| The contact area between the Alxa block and the Ordos block in the western part of the North China block is an active fault zone with strong deformation.Its also located in the northern end of the North-South Seismic Zone of the Chinese mainland.The Helanshan fault system in this contact area is basin-range structural controlled by strike-slip and normal faults.In 1739,the Pingluo M8 earthquake that occurred in the Yinchuan basin was caused by normal fault activity.Geophysical detection could help image extension characteristics of multiple faults and contact relationship of the Helanshan fault system and the blocks on both sides.The results could help understand scientific issues such as the seismogenic structure and environment of the 1739 Pingluo M8 earthquake and the interaction relationships between blocks(e.g.,Alxa block,the Ordos block,and the Tibetan Plateau).The Magnetotelluric(MT)method is sensitive to low resistivity anomalies and not shielded by a high resistance layer.The 3D electromagnetic inversion could recover the actual deep electrical structure under a complex structural environment.In 2019,34 broadband MT data with good quality were acquired along the YCL3 profile crossing the Helanshan fault system and its adjacent areas along an NW-SE direction.Combined with the HYFN profile,an existing MT profile along near EW-direction,at the southern end of the Helanshan fault system,the deep structural characteristics of the contact area between the Alxa block and the Ordos block were revealed.The phase tensor decomposition technology was used to carry out qualitative analysis of its dimensionality and electrical structure of two profiles.Stable and reliable 3D electrical structure images were obtained by using the 3D inversion code Mod EM system for the two profiles separately.The deep electrical structure characteristics and fault extension patterns of the area were analyzed based on other geological and geophysical research results.The seismogenic environment and structure of the 1739 Pingluo M8 earthquake were discussed.The main new results of the paper are as follows:(1)The MT data at about 500 stations in Yinchuan,Yuncheng,Hebi,and Zhangjiakou are processed and analyzed in this paper.The data has been statistics and sorting out to 45 typical MT data under 7 kinds of strong electromagnetic interference environments(e.g.,high-speed railway,electrified railway,wind power station,photovoltaic power station,large-scale high voltage power grid,colliery,and city).A variety of modes of fine spectrum processing were carried out.The results show that the electromagnetic interference of high-speed railway,electrified railway,and photovoltaic power station caused mainly near-field interference which the apparent resistivity in the mid-frequency band is 45° ascending,and the value of the phase tends to 0.The large power transmission networks,colliery,and the city cause a phenomenon of that discrete "outlier" of a single-or multi-frequency appears on apparent resistivity curve.The wind power station brings very little influence on the MT data.When MT detection is carried out near the strong electromagnetic interference source in a short distance,the data acquisition time should increase by at least two nights.Using high-quality remote reference data for remote reference and non-Robust processing,combined with fine spectrum selection,we can effectively obtain qualified and even excellent MT data.(2)The YCL3 profile provides a deep electrical structure of Alxa block,Helanshan uplift,Yinchuan basin,and Ordos block.The deep electrical structure reveals that the distribution characteristics of each structural unit are inconsistent.The Alxa block and the northwestern margin of the Ordos block show a relatively stable layered structure style,while the Helanshan uplift and the Yinchuan basin within the Helanshan fault system show typical basin-range structural features of high-resistance bodies in the mountain and low-resistance bodies in the basin.The Helanshan uplift presents a stable high-resistance characteristic integrally,with low-resistance bodies on the east side of its northern section.The Yinchuan basin is characterized by a faulted basin style which deep in the west and shallow in the east.(3)The Eastern Helanshan fault and the Yellow River fault are obvious large-scale electric boundary zones in the Helanshan fault system.The Eastern Helanshan fault is characterized by a deep fault extending to the lower crust with a gentle dip angle in the east.The Yellow River is characterized by an obvious electric boundary zone penetrating into the middle and lower crust with a steep west.The two faults form a "negative flower" structure in the form of an east-west boundary in the deep part.The Yinchuan fault may continue to extend northward in a blind state and is the core fault in the "negative flower " structure.The Pingluo earthquake occurred in the transition area from the low resistivity body in the Yinchuan basin to the high resistivity body in the Helanshan uplift,which was caused by the joint action of the "negative flower" fault system composed of multiple faults.(4)The structural features of the fault depressions on the east and west sides of the Helanshan uplift are obvious,indicating that the Alxa block and the northwestern margin of the Ordos block are in a state of tension,providing a development environment for the development of strike-slip and normal faults in the Helanshan fault system.In contrast,the Haiyuan arc-shaped structural belt is located on the southwestern margin of Ordos.When the northeastern edge of the Tibetan Plateau and the southwestern margin of Ordos are pushing,the deep part of the area is broken and deformed violently,which leads to the tectonic environment of strike-slip and thrust nappe faults in the area. |
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