郯庐断裂带中南段及邻区Rayleigh波相速度与方位各向异性

RAYLEIGH WAVE PHASE VELOCITY AND AZIMUTHAL ANISOTROPY OF THE MIDDLE-SOUTHERN SEGMENT OF THE TAN-LU FAULT ZONE AND ADJACENT REGIONS FROM AMBIENT NOISE TOMOGRAPHY

与方位各向异性郯庐断裂带中南段及邻区处于华北克拉通和扬子克拉通的结合带,是研究陆内碰撞、大陆聚合及生长、地球动力学和岩石圈形变等问题的天然实验场。为获取研究区内更为精细的壳幔S波速度和方位各向异性的分布,文中利用郯庐断裂带中南段及邻区范围内共261个固定宽频带地震台站2a记录的连续波形数据,利用时频分析法提取了5~50s周期的Rayleigh面波相速度频散曲线。之后,将研究区域划分为0.25°×0.25°等间隔的网格点,并采用Tarantola方法反演获得了研究目标区内的二维Rayleigh波相速度与方位各向异性的分布图像。此外,还分析研究了6个具有代表性周期的相速度及方位各向异性分布图像,这些图像揭示出郯庐断裂带中南段及邻区壳幔速度结构的横向非均匀性及方位各向异性的空间差异特征。研究结果表明,相速度的分布特征与地质构造单元有着较好的对应关系。在地壳浅部(6~10s),拥有较厚松散沉积层覆盖的盆地和基岩出露的造山带分别呈现为低速异常和高速异常。随着周期递增(15~20s),地壳介质受浅部沉积层的影响有所减弱,河淮盆地、苏北盆地等平原地区局部出现了高速异常。下地壳至上地幔(25~30s)的相速度分布受地壳厚度的影响,总体上与莫霍面埋深呈负相关,如莫霍面厚40km的大别造山带由短周期高速转为低速分布。由于研究区内各个地质构造单元所处的构造环境存在差异,Rayleigh波相速度的方位各向异性也存在较为明显的空间差异性。总体上,各向异性的强度随着周期(深度)的递增而有所增强,快波方向更具规律性。综合苏北盆地自地壳浅部至岩石圈地幔的连贯低速分布及方位各向异性的一致性,我们认为其下方可能存在着壳-幔强耦合的现象。

The middle-southern segment of the Tan-Lu fault zone and its adjacent area is located in the joint zone of the North China craton and Yangtze craton.It is a natural test ground for studying the problems of intracontinental collision,continental convergence and growth,geodynamics and lithospheric deformation.Although early research involved the central-south section of the Tan-Lu fault zone and its neighboring areas,it is difficult to carry out a detailed discussion on the S-wave velocity and azimuthal anisotropy in the middle and south section of the Tan-Lu fault zone and its adjacent areas,due to different research purposes and objects,the limitation in selecting research scope or the lack of resolution.To obtain more detailed crust-mantle velocity structure and azimuthal anisotropy distribution characteristics in the study area,this paper uses waveform data recorded by 261 fixed wideband seismic stations in the middle-southern segment of the Tan-Lu fault zone and its adjacent zone for two consecutive years.The phase velocity dispersion curve of Rayleigh surface wave with 5~50s period was extracted by time-frequency analysis.Then,the study area was divided into 0.25°×0.25°grids,and the two-dimensional Rayleigh phase velocity and azimuthal anisotropy distribution image in the area was retrieved using the Tarantola method.The phase velocity and azimuthal anisotropy distribution images of 6 representative periods were analyzed.These images reveal the lateral heterogeneity of the crust-mantle velocity structure and spatial differences in azimuthal anisotropy in the middle-southern segment of the Tan-Lu Fault and its adjacent areas.The results show that the distribution characteristics of phase velocity have a good correspondence with geological tectonic units.In the shallow part of the earth s crust,the basins covered by thick unconsolidated sedimentary layers and the bedrock exposed orogenic belts show low and high velocity anomalies,respectively.With the increase of the period(15~20s),the influence of the shallow sedimentary layer is weakened,and the high-speed anomaly appears in some plain areas such as the Hehuai Basin and Subei Basin.The distribution of phase velocity in the lower crust and upper mantle(25~30s)is affected by the thickness of the crust,which is inversely related to the burial depth of Moho surface.For example,the Dabie orogenic belt with a thickness of 40km changes from a short period high-speed to a low-speed distribution.Due to the differences in the tectonic environment of each geological structural unit in the study area,the azimuthal anisotropy of Rayleigh waves has obvious spatial differences.In general,the strength of anisotropy increases with increasing period(depth),and the direction of fast wave is more regular and followable.Based on the consistent distribution of low velocity and azimuthal anisotropy from the shallow crust to the lithospheric mantle in the Subei Basin,we believe that there may be a strong crust-mantle coupling phenomenon.The results obtained by different seismic anisotropy observation methods are different manifestations of anisotropy.However,due to the one-sided and low-resolution problems of single observation method,it is necessary to carry out joint inversion or comprehensive multiple observation methods.