Study On Contemporary Tectonic Deformation In North China By Using Three-dimensional Simulation And Parallel Computation

There are dense population and many big cities, and politics, economy, culture, and transportation are well developed in North China. This area is one of the regions with complex geological structures, unique structural deformation and very active earthquake activity, where seismic intensity and frequency are very high, so North China is also one of the regiongs with highest research level. Based on geological data, geophysical data, GPS observations and parallel computation, this thesis studies the tectonic deformation, the nature of fault movement, the law of seismic activity and the influence of earthquake in North China by means of numerical simulation.The development of numerical simulation has a history of over 50 years, and the analysis of tectonic deformation in North China by numerical simulation has also made a lot of important results. But many problems still exist: 1) Due to the limitations of computer hardware and software, the previous model used a coarse grid and has a low accuracy. 2) Faults play an important role in the tectonic movement of continental lithosphere. The faults in North China are very active and distributed densely, but most existing studies on North China only consider several major faults. 3) A large earthquake has a great influence on ground motion, deformation and regional tectonic stress field. The influence is not just in the region near the epicenter,but sometimes transfers a far region. For instance, how the Wenchuan Ms8.0 earthquake in 2008 affected the tectonic deformation and future seismic activity in China, especially in North China, is a problem worthy of study. 4) In numerical simulation experiments, the comparison of simulation results, which is an important part of the experiment and always used to test the rationality of numerical models and reliability of data, largely tests the credibility of the numerical simulation results. Unfortunately, this part is often neglected by many researchers, or only a small part of results are compared to prove the rationality of numerical models.Aimed at the above problems, the thesis focus on the following aspects of works:(1) Three-dimensional dynamic model of North ChinaThe simulation is based on ANSYS parallel software platform. According to active tectonic block region division and distribution of active faults in North China, combined with GPS data, the range of the numerical model is defined as 99.8°~121.4°E, 27.9°~42.3°N, which contains a majority of the North China active tectonic block region and a part of other block regions including the Tibetan Plateau, the Xiyu, South China, and the Northeastern Asia.The North China active tectonic block region consists three active tectonic blocks: Ordos, North China Plain, and Eastern Shandong-Yellow Sea block. The main active fault zones in North China include the arc-like fault zone in southwestern Ordos, Yinchuan-Jarantai normal fault zone, the Hetao graben system, the Zhangjiakou-Bohai Sea fault zone, the Weihe graben system, the Shanxi graben system, Tanlu fault, the Tangshan-Cixian fault zone and the Anyang-Xuzhou fault zone. The model considers all known late Quaternary active faults in North China and every fault is assumed to be 5km wide.The model is divided into 416582 elements whose average side length is 25km with 582392 nodes. Using GPS data of two periods 1999~2004 and 2004~2007 as the boundary constraint, the tectonic deformation in North China is numerically simulated.(2) Simulation of crustal movement velocity and analysis With the GPS boundary conditions, the reliability of the model and simulation calculation is evaluated by comparing the movement velocity of simulation results with GPS observations.The results show that the velocity of crustal movement in North China decreases from east to west, and increases from north to south overall. The velocity is small in north of North China and Yanshan block, while it increases in the south central North China and South China block. In addition, compared to 1999~2004, the velocity of the blocks in North China accelerates by 1mm/a approximately in 2004~2007. It is an almost match between simulation results and GPS observed velocity field. However, there are major diffferences at a few sites in the movement direction and/or velocity size, which are mainly distributed in the northeastern part of North China Plain block probably due to the Wenan Ms5.1 earthquake on 4 July 2006, or the boundary effect of simulation and GPS observation error.(3) Simulation of the slip of faults and analysisConsidering all known late Quaternary active faults in North China in the model and according to the simulation results, this study compares the slip of faults obtained from simulation with that from geological survey. The result shows that for the 48 strike-slip faults in statistics, the coincidence rate is 89.6% for 43 faults using 1999~2004 GPS observation and it is 93.8% for 45 faults using 2004~2007 GPS observation. It shows a certain rationality of the numerical model, since the simulation results can well reflect the strike-slip movement of faults. In addition, this work compares the slip of faults obtained from the simulation using 1999~2004 GPS data with that from the inversion of the horizontal displacement field using high-precision GPS data by YangGuohua, and the results coincide well.(4) Simulation of stress and strain fields and analysisThe simulation results show diagrams of the minimum principal stress, the maximum principal stress, the minimum strain and the maximum principal strain in North China in 1999~2004 and 2004~2007. The horizontal strain direction in North China is generally consistent. The direction of minimum principal strain is NNW-SSE and the direction of maximum principal strain is NEE-SWW, which accord with the strain direction obtained from inversion of focal mechanism solutions, GPS observations, etc. by previous work. Comparison of the principal strain field of the North China and adjacent areas obtained from the simulation with that from other research exhibits a good agreement.(5) Analysis of the influence of the Wenchuan earthquake on the tectonic deformation in North ChinaBased on the verification of the model’s rationality, this work preliminarily simulates the effect of the Wenchuan earthquake on the stress-strain field and the slip of active faults in North China. By means of the comparison between the stress and strain fields in North China caused by the Wenchuan earthquake and the basic fields derived from GPS data, it is found that the directions of the principal strain for the both fields have big differences(>45°) in North China Plain and Eastern Shandong-Yellow Sea block. It means that the strain field is affected by unloading due to the Wenchuan earthquake. While the directions of the principal strain are similar to each other in Ordos, therefore, where the strain field is loading due to the earthquake. Comparing the slip of faults caused by the Wenchuan earthquake with that by GPS constraint, it is found that the faults have same direction in the Yinchuan-Jarantai normal fault zone and the Shanxi graben system and the faults have opposite directions in the Hetao graben system, the Anyang-Xuzhou fault zone and Tanlu fault.Through three-dimensional simulation of tectonic deformation nowadays in North China, the main conclusions are as follows:(1) The velocity of crustal movement in North China decreases from east to west, and increases from north to south. It is almost a match between simulation results and the GPS observed velocity field. However, there are great diffference for a few sites in the movement direction and/or velocity size, which are mainly distributed in the northeastern part of North China Plain block probably due to the Wenan Ms5.1 earthquake on 4 July 2006, or boundary effect of simulation and GPS observation error.(2) The average velocities of most blocks in North China are similar between simulation results and GPS observations. Comparing data of 2004~2007 with 1999~2004, whether the simulation results or GPS observation, the average velocity increases by 1mm/a, and it reflects the acceleration of crustal movement in North China in 2004~2007.(3) It shows a certain rationality for the numerical model, since the simulation results can well reflect the strike-slip movement of faults. The results show that, in the 48 strike-slip faults in statistics, the coincidence rate is 89.6% for 43 faults using 1999~2004 GPS observation and it is 93.8% for 45 faults using 2004~2007 GPS observation. (4) The simulation results show the horizontal strain direction in North China is generally consistent: the direction of minimum principal strain is NNW-SSE and the direction of maximum principal strain is NEE-SWW, which accords with the strain direction derived from inversion of focal mechanism solutionss, GPS observation, etc. by previous work. The results also show that, the active faults have a great impact on the simulation results, which are the areas of concentrated stress and strain.(5) The results show that, the Wenchuan event posed an effect of unloading on the strain field in eastern Ordos, North China Plain and Eastern Shandong-Yellow Sea block, and loading on the strain field in western Ordos. The motions of same direction caused by the Wenchuan shock occur on the Yinchuan-Jarantai normal fault zone and the Shanxi graben system and that of opposite directions take place on the Hetao graben zone, the Anyang-Xuzhou fault zone and Tanlu fault.