A Study On The Deep Electrical Structure Along The Profile From Menglian To Luoping In The Southern Segment Of The South-north Seismic Belt In China

Yunnan province lies in the southeastern of the Tibetan plateau. It abuts the vastEurasian continent to the north and faces the Indian plate in the south. According totopography, Yunnan can be divided into eastern and western parts. The western partof Yunnan is the extension of the Tibetan plateau and the eastern part belongs to thesouthwestern Yungui Plateau. n most of Yunnan, the terrain is very complex featuredby alternating mountains and valleys, and only appears gentle relief in the south nearthe borders.Chinese mainland continent is one of typical regions with active tectonicmovement in the world, while Yunnan is one of the areas in China mainland whichare prone to frequent major earthquakes. It is located at the eastern margin of thecollision zone of the Indian and the Eurasian plates. Due to northeastward motion ofIndian the plate, the Tibetan plateau moves to north and east. Such a motion isobstructed by the Yangze block along the eastern margin of the Tibetan plateau,causing the crustal material to escape to south, which continues at present. Majority ofYunnan’s crust is made up of these eastward moving materials. In such a specialdynamic context, many major earthquakes occur in Yunnan, thus this area has becomeone of the focused targets in research of seismology and active tectonics.Yunnan is the southern section of the north-south seismic belt which stretchesthrough central China mainland. Several great earthquakes of recent years occurredin this the north-south seismic belt. Many studies have emphasized the significanceof this north-south seismic belt in research of lithospheric structure and seismicactivity in China and adjacent regions. As part of the project named HimalayaProgram: Probing electric structure beneath the southern section of the north-southseismic zone, a magnetotelluric (MT) profile from Luoping to Menglian wasdesigned, which crosses southern Yunnan with a length of560km. Along this profile,data collection has been carried out for MT, LMT and ELMT measurements. This thesis uses general MT and long-period MT data to study the deep structurealong the Menglian-Luoping profile. The author makes a full analysis of the datausing the advanced MT data processing and analysis techniques. The electricalstructure of crust and upper mantle along the profile is obtained by the2D NLCGinversion method. And in combination with the geological data his thesis constructsan electric model and attempts to give geological interpretations to it. The primarycontent and conclusions of the thesis are summarized below.1. Data processingThe software SSMT2000and MTEDITER are used for the data processing of theoriginal time series of114sites on the Menglian-Luoping MT profile. Then theapparent resistivity and impedance phase curve of these sites are obtained. Theimpedance tensor decomposition technique is employed to process the Swift, Bahr,GB and phase tensors. Qualitative analysis has been performed to the apparentresistivity and impedance phase curve and the skewness for the primary lateralchanges of electrical conductivity structure. By analyzing the structure characteristicsand main axes of electricity in the target area, the strike of the regional electricalstructure is determined, which is perpendicular to the MT profile in combination withregional geological structure. The2D NLCG method is chosen for inversion andinterpretation after rotating the data directly to the survey orientation.2. Two-dimensional inversionIn this thesis, the inversion parameter on2D NLCG inversion is chosen by thestudy of varied polarization modes, the regularization factors and the t value of floorerrors. After times of inversion the best2D inversion model is determined by analysisof these parameters.The2D initial model in which the resistivity is100is produced by usingAutomatic Generating Technique for the grid with topography of central measurementpoints after several kinds of tests, and TM mode is determined. The regularizationfactor is set to be50, and the threshold value of errors in TM mode is2%. The ultimate model of the target zone is obtained after59iterations. The RMS value is3.76.3. Data interpretationBased on the final inversion model of the target profile, the location and theextension of the main faults and boundary have been determined. At the same time,this work analyzes the seismogenic environment of the major earthquakes throughthe distribution features of the low-resistivity layer in crust. It suggests that theelectrical structure gives the evidence for low-resistivity channel which is consistentwith previous research results in recent years.4. Seismogenic environmentThe MT profile in southern Yunnan crosses through three seismic areas, whichare the Gengma-Lancang meizoseismal area, Simao-Puer meizoseismal area andTonghai meizoseismal area. They have the same feature in deep electrical structurewhere the low resistance area and high resistance gradient zone are probablyassociated with major earthquakes. It would be helpful for characterizing andunderstanding the seismogenic environment in this region.