| The Haiyuan-Liupanshan tectonic zone (HLTZ) is located at the junction of the northeast margin of the Tibet Plateau and southwest margin of the Ordos block. It is a gradient zone of crustal structure differentiation and geophysical field changes as well as one of the areas with most intensive tectonic deformation since late Quaternary in our country. Studying the deep structure of this tectonic zone and its contact relation-ships on either side would help understanding the expansion deformation mechanism and the deep dynamic environment of the northeastern Tibet plateau.An MT survey across the HLTZ has been carried out in 2013, that was sponsored by the Project of State Key Laboratory of Earthquake Dynamics "3D deep structure and seismogenic environment of the connect area of North-South Seismic Belt and west Qinling" (LED2013A01) and the Seismic Industry Special Research Project "China earthquake science array detection-the northern section of the north-south seismic belt"(20130811). On the basis of previous data, this survey collected data along three high density MT profiles (189 sites), which separately cross the HLTZ at Guyuan segment (north), Longde segment (middle) and Huating segment (south). The Phoenix MTU-5A five-component MT observing systems were used to collect data. Several methods including "robust", remote reference, and phase tensor decomposition were used for data processing. The NLCG two-dimensional inversion was conducted to image deep electrical structure.2D forward modeling and 3D inversion were calculated to verify the 2D inversion results.An Ms6.6 earthquake occurred during data collection at Minxian-Zhagnxian in the West Qinling area. There were three MT sites in working status with a distance of about 300km to the epicenter. The coseismic electromagnetic signal and seismoelectromagnetic signal recorded were displayed. These singals were compared with the seismic waves recorded by nearby seismic stations. Combining numerical simulation and inversion results of MT profiles, this thesis tried to analyze the relationship between deep structure and seismoelectromagnetic signal.The primary results of this thesis are summarized below.(1) Downward extensions of faults. In the study area, the Yueliangshan south-pidmont fault, Haiyuan-Liupanshan fault, Qingtongxia-Guyuan fault and Huining-Yigang fault are all major electrical boundaries. At the Guyuan segment, the Haiyuan- Liupanshan fault and Yueliangshan south-pidmont fault are low-resistance bands with certain widths and tilt to southwest. Nearby them there are two hidden low-resistance bands. These four low-resistance bands aggregate into low resistivity layer at about 25km depth in the lower crust, forming a "positive flower" structure. In this segment, the Qingtongxia-Guyuan fault separates low and high resistance. At the Longde segment, the Yueliangshan south-pidmont fault dips to northeast, and the Liupanshan fault dips to southwest, both, as low-resistance bands, also converge in the lower crust. In this segement, the Qingtongxia-Guyuan and Weizhou-Anguo faults differ less in electricity. At the Huating segment, the "flower" structure is vanished in the Liupanshan fault that is a single low-resistance band dipping shouthwest.(2) Deep electrical structure of blocks. The three MT profiles cross the Longzhong basin, the arc-like tectonic belt and the Ordos block. They revealed a mosaic pattern of low-resistance bands in high-resistance background. The low-resistance bands extend downward and converge in the low-resistivity layer in the lower crust. The Longzhong basin has good electrical stratification, with similar structures from inversion results of the profiles. High resistance characterizes the shallow subsurface of several hundred meters, and a low resistance layer is present at about 25km depth. The west margin of the Ordos block shows a complete high-resistivity body, while the Ordos basin is of a typical style of Cenozoic tectonics, which has a low-high-low layered resistivity structure.(3)Deep electrical structural characteristics and crustal deformation. The high-or low-resistivity bodies appear alternatively in the Longzhong basin'crust, which is on the southwest side of the HLTZ. And the crust on its northeast side is a complete high resistivity body. The active tectonics research found that the left strike-slip displacement of the Haiyuan fault zone is converted into crustal shortening of the Liupanshan fault zone. GPS observations suggest that current tectonic deformation is distributed in a range of hundreds of kilometers west of the Liupanshan. Such a deformation state can be explained well by deep electrical structure:the crust on the southwest side of the HLTZ is vary broken, thus relatively easily to deform by the northeastward pushing of the Tibetan plateau, while the crust on the northeast side of HLTZ is intact and hard to deform.(4) Features of deep structure beneath the Liupanshan tectonic zone and seismic risk. From deep electrical structure images, it is inferred that the Longzhong basin is a stress transfer carrier, because a low-resistance layer, likely also low-viscocity layer, exists in the lower crust below this basin. The Liupanshan tectonic belt and the block on the northeast side are of high resistance, implying a capacity of stress accumulation. The layering stable Ordos block acts as a barrier to crustal motion. So the deformation features and deep structural environment of the Liupanshan tectonic belt seems similar to the Longmenshan before the 2008 Wenchuan earthquake to some extent, sugesting its seismic risk should be vigilant.(5)Observation and analysis of seismoelectromagnetic signal and coseismic electromagnetic signal during the 2013 Minxian-Zhangxian Ms6.6 earthquake. When the 2013 Minxian-Zhangxian Ms6.6 earthquake occurred, three sites 300km distant to the epicenter were just implementing MT measurement. The MT instrument not only record the coseismic electromagnetic signal produced by "electromagnetic driving with shock effect" through seismic wave'propagation, but also recorded the seismoelectromagnetic signal excited by the source rupturing. These seismoelectro-magnetic signals are recorded around 12 second after the earthquake occurred, lasting about 4 seconds, much earlier than P-wave arrivals. It's considered a kind of useful warning signal which deserves further study. |
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