Study On Crustal Velocity Structure In Middle Tianshan And Its Adjacent Areas Based On Ambient Noise Surface Wave Direct Inversion Method

Tianshan orogenic belt is a natural testing field to study the intracontinental orogenic effect.Influenced by the remote effect of Indian-Eurasian plate collision,the intraplate tectonic action occurred in the northern and southern Tianshan seismic belt,resulting in strong seismicity,which makes further understanding of the deep structure of Tianshan orogenic belt and the coupling relationship between basin and mountain is particularly important.The ambient noise tomography is not limited by the distribution of natural earthquakes,and the 3D velocity structure of underground media can be retrieved by using continuous and stable noise signals.In this study,the middle Tianshan Mountains and its adjacent areas in China are used as the research area.Based on the continuous noise data recorded by 85 stations in Xinjiang network,Kazakhstan network and mobile station in this area(40° – 49°N,79° – 93°E)from 2017 to 2019,and combined with the extraction method of surface wave dispersion curve based on image analysis technology,the Rayleigh wave phase velocity dispersion curve of 6 –52 s is obtained.The direct inversion method of surface wave is used to study the 3D S wave velocity structure of the crust in this area.Compared with the existing ambient noise tomography results,this study extracts the dispersion curves of surface waves from more stations,and uses the cluster analysis method to control the quality more strictly.The ray path in the inversion process is closer to the real situation,so the velocity model is more refined.It is of great significance to deeply understand the structural characteristics of the middle Tianshan Mountains and its adjacent areas and the coupling relationship between basin and mountain,which is helpful to further understand the relationship between strong earthquake tectonic activity and deep structure.The main results obtained in this paper are as follows :(1)The velocity distribution of S wave in shallow crust is related to the thickness of sedimentary layer in tectonic unit.The northern margin of Tarim Basin and Junggar Basin show low velocity,and Tianshan orogenic belt shows high velocity.In the middle and lower crust,there are low velocity bodies wrapped by high velocity anomalies beneath the Tianshan orogenic belt,and their values are similar to those of the middle and upper crust of the basin,which may be the result of migration of shallow materials caused by tectonic activities.Near the Moho surface,the Tianshan orogenic belt shows relatively low velocity,which represents that it is a relatively ' soft ' block with low mechanical strength,and is more prone to deformation and uplift under the extrusion of ' rigid ' blocks on both sides.(2)The crustal thickness of Junggar Basin and Tianshan orogenic belt is between45-52 km and 50-62 km,respectively.Along the north-south direction,the Moho surface of Tianshan Mountains is obviously concave,showing a relatively small dip angle and a relatively and gentle shape.The wide crust-mantle transition zone is a region with strong tectonic activity.There is a material exchange in the form of highdensity mineral component subsidence and low-density mineral component rise near the Moho surface and the top of the upper mantle.The resulting upper mantle deformation is subjected to additional stress at the bottom of the Tianshan crust,which makes the regional structure unstable.(3)There are differences in the basin-mountain coupling relationship in different regions of Tianshan orogenic belt.From west to east,the basin-mountain junction on both sides of the Tianshan Mountains gradually transforms from the bidirectional subduction of the Tarim Basin and the Junggar Basin to the unidirectional subduction of the Junggar Basin.Between 82°-86.5°E,Tarim Basin and Junggar Basin subducted bidirectionally below Tianshan Mountains.Between 86.5°-88°E,Junggar Basin subducted southward to Tianshan Mountains.From west to east,the coupling relationship between different basins and mountains revealed the differences in tectonic movement in different regions in the middle Tianshan Mountains since Cenozoic.(4)Using the obtained velocity model,the relationship between the precise relocation results and velocity structure of Hutubi Ms6.2(2016),Nileke Ms6.0(2011),Xinyuan-Hejing Ms6.6(2012),Jinghe Ms6.6(2017)four moderate and strong earthquakes is explored.Hutubi Ms6.2 and Nileke Ms6.0 occur in the boundary area of high and low velocity distribution.Xinyuan-Hejing Ms6.6 and Jinghe Ms6.6 are distributed in the relatively high velocity side of the underground medium.Affected by plate collision,brittle rocks in the high speed area are ruptured,which leads to earthquakes and confirms the research results of seismic seismogenic structures.