Research On Crustal S-wave Velocity In The Central And Eastern Qiangtang Based On Background Noise

The surface wave information between stations can be obtained by extracting the Empirical Green's Function based on the ambient noise cross-correlation function method.Compared with the surface wave theory of traditional natural earthquake,this method is not constrained by the spatial and temporal distribution of earthquakes and able to extract the short-period surface wave dispersion below 20 s by calculating the long-term ambient noise cross-correlation function,which is conducive to study the velocity structure of the upper crust.Thus,the ambient noise cross-correlation function method has been an important means widely used in inversion of the crust/upper mantle velocity structure in recent years.Located in the northern part of the Qinghai-Tibet plateau,the Qiangtang block is considered to be a key area to study the collision boundary between the Indian plate and the Eurasian plate.since the Cenozoic era,Qiangtang block has been through the intense tectonic movement,cause its crustal structure is very complex.Therefore,the study of the crustal structure of Qiangtang is helpful to make clear the geological changes along with the huge thick crust of the Qinghai-Tibet plateau forming.Based on the 2017 annual continuous waveform data recorded by 30 broadband temporary seismographs deployed along the Qinghai-Tibet Highway in the central-east part of Qiangtang along with 4 static stations(Naqu station,Zaduo station,Tuotuohe station and Qumalai station),this thesis extracts the Rayleigh surface wave phase velocity dispersion of 2~40s by cross-correlating the ambient noise and then obtaining interstation Empirical Green's Function.Finally,the 3D shear wave velocity structure of east-central Qiangtang crust is obtained by using the surface wave direct inversion method based on the wavelet domain sparse constraint.Afterwards,the influence of different initial models on the inversion results is discussed and the conclusions are fellow:(1)Low-velocity layer is widely distributed in the central-east of Qiangtang from surface to the depth of 10 km,and its thickness in northern part is greater than that in southern part,which,according to the analysis,has something to do with the thick sedimentary layer in this area.(2)In the depth of 10~20km,there is an obvious high-speed object located below the central uplift belt,which is considered the Marine rock mass of the lower crust that upwelling during the suture zone forming.Consequently,the location of LongmucuoShuanghu suture is deduced correspondingly.(3)In the depth of 20~35km,there is characterized by a consecutive low velocity below Qiangtang block,not connected with the Lhasa block.It is believed,the lowvelocity layer proves the partial in the crust of the Qiangtang is subsistent.