Study On Tectonic Deformation And Seismogenic Mechanism Of The Minle-Yongchang Active Fault In The Hexi Corridor

The Hexi Corridor is located at the forefront of the northward extrusion thrust of the Qinghai-Xizang block.The late Quaternary active structures in this area are widely developed,and it is an important area for studying the structural deformation characteristics and seismicity of the plateau.These studies can provide important references for the study of continental dynamics,risk determination of large earthquakes,and earthquake disaster prevention.Among them,the concealed fault——Minle-Yongchang fault,located in the middle and eastern part of the Hexi Corridor,that developed on the south side of the uplift of Dahuang Shan in the basin belongs to the seismogenic structure of the 2003 Ms6.1 and Ms5.8 earthquakes in Minle-Shandan,Gansu.However,until now the understanding of the geometry and the latest activity features of this fault is very limited.The inference of the geometric distribution is only based on topographical differences,and there is barely any study on the fault activity profile.Therefore,it is of great significance to conduct in-depth and quantitative studies on the tectonic activities,deformation characteristics,and seismogenic mechanisms within the foreland basins that was overlooked in the past.In this paper,the methods used are active tectonics,tectonic geomorphology,and seismology.On the basis of preliminary data collection and detailed interpretation of aerial photos,the synthesis study are carried out to investigate the new activity characteristics of the Minle-Yongchang fault within the Hexi Corridor Basin,thanks to a series of research such as field surveys,deformation measurements,and seismic relocation,I came out with some quantitative parameters such as the late Quaternary tectonic activity patterns,thrust rates,and deep structural characteristics.During my research I took especially the Minle-Shandan earthquake as an example to analyze and discuss its seismogenic mechanism.The main conclusions are summarized as follows:1.The Minle-Yongchang fault is an almost east-west compressive thrust fault.In the west,the Late Quaternary tectonic deformation pattern is the reverse fault-active fold belt.There are obvious river terraces and other geomorphological deformation phenomena,and the overall strike of structural deformation is 311°.Among them,the uplift volume of T₂ and T₃ terraces is about 2m and 22m,respectively,while the terrace ages are 10ka and 106ka,indicating that the Minle-Yongchang fault has been continuously active since late Pleistocene,with an average uplift rate of?0.22±0.05?mm/a or so;2.According to the results of the seismic relocation carried out by various methods,the fault parameters of the seismogenic fault of the Minle-Shandan earthquake in 2003 were obtained by inversion of the source parameters in the study area.Its seismogenic fault strikes at 311°,dips at 14°,tends toward NE,and has a depth range of 18 to 20 km.It is a low-angle reverse fault.The result of the focal mechanism solution of the main shock is similar.In addition,the focal mechanism of the two main shocks shows that the P-axis direction is about 40°,which is the same as the direction of the main compressive stress in this region,indicating that the Minle-Yongchang fault is the result of the expansion of Tibetan Plateau to the NE direction;3.The post-earthquake field investigation,the distribution of aftershocks and the deformation of terraces,all those indicate that the Minle-Yongchang fault is the seismogenic fault of the 2003 Mingle-Shandan earthquake.This earthquake is a typical fold earthquake;4.Stereoscopic image pairs play an important role in the research of active structures,and can provide important guidance for determining the spatial position of structural deformations and field investigation.A combined and complete research on deep and shallow structures by using various methods such as the active structure,tectonic geomorphology and seismology can help us thoroughly understand the interactions among faults.