Present Grustal Vertical Movement Of Eastern Tibetan Plateau And Coseismic And Postseimic Vertical Deformation Of Two Typical Earthquakes

The collision between Indian and Eurasian plates that occurred in the earlyCenozoic not only formed the Himalayan orogenic belt, but also created the Tibetanplateau, which is famous for the unique elevation, geomorphology, geologicalenvironment and natural environment in the world. Since the mid-19th century Prattand Airy founded the isostasy theory, the formation, evolution and uplift mechanismof the Tibetan Plateau is one of the focused international studies of continentaldynamics. At present, many scientists have proposed a variety of geophysicaldynamic models and attempted to explain the uplift of the Tibetan Plateau. Thesemodels have played a positive inspiration and impetus to understanding of the upliftmechanism. Different models suggest rather different crustal deformation patterns inthe vicinity of the plateau, especially around its east margin. Thus, precisemeasurement of the three-dimensional crustal deformation in this region can provideimportant quantitative numerical boundary conditions for the dynamic evolution, andhelp to understand the kinematic and geodynamic models.As the eastern border of the Tibetan Plateau and the central and southern section ofthe Chinese-Mongolian continental mid-axis tectonic belt, the northern part of theeastern margin of the Tibetan Plateau is located on the western margin of the Ordosblock, and its central part crosses Qinling and through the Longmen Shan, its southernpart goes through the Anninghe-Xiaojiang fault. The eastern margin of the TibetanPlateau separates stable blocks of the Ordos, Sichuan basin and South China from thestrong uplift of the Tibetan Plateau. The deep geophysical exploration shows that thecrustal thickness of the Tibetan Plateau is about60⁷0km, while the crustal thicknessof North and South China is only about40⁵0km. The eastern margin of the TibetanPlateau is the gravity gradient and abnormal crustal thickness zone. Late Cenozoicand contemporary tectonic deformation on both sides of the eastern margin of theTibetan Plateau is obviously different. Large-scale active faults and strong earthquakes mainly occurred in the west of the zone, while its east does not hostlarge-scale active faults and the earthquake activity is much lower. On May12,2008,a devastating Ms8.0earthquake occurred in the Longmen Shan region, the middle partof the eastern margin of the Tibetan Plateau. This earthquake proves the tectonicactivity in the eastern margin of the Tibetan Plateau.Because of the complicated seismic geological structure in the Tibetan Plateau, it isdifficult to determine the tectonic deformation field by the use of geologic andgeophysic method in a short period. However, with the development of GPStechnique in recent years, it provides an unprecedented effective approach foracquiring the velocity field of the tectonic deformation. The image of thehigh-resolution velocity field for the eastern Tibetan Plateau has been obtained byGPS. Although GPS measurements can provide three-dimensional crustal deformation,due to the atmospheric refraction, the antenna phase centers of satellite and receiver,the accuracy of vertical positioning is lower than the horizontal. On the other hand,with the operation of Crustal Movement Observation Network of China （CMONOC）in1999, the observation history of GPS in China is only more than a decade. Bycontrary, the precise leveling networks were measured after the1966Xingtaiearthquake for nearly50years. Therefore, the precise leveling measurement is still amajor technique for obtaining the crustal vertical deformation which could be studiedin various fields of earthquake science research.Using precise leveling observations to study the vertical crustal deformation of theeastern margin of the Tibetan Plateau is a main theme through out this thesis, and thecontent can be divided into the following two aspects.1Present crustal vertical deformation of the eastern margin ofTibetan PlateauBased on the regional leveling observations collected in the eastern margin of theTibetan Plateau, we obtain the present crustal vertical velocity field, which couldprovide important basic information for the long-and middle-term seismic riskprediction. Combining the vertical velocity field acquired in this work and horizontal velocity obtained by previous studies, we analyze the characteristics of thethree-dimensional crustal movement of the eastern Tibetan Plateau, and discuss thegeodynamic mechanism of crustal deformation. The primary conclusions are asfollows.（1） The precise leveling data observed around the eastern margin of the TibetanPlateau are collected, including the leveling networks used in monitoring tectonicdeformation of main active faults in China since1970, the national leveling networksof China observed in1980s and1990s separately, and the central and western Yunnanregion surveyed between2010and2011which is a part of “Integrated GeophysicalField Observation–the eastern margin of Tibetan Plateau” project. All the heightdifferences between adjacent two benchmarks are plotted over time, so the instablebenchmarks caused by earthquake events or ground water drawing could be removed.Finally, there are3439benchmarks of which the ratio of first-order height differencesis97.5%, and the ratio of second-order is2.5%.（2） Before overall adjustment, we choose the leveling network located in the centraland western Yunan region （south of26°N） to estimate the prior uncertainty of eachkilometer of height difference. This leveling network was surveyed between2010and2011, and the movement of benchmarks can be considered as zero. So we can get theadjusted height elevation using the static adjustment. The result shows that theuncertainty of one kilometer of height difference is1.2mm.Because of the strong tectonic deformation in the studied region and the complexityof leveling data, the linear dynamic adjustment model is used to estimate the unknownparameters. The vertical velocities of9GPS stations within this region are as a prioriconstraints which can effectively reduce the accumulated systematic errors along theleveling routes. As a result, the present crustal vertical velocity field image is obtained.It shows that the posteriori uncertainty is0.97mm, which is consistent with the prioruncertainty. This indicates the reliability of velocity field result from a small region.（3） The trend of long-term crustal vertical movement obtained in this thesis isconsistent with existing results inferred from geological methods, GPS and levelingmeasurements. Most regions of the eastern margin of the Tibetan Plateau are in the status of uplift, especially Gonggashan uplifts at a rate of5.7mm/a, and the uplift rateof the western Qinling is up to6.4mm/a.（4） The vertical velocity profile across the fault can be used to estimate its verticalslip rate, which is difficult to derive from the geologic method and can providequantitative constraint. The result shows that the vertical slip rate of the LongmenShan fault is up to3.4±0.4mm/a, followed by the Daliang Shan fault with the rate of2.0±0.4mm/a. The vertical slip rates of the Helan Shan, Liupan Shan, Longriba andXiaojiang faults are between1and1.6mm/a. There is no significant vertical slipacross the Zemuhe and Red River faults.（5） The wavelet decomposition technique is applied to obtain different wavelengthsof the vertical velocity field for the eastern margin of the Tibetan Plateau. The crustalvertical deformation of long wavelength （500-1000km） can be related to the deepmantle under the Tibetan Plateau, while the regional deformation of short wavelengthmay be related to the crsutal deformation.（6） Combining the present crustal vertical velocity field with horizontal velocityfield obtained in the eastern margin of the Tibetan Plateau, we investigate thecharacteristics of crustal movement and corresponding dynamic mechanism. Thehorizontal material influx result shows that the movement of upper and lower crust israther different in most regions, namely, to maintain the uplift rate of each sub-blockrequires the influx rate of the lower crust and upper mantle higher than that of theupper crust, which provides the evidence for the existence of lower crustal flow in thisregion. For example, the uplift rate of the western Songpan-Ganzi block is0¹mm/a,the uplift rate of its central part increases to2³mm/a, and the vertical rate of itseastern part which is close to the Sichuan Basin drops to0¹mm/a. However, theaverage horizontal movement of material in the lithosphere can only lead to an upliftrate of⁰.7mm/a. This vertical movement may reveal the existence of channel flowin the middle and lower crust under the Songpan-Ganzi block. While the TibetanPlateau moves eastward, due to the blocking of stable crust of the Sichuan Basin, thelower crustal material accumulated under the Longmen Shan and western Sichuanplateau in the way of plastic flow. Therefore, the middle and lower crust of the western Sichuan plateau thickened significantly, and exerted a vertical uplift to theupper brittle crust, which resulted in the uplift of the Longmen Shan and westernSichuan plateau.（7） The regional three-dimensional velocity field indicates that the Helan Shan isuplifting and the Yinchuan graben is subsiding. The crustal shortening of the LiupanShan region is the primary drive for uplift. The subsidence of the central and southernSichuan-Yunnan fragment is caused by the near east-west extension.（8） Through the regional vertical velocity field, combining with epicenterdistribution of more than Ms5.0earthquakes, we find an abnormal zone near theYongde-Zhenkang town with a high uplift rate, which is an earthquake gap on theLongling-Lancang fault.2Coseismic and postseismic vertical deformation of two typicalearthquakes occurred in the eastern margin of the TibetanPlateauLeveling data are used to analyze and study the coseismic and postseismic verticaldeformation of two typical earthquakes that occurred in the eastern margin of theTibetan Plateau. The main conclusions are described as follows.（1） We model leveling deformation data observed following the1990Ms7.0Gonghe earthquake in Qinghai Province to infer postseismic deformation sources andmechanisms. Using coseismic vertical displacements and previous study resultobtained from seismic body wave inversion, we update the main shock rupture model.Time series of elevation change between adjacent benchmarks can be modeled by alogarithmic or an exponential relaxation function. To model the postseismicdisplacements, we utilize the raw observations of elevation differences betweenadjacent benchmarks, not their integrals with respect to a reference benchmark, toconstrain a dislocation model in a continuum. It makes full use of the leveling dataand effectively reduces biases introduced from cumulative errors due to dataintegration. The postseismic modeling result suggests that two mechanisms operate simultaneously to produce the postseismic vertical deformation observed at thesurface: afterslip on the coseismic rupture fault plane of the main shock and itsperipheral extension, particularly upward into the sediment layer above the mainrupture, and viscoelastic relaxation of the lower crust and upper mantle, with aviscosity of9×10¹⁹Pa·s. The result suggests more brittle and less viscous lower crustand upper mantle underneath the Qaidam basin than some of previous studiesenvisioned.（2） Using the coseismic displacements from GPS and near-field precise levelingmeasurements before and after the2008Ms8.0Wenchuan earthquake, the cosesimicfault geometry and slip distribution are modeled. Besides the similar results withprevious studies, we find that the slip at the depth of15¹8km under the Beichuanfault is as large as2⁴m. The postseimic relaxation process is acquired by utilizingthe GPS data observed around the Longmenshan fault between2008and2011. It ishelpful to understand the rupture mechanism of this quake, and provides basicinformation for the physical mechanisms of postseismic deformation. The evolutionof postseimic vertical deformation is derived based on precise leveling measuredbetween2008and2011near the rupture.