Present-day Crustal Deformation Of Western Sichuan Inferred From Geodetic Observations

Western Sichuan Province is geographically located in the eastern margin of the Tibetan Plateau and the joint area among the Sichuan-Yunnan rhombic block, Bayan Kala block and Southern China block. It also straddles the middle and south sections of the north-south seismic belt in the Chinese mainland.Because of continuous subduction of the India plate beneath the Eurasian plate and collision between the two the plates from the middle Eocene epoch ( about 55 million years ago) , especially the extrusion of the material from the central Tibet Plateau and the indentation of the Asam wedge, this region is one of the most outstanding tectonic areas in Chinese mainland with active faults, complicated structures and frequent major earthquakes. And thus, a lot of investigations and research work have been done in this area. However, because of the difficult natural condition, only few a small number of main active faults have been studied sufficiently in the field work, such as Xianshuihe fault, Anninghe fault, Zemuhe fault. The other active tectonics, like second-class active faults, active folds, shear zones and nappe structures, are still lack of complete studies. Fortunately, with the extensive application of GPS in this region in the last two decades, together with the accumulation of traditional leveling data, it is now possible to infer the movement styles of various active structures using the highly precise and densely distributed crustal deformation observation data, via geophysical models describing the relationship between tectonic movement and crustal deformation such as the Okada's elastic half-space dislocation model. And this kind of work can provide us a special way to understand the geodynamic mechanism of the crustal deformation and tectonic movement of the southeastern margin of the Tibetan Plateau.In this thesis, first a 3D geometric model is established for the main active faults in the western Sichuan Province based on the results of geological reconnaissance, geophysical investigation and geodetic observations of other scientists. This fault model includes the spatial distribution, segmentations, dip directions and angles, locking depth and slip rates of the following 25 faults: Garzê-Yushu fault, Xianshuihe fault, Anninghe fault, Zemuhe fault, Daliangshan fault, Xiaojiang fault, Mabian fault, Huayingshan fault, Longmenshan fault, Longriba fault, Maqu fault, Minjiang fault, Zhongdian fault, Jinshajiang fault, Batang fault, Weixi-Qiaohou fault, Red river fault, Lancang Jiang fault, Chenghai fault, Lijiang-Xiaojinhe fault, Litang fault, Nantinghe fault, Longlin-lancang fault, Nujiang fault, Jinhong fault. Based on this model, this work then inverts the present-day slip rates of the main active faults in the region using the Okada's 3-D elastic half-space dislocation model with the best fit to the crustal deformation observations of 324 GPS stations and 474 leveling points. The inversion software is modified from the MAIN113 which was programmed by the United States Geological Survey (USGS). The inversion results are well consistent with the ones from geological methods.For some faults with big uncertainties in their geological slip rates, we got their slip rates as follows: Mabian fault, 2.6±0.8 mm/a (left-lateral) and 0.7±0.7 mm/a (thrust); Batang fault, 4.1±1.4 mm/a (right-lateral) and 0.4±0.9 mm/a (normal); the northern segment of Jinshajiang fault, 1.8±1.4 mm/a (right-lateral) and 1.2±0.9 mm/a (normal); the middle segment of Jinshajiang fault, 2.5±0.8 mm/a (right-lateral) and 1.6±0.7 mm/a (thrust); Zhongdian fault, 1.5±0.9 mm/a (right-lateral) and 1.5±0.9 mm/a (normal).; the eastern segment of Longling-Lancang fault, 2.0±1.6 mm/a (right-lateral) and 0.9±0.9 mm/a(thrust), the western segment of the fault, 0.7±0.9 mm/a (right-lateral) and 1.0±0.9 mm/a (thrust). These slip rates from Geodetic data give an important reference to the study of the active faults and earthquake hazard analysis.In addition, the fitting residuals of the velocities between the model and the observation from the GPS and leveling revealed that the crustal deformation features of the Western Sichuan Province can be well described by the continuous slips of a series active faults below the locking depth, although the fitting residuals of some regions show systematic discrepancy, which may result from the material property discrepancy of different areas and the effects of channel flow in the middle or lower crust in the area.As a tentative, this work also tries a block model, which is actually an extremely simplified dislocation model, to explain the features of the regional crustal deformation via the translation and rotation of 11 sub-blocks, i.e. Aba sub-block, Longmenshan sub-block, South-China block, Yajiang sub-block, Shangri-la sub-block, Central Yunnan sub-block, West Sichuan and Yunnan flow zone, Baoshan sub-block, Ximeng sub-block, Mabian sub-block and Daliangshan sub-block. The result shows that the crustal deformation of Western Sichuan Province can also be well described by the rigid movement of a series of sub-blocks bounded by active faults.In order to understand the dynamic mechanism of the crustal deformation in the Western Sichuan Province, this work also tries to use FEM (finite element method) and 2-D plane strain model to simulate the regional crustal velocity field and strain rate of this area. The simulated results reasonably demonstrate the main features of the crustal deformation form GPS observations. In addition, as the base of this research, this thesis gives an overall review to the studies of the tectonic movement and crustal deformation of the Tibetan Plateau, including the research history, current situation, main theories and focused issues. It also review the GPS technology with the emphasis on its applications to geodesy, such as the strategies about GPS data processing,data combination and error reduction, etc.Comparing with the similar work in this region of other researchers, this work has two improvements: One is that it uses more dense GPS data, especially the dense GPS profiles, in the dislocation model to inverse the slip rates of active faults. These dense GPS data obviously gave a better constraint on the horizontal deformation. The other is that it uses widely distributed precise leveling data in the dislocation model together with GPS velocity data. The join of these leveling data can give a effective constraint on vertical component. These two improvements make the results more reliable in the inversion of the slip rate of the active faults concerned.