Body Wave Anisotropy Tomography Of Northeast China And Northeast Of Qinghai Tibet Plateau

Seismic body wave travel time tomography is a kind of inversion technology which uses the arrival time of seismic body wave as observation data to inverse the seismic wave velocity distribution of underground media.Based on the assumption of symmetric weak anisotropy,by introducing anisotropic parameters into the travel time inversion equation,the joint inversion of velocity disturbance and anisotropic properties can be carried out simultaneously.The anisotropic analysis of 3D media is helpful to further explain the dynamic problems such as regional structural evolution,deep material deformation and dynamic model.This paper reviews the theoretical development,technical implementation and numerical simulation methods of volume wave anisotropic tomography in recent years.Based on the previous work,we further completed the numerical realization process of 3-D body wave anisotropy inversion calculation and resolution detection based on damped least square method,and the test of artificially events is carried out.Based on this method,teleseismic 3-D volume wave anisotropy tomography was carried out in Northeast China and northeast margin of Qinghai Tibet Plateau,respectively.The 3D velocity structure and anisotropy distribution of the upper mantle in the two regions were obtained.1.Based on the assumption of axisymmetric weakly anisotropic medium,the inversion function of 3D velocity perturbation and anisotropic parameters is realized by introducing two anisotropic parameters into isotropic traveltime tomography inversion program.According to the different resolution character of 3D velocity structure and anisotropic parameters,the inversion program allows the mesh model with velocity perturbation and anisotropic parameters to be set independently.The influence of station distribution and damping coefficient on the inversion of 3D velocity structure and anisotropic parameters is analyzed and discussed.The influence of station distribution may be more concentrated in the shallow region.The selection of damping coefficient needs to consider the root mean square of travel time residuals and the smoothness of the model.2.Using the teleseismic body wave travel time data of NECESSArray array,the velocity perturbation structure and anisotropic distribution images of upper mantle in Northeast China are obtained.The results show that the velocity perturbation and anisotropy distribution in Northeast China have obvious lateral heterogeneity.Below420 km,the low velocity anomalies in the lower part of the Aershan area converge with those in the Songliao Basin,and the FVD is distributed in the NW direction,which indicates that the two may have the same deep heat source supply and have the same relationship with the deep dynamic process of the Pacific Ocean.Within 60 ?240km,there are different anisotropic distribution on the East and west sides of the Aershan area,which is related to the combination of ancient blocks and the extension deformation of the basin in the later stage.In the Songliao Basin,the there is high velocity perturbation anomaly in most area,and there is low velocity perturbation anomaly within the depth of 120 km in the central area,which may be related to the upwelling of thermal materials from the asthenosphere;at 60 km below the basin,FVD in the South and central part of the basin is distributed in the direction of E-W and NE-SW in the East,it is inferred that it may be affected by the relative movement along the amalgamation zone between the North China Craton and Songnen block and depth fracture.Below 420 km,FVD is mainly distributed in NW direction,which is similar to SKS resault,which may indicate that the source depth of SKS anisotropy is deep,and the formation mechanism is related to the westward subduction of the Pacific plate.The distribution of FVD within 180 km below the Changbai volcanic area is consistent with the strike of the continant amalgamation zone,indicating the influence of the amalgamation process on the local tectonic deformation;below300km,it shows consistent NW trending FVD,which is inferred to be related to the westward subduction of the Pacific plate;there is a low velocity perturbation anomaly area in the northwest of the volcanic area within 520-620 km,but the amplitude of azimuthal anisotropy is large,and the overall trend is consistent.It is not related to the mantle plume from the deep,but may be related to the deep dehydration of the stagnant Pacific slab.3.Based on the teleseismic data of the second period of seismic scientific array,the anisotropic tomography of the northeast margin of the Qinghai Tibet Plateau iscarried out.The results show that the characteristics of velocity perturbation and anisotropy change obviously in horizontal plane,being difference distribution between the Qinghai Tibet Plateau and adjacent blocks.There is a low velocity anomaly about 300 km deep beneath the northeastern margin of the Qinghai Tibet Plateau.It is inferred that the Tibetan Plateau is softer and hotter than the surrounding ancient blocks within this depth range.From 360 km to 420 km,the low velocity anomaly under the Tibetan Plateau gradually transforms into a large area of high velocity anomaly,and extends to the deep mantle.The anisotropic FVD is basically consistent with the relative movement direction between the Qinghai Tibet block and the surrounding blocks in the range of 60?120km.Within range of 120?420km,FVD changes to NW direction in most areas,which is basically consistent with SKS results.Within the Ordos block,there is mainly high velocity perturbation anomaly,the anisotropy amplitude is relatively low,and the FVD is in E-W direction,indicating that the block is a relatively stable block.There is a relatively low velocity value in the western margin of Ordos block,and the anisotropy changes significantly with the depth.It is inferred that there may be some deformation in the western boundary of Ordos block.Under the Alxa block,the results show that the whole area has the characteristics of high velocity perturbation anomaly,and the northeast part presents the characteristics of low velocity perturbation anomaly,which extends to the depth of600 km,indicating that the Alxa block has relatively stable characteristics.Under the Yangtze block,0 ? 360 km,the high velocity perturbation anomalyies are dominant.The anisotropic FVD distribution is different between the interior of the block and the edge of the block,indicating that the interaction between blocks has a significant impact on the anisotropy distribution.Below 420 km,there is a low velocity perturbation anomaly.It is preliminarily inferred that there may be softer mantle material characteristics or local high-temperature thermal material activity in this depth range.