of main faults( ～0.1mm/year in 3D model results, ～0.3mm/year in 2D model results) are consistent with geological observations and deformation survey across those faults. In 3D models calculation, the stress field in depth of 10～15 Km shows that the maximum and the least principal stress axis is nearly horizontal, the principal tensile stress is about 2～8 times of the principal compressive stress, and their orientations are NNW and NEE, respectively, which are consistent with focal mechanism results and field stress survey. The calculated strain rate in the whole region is rather low, ～10^-9/year, which is in accord with former study. Besides, there are three north-east trend high gradient shear strain zones along Shanxi fault zone, Tangshan-Hejian-Cixian fault zone and Tanlu fault zone, respectively. However, in 2D result, the predicted horizontal slip velocity of Zhangjiako-Bohai fault zone is 1.35～ 1.45 mm/year, more higher than that of other faults and 3D model results. 2D model and 3D model calculations imply that the present-day horizontal lithosphere deformation movement pattern in North China region is controlled by the movement of large tectonic blocks surrounding the region. And the influence from the vertical and lateral heterogeneity of lithosphere is secondary. At the same time, Controlled by large tectonic blocks around, the slip features of the main faults within the region inherit their fashions of geological time scale and assume rather low slip velocity, except Zhangjiakou-bohai fault zone which needs to pay more concern.Based on 3D model (3DCM_I/3DCM_II), taking power law constitutive relation of rock into account, and using elastic-creep static structural finite element analysis method, the tectonic process during later geological era (～4Ma) is simulated to study the dynamic environment and driving forces of the present-day crustal movement evolvement in North China. In the calculation, the GPS observations surrounding the model area are taken as boundary condition, and the rheology of lithosphere, gravity factor, displace-strain nonlinearity induced by large deformation are also considered. Through choosing boundary conditioning for side faces and bottom of model, together with adjusting rheologic parameters of model element, the influence factors such as the plate tectonics, surrounding block movement, faults activity, and the lithosphere heterogeneity are analyzed to find the most plausible calculation that fits GPS observations and tectonic stress information within the region. The thesis gives 22 model calculations. The results shows that when the nodes on upper side faces of the models move little faster than that of the lower side faces, coupled with the drag movement on the model bottom, the simulation brings its better accordant with observation. Under such conditioning, the lowest average discreteness between the predicted surface velocities and GPS observations is 1.1529mm²/year (model 3DCM_I), 1.1451mm²/year (model 3DCM_II), respectively. It's lower than elastic structure finite element analysis result. And the predicted fault slipping rates is consistent with geological information. Under the same boundary conditioning, 3DCM_I model result is better than 3DCM_I model and the simulation including fault slipping are better than the opposite one. The predicted stress field in the depth of 10-15km shows that the least compressive principal stress axis is NNW trend, nearly horizontal. The maximum compressive principal stress axis is NEE trend, nearly horizontal in south area and nearly vertical in north area. The orientation of the predicted principal stress axis is accord with existing information. However, the state of the predicted stress axis is different with the existing cognition to some extent, which favors the maximum and least principal stress axis all being horizontal. The predicted stress field variation feature with depth is obviously controlled by boundary conditioning ( dynamic setting ). The influence of faults on the predicted stress field only focuses on local area or within fault itself.In order to study the surface deformation influenced by local construction movement, especially by the activity of blind faults, finding the method to obtain such movement message, the thesis develops GPS differential deformation analysis method. Using this method, the GPS observations in North China region and China Continent are analyzed, respectively. The differential velocity field between the simulation and GPS observations in North China shows that Zhangjiako- Bohai fault zone and Tangshan-Hejian-Cixian fault zone are recent intensively active area, and need closely observe against strong earthquake in future. As for China continent, three models (homogenous model, block model and multi-driving model) are tested for GPS differential deformation analysis. The results show that the present-day tectonic movement in China continent is controlled by multi-driving forces. Among them, the northward indention of India plate and the mantle convection induced dragging force on the bottom of lithosphere play important roles. The former determines the tectonic fashion and surface movement pattern in west continent including Tibet and surrounding region. The latter exerts significant influence on east continent including North China region and South China block. In addition, the influence from the main fault zones, such as Altyh fault zone and Zhangjiakou-Bohai fault, exert their influence on the local surface horizontal movement and should not be ignored.Assembling the digital simulations above, we can draw the conclusion as follows. The regional dynamic environment and driving forces of lithosphere deformation in North China is very completed. In one hand, under the subduction of Pacific slab and the northward indention of India plate, the movement of Ordos active block, South China active block and East-North active block controls the surface movement pattern of North China region. In another hand, mantle convection process may directly influence the surface deformation movement of the region. At the same time, the influence from present-day large active fault systems such as Zhangj iakou-Bohai fault and lithosphere heterogeneity, especially the existent of mid-crustal shear zone, should not be ignored.