Electrical Resistivity Structure And Tectonic Implications Of The Eastern Section Of Kunlun Fault And Jiuzhaigou Earthquake Region

The Cenozoic India-Eurasia collision has driven the uplift of the Tibetan Plateau and bring a profound impact on the structure of the East Asian region.During the uplift of the Tibetan Plateau,the eastern and northeastern margin of the Tibetan Plateau has intense deformation and resulted in the development of a complex set of crustal faults.The Kunlun fault is a critical strike-slip fault in the eastern margin of the Tibetan Plateau.The Kunlun fault is suffering a decrease in slip rate and branches out into serval secondary fault.The eastern margin is also prone to strong earthquakes.The 1973 Huanglong earthquake and 1976 Songpang earthquake occurred in this region.In 2017,the Jiuzhaigou M7.0 earthquake near the eastern section of Kunlun fault had caused severe damage.This study selected in the eastern section of the Kunlun fault and the Jiuzhaigou earthquake zone as the research area.In this dissertation,we have discussed the seismogenic fault of the Jiuzhaigou earthquake,the seismogenic context of such strong earthquakes near the eastern section of Kunlun fault,the extension characteristics of the eastern section of Kunlun fault,the relationship between the electrical structure and the decreasing in slip rate,and the contact relationship between several blocks in this region.With the development of the 3D inversion technology,using 3d inversion to invert using densely distributed Magnetotelluric(MT)data can effectively recover real 3D electrical structures under complex structural environment,thus revealing the extension characteristics of the underground structure and deep contact relationship.We carried out a MT survey in the research area.273 MT sites were obtained in this region,formed a density array that covers the Jiuzhaigou earthquake region,and four profiles cross four crucial sections of the eastern section of Kunlun fault.We use the phase tensor decomposition method and induction vector to analyze the dimensional and electrical structure of this region.We use the 3D inversion program Mod EM to invert the MT data with a different type of data,coordinates,and parameters.The reliability of the selected result model is verified by the synthetic data inversion test and model sensitivity forward test.The geological,geophysical,and deformation data are closely combined in the interpretation and analysis of geological structures.The main conclusions of this dissertation are listed as follows:(1)The 2017 Jiuzhaigou earthquake occurred in the boundary between the high conductivity layer and the high resistivity body.The hypocenter located near the endpoint of the upward movement high conductivity layer of the Songpan-Ganzi block.The Huya fault exhibit an electrical boundary extend to deep.The Huya fault and Tazang fault form an imbricate fan structure and extend into the high conductivity layer in the mid-lower crust.The buried north Huya fault is the seismogenic fault of the Jiuzhaigou earthquake.(2)Both the 1973 Huanglong earthquake and 1976 Songpan earthquake series occurred near the endpoint of the upward movement high conductivity layer of the Songpan-Ganzi block.These earthquakes have similar seismogenic contexts with the Jiuzhaigou earthquake,which suggested that the upper crustal deformation is controlled by the high conductivity layer in the mid-lower crust.The dynamic of strong earthquakes are from the weak mid-lower crust.Due to the uneven resistivity structure along the Kunlun fault-north Huya fault-Huya fault,the stress accumulation capacity of different sections is different,which finally shows that strong earthquakes occur at various locations along the belt.(3)The deep extension in different sections of the Kunlun-Qinling fault system is different.The Maqu section of Kunlun fault exhibits an electrical boundary dipping steeply and extend to deep.The Tazang fault,Bailongjiang fault,and Guanggaishan–Dieshan fault form an imbricate fan structure that extends from southwest to northeast.These faults merge and extend into the high conductivity layer in the mid-lower crust.The alternating high and lower resistivity crustal structure near the eastern terminus of the Kunlun fault is the reason for decreasing the slip rate along the Kunlun fault.The structure of high and low resistance absorbs and disperses the slip rate of the East Kunlun fault so that the slip rate is distributed on multiple faults.(4)The Songpan-Ganzi block has a high conductivity layer with low viscosity,which suggests that the material in the Tibetan Plateau is moving toward the east and northeast.This layer is shallow in the southeast and northeast,indicated that the movement toward the Longmenshan fault zone,West Qinling orogenic zone,and Bikou block has been blocked and resulted in the high conductivity layer accumulate here and surge upwards.This is the dynamic of frequent strong earthquakes and surface uplift in this region.This study found the eastern and northeastern boundary of the high conductivity layer.However,the southern boundary of the high conductivity layer and the contact relationship between the high conductivity layer and the Longmenshan fault zone needs further study.