| In the east of China,there are two famous giant linear tectonic zones in the near north-south direction,namely,the Tanlu fault zone and the North-South gravity gradient zone.They are located in the most severely damaged area in the eastern part of the North China Craton and are also very important structural boundary zones,so they are hot areas of geoscience research.Among them,the Tanlu fault zone(TLFZ)through eastern China on a large scale,spanning many active blocks with different evolutionary histories.It is also an active fault zone with an early formation history and experienced multiple changes in the nature of activities in different tectonic periods.Due to the complex structure,geological survey and seismic activity intensity have obvious segmentation characteristics.It is the obvious abnormal zone in the results of geophysical fields such as gravity and magnetic field,velocity structure,and electromagnetic structure,and it is also a fault zone with a high degree of research and great controversy.In the middle and late Mesozoic,the Yangzi plate collided with the North China plate,and the paleo Pacific plate subducted to the northwest.Large-scale sinistral strike-slip movement occurred in the TLFZ,staggering the Sulu orogenic belt and the Dabie orogenic belt by more than 400 kilometers.The Hefei-Suqian segment of the TLFZ connects the Dabie and Sulu orogenic belts and is also the boundary fault between the North China and Yangzi plates,which is related to the collision process of the two plates and the original mechanism of the TLFZ and has important scientific research value.The seismicity in the Hefei-Suqian segment is weak as a whole.In modern times,small and medium-sized earthquakes with shallow sources are mainly distributed on both sides of the fault zone,and few earthquakes on the main trunk,which are segmented,zonal,and uneven.There are also some seismic gaps.Whether it is the earthquake silence period caused by fault locking or the lack of deep environment or external driving force for strong earthquakes in this region needs to carry out deep structure analysis and research.The Hefei-Suqian segment of the TLFZ and its adjacent area are selected as the study area in this paper.The magnetotelluric array data of 185 survey points covering the study area are collected.Through the analysis of the characteristics of the apparent resistivity and impedance phase curves of the survey points,the phase tensor decomposition and the real induction vector are used to qualitatively analyze the dimensionality of the study area and the electrical structure characteristics of different structural units.Then using the three-dimensional electromagnetic inversion program of Mod EM based on the nonlinear conjugate gradient method,and comparing the threedimensional inversion results of different data combinations,different coordinate systems,and whether with the terrain.The sensitivity tests of key structures and effective depths of the selected electrical structure model were carried out,and finally,the reliable and high-precision three-dimensional electrical structure distribution image of the crust scale in the study area is obtained.To further study the deep electrical structure characteristics of different structural units in the study area,the unit contact relationship,and the crustal structure analysis of the main faults,combined with the geological survey results,seismic imaging,small earthquake positioning,geodesy,and other geophysical data of the survey area,the geological structure interpretation,seismogenic environment,and activity analysis is further carried out.The main research results of this paper are as follows:(1)The three-dimensional magnetotelluric inversion can use a variety of combinations of electromagnetic responses to obtain the electrical structure model.When conducting the three-dimensional inversion,the data combination should be comprehensively judged according to the electromagnetic environment and data quality of the survey area.It is not generally believed that the more types of data can obtain the more comprehensive underground electrical structure information.Through the comparison of inversion results of various data combinations and the inversion test of forwarding model data,it is found that the combination of anti-diagonal apparent resistivity and impedance phase data has more advantages in selecting data and setting threshold error.When the survey points are evenly distributed and the electromagnetic interference noise is large,it can be considered to use the combination of anti-diagonal apparent resistivity and impedance phase data for three-dimensional inversion,which may have more advantages in the percentage of data involved in inversion,lowfrequency data utilization,relative root mean square error,deep resolution,etc.(2)The three-dimensional crustal electrical structure model in the study area is obtained by using the combination of apparent resistivity and impedance phase data.From the 3D electrical structure,the Hefei-Suqian segment of the TLFZ is a vertical,deep,and large fault zone that cuts through the crust and reaches the Moho surface.The extension depth of other major faults in the adjacent area is relatively shallow,and are not exceed the depth of the middle and upper crust.The high-resistivity and the lowresistivity anomalies are distributed in shallow areas on both sides of the study area,maybe affected by small faults and structural development,while the deep electrical characteristics are low in the west and high in the east with the TLFZ as the boundary.The electrical distribution characteristics are related to different tectonic units: the Zhangbaling uplift and Sulu orogenic belt show high resistivity anomalies with deep depth,there are high resistivity bodies with a depth of about 20 km under the Bengbu uplift area,and the electrical structure of Guzhen basin and Hefei basin is stratified,showing low resistivity anomalies in the middle and lower crust.(3)From the deep electrical structure,it is shown that the TLFZ has the characteristics of segmentation.The separation of the two high resistivity bodies in the north and south near Jiashan caused obvious subsection changes in the deep electrical structure of the TLFZ.The relatively low resistivity area between the high resistivity bodies hindered the transmission of stress on the fault,resulting in strong and weak changes in the structural deformation of the fault zones on the north and south sides of Jiashan.(4)It is explained that the Sulu orogenic belt and Zhangbaling uplift area are the seismic blank areas from the deep electrical structure.The moisture content of high conductors in the crust is calculated and analyzing th Xe action of high conductors extends to the surface at the time of the earthquake.Combined with the electrical structure results of the Dabie orogenic belt,and the high resistance of Zhangbaling uplift extends to the southwest Dabie orogenic belt and passes through the TLFZ,easy to accumulate stress,but the frequent occurrence of small earthquakes in this area may be also conducive to the release of stress.The Sulu orogenic belt high resistivity body distributed along the east side of the TLFZ deflects westward in the Sihong-Jiashan section,making the fault zone pass through the high resistivity body,which may become an obstacle to fault activity and cause stress accumulation,and become an important deep background conducive to the occurrence of future earthquakes.Therefore,should be paid attention to the Sihong-Jiashan section especially. |
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