| The main motive force in Sichuan-Yunnan region is derived from collisions between the Indian plate and the Eurasian plate. It is the combination of different order force sources. There is the strongly lateral extrusion which comes from the direct effect of Indian plate, in the west of Yunnan, but it is the result of stress of the multilevel conversion in the east of Yunnan. Subduction of the Indian lithosphere under Eurasia plays an important role in the tectonic evolution of the Tibetan plateau and surrounding regions, thickening and shorting of Tibetan plateau. Eastward extrusion movement of Qiangtang Block is obstructed by the Sichuan Basin, which drives the Sichuan-Yunnan Block continued to the southeast movement. Puer seismic area is located in Simao Block which is situated in the structure transition zone between Tibetan plateau and Yangtze platform. There are several large faults including Lancang River fault, Wuliangshan fault and Red River fault, which control the formation and development of the regional crustal evolution. The form of the present crustal movement is the rotate and tensile deformation of several micro-block.Since 1884, it happened 11 times earthquakes with magnitudes above 6, not an earthquake with magnitudes above 7 in Puer seismic area. These earthquakes are main shock-aftershock and group of earthquake type. In 2011-2012, we have been collected more than 270 MT observation points in Sichuan-Yunnan region, and more than 260 MT measured points in Puer seismic area, respectively, which is supported by China National Special Fund for Earthquake Scientific Research. This work establishes the subsurface three-dimensional(3D) electrical resistivity model in Puer seismic area by MT data collection, processing and inversion, and the thermal and compositional structure of Sichuan-Yunnan by jointly inverting Rayleigh wave dispersion data,geoid height, topography, and surface heat flow with probabilistic(Bayesian) Monte Carlo method. Based on the attained electrical resistivity model and thermal structure,the fluids contents in the upper mantle are estimated in terms of laboratory studies of rock conductivity. In combination with the electrical resistivity model, the fluids contents, other geophysical observations and petrological studies, this thesis attempts to explore the dynamic process and mechanism of earthquake preparation in Puer seismic area. The main research content and results of this thesis are summarized below.1. MT analysisThe magnetotelluric data were acquired at 274 sites along 8 profiles, the average distance about 2km. The profiles of L1-L6 laid along the direction of NE80°, almostperpendicular with the Lancang River fault and the Wuliangshan fault. The direction of the L8 profile is NE60°. The direction of the L7 profile is nearly perpendicular with the L1 profile. The GB tensor decomposition, phase tensor and induction vectors are used to investigate dimensionality and geoelectric strike direction. MT data in Puer seismic area are characterized by three dimensions and a reasonable geoelectric strike direction is determined together with the geological strike. The N15°W can represent the advantage of geoelectric strike direction. MT data can be decomposed into the transverse electric(TE) mode and transverse magnetic(TM) mode by rotating the impedance tensor into the regional geo-electric strike direction. To know the variations of the apparent resistivity and phase along the profile, the qualitative analysis is made for the rotated MT data.2. MT inversionBefore MT data are converted to an electrical resistivity model, data quality is assessed using one-dimensional Rhoplus algorithm and bad data points induced by strong culture noise are removed. The 2D NLCG inversion is implemented for the edited data. During the inversion, a significant number of inversions are performed by choosing different control parameters(data mode, regularization factor, data error floor). The initial models were constructed with 100Ω.m uniform half-space and an incorporating topography. After multiple inversions and model validation, the final model is chosen with the regularization factor of 30. The error floors are set to 5% for TM apparent resistivity and phase in the NLCG inversions. We employed the Mod EM for 3D modelling an inversion. The study area was discretized with a 2km grid in the core, padded with 10 cells on all edges, with widths increasing by a factor of 1.5outward to the boundary. Vertically, 63 layers were used, starting from 20 meters and increasing logarithmically with difference factors. The discretization resulted in a106×74×70 grid, in the x, y and z directions, including 7 air layers. The initial models were constructed with 100Ω.m uniform half-space. A large number of 3D inversion are performed by choosing not-rotated and rotated data, including off-diagonal impedance, full tensor elements and apparent resistivity and phase, respectively. An iterative inversion procedure with manual intervention is introduced in order to obtain better data-fitting. In fact it is a multi-stage inversion, when the current inversion procedure is stopped, the model with minimum RMS is selected as the initial model for a renewed inversion by keeping other parameters unchanged. Finally, the model deriving the off-diagonal impedance has been chosen. The error floors are assigned5%*|Zxy·Zyx|1/2. The normalized root mean square misfit of final model required morethan 180 iterations is 1.05. The main electrical anomalies in the final model of 2D and3 D inversion have been delineated and described in detail.3. Interpretation of the electrical resistivity modelCoupled with geological and other geophysical data, the 3D electrical resistivity model and its tectonic implications are analyzed in detail. The 3D electrical structure model shows that two low resistivity zones are located on the middle and lower crust either side of the Wuliangshan fault. The low resistivity zones are interpreted as due to aqueous fluids and partial melts, perhaps caused by asthenosphere upwelling. The high resistance beneath Wuliangshan fault is mainly comprised of the stable metamorphic crystalline basement. The seismic sources are mainly distributed in the boundary between high resistance and low resistance. It is interpreted that the mechanical properties of the low resistivity rocks are mainly weak, which is not conductive to accumulation of stress, however the adjacent high resistivity rocks that are rigid, are prone to brittle fracture. Under the action of stress, the deformation each rock is inconsistent, so it will inevitably lead to the stress concentration in the high resistivity rock. The fluid located inside rock can reduce the strength of the rock. The earthquake will occur when the rock reaches its limit for deformation.4. The thermochemical structure and fluid content of the lithosphere mantleThere are two significant heat anomaly area in the thermochemical structure of the lithosphere and upper mantle beneath Sichuan-Yunnan region from multiobservable probabilistic inversion. The high heat region in southwest of Yunnan is bound up with the mantle thermal disturbance caused by the eastward subduction of Indo-Burmese Wedge. The heat anomaly nearby the Red River fault is connected with the shear movement and the mantle magmatic intrusion. Based on the electrical resistivity model derived from MT data of Menglian-Luoping profile and the thermochemical structure, the water content and melt fractions in range of 40-100 km depth for three stations, located at the Puer basin and the Wuliangshan fault,respectively are estimated using the laboratory studies of rock conductivity, a modified Archie’s equation. It shows that the lithosphere mantle beneath Puer seismic area is rich in water and melt fraction. The Mg#(90-91) of lithosphere mantle is high than the original upper mantle. So we can determine that the lithosphere mantle beneath Puer seismic area is in partial melting or used to be in partial melting.Combination with electrical structure model, the low resistivity zone in upper mantle is caused by partial melt. The fluid can greatly reduce the strength of the bottom of the lithosphere mantle.5. The mechanism of earthquake preparation and the dynamic processIntegrating the 3D electrical resistivity model, the fluids estimated, the xenolith studies of water contents, seismic imaging results and the thermochemical structure of the lithosphere mantle, we have more comprehensive understanding about the mechanism of earthquake preparation beneath Puer seismic area. The partial melt fraction and the temperature of lithosphere beneath the seismic area is higher. Mantle material upwelling, so the upward stress is stronger. Due to the high resistivity rock above the upper crust, the most of the energy from the middle and lower crust and upper mantle is not released, causing the accumulation of crustal stress on fracture intersections, which is boundary between the high resistivity rock and the low resistivity rock. At the same time, the upper mantle asthenosphere is moving toward northeast direction, while the movement direction of crust is southeast, accelerating the rate of stress accumulation. The earthquake will occur because of the instability of the stress triggered by the other external forces. It is difficult to form a large scale fault and a large accumulation of stress that the upper crust electrical structure is broken, so the large earthquake is difficult to occur. |
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