青藏高原东北缘地壳S波速度结构与各向异性研究

The northeastern Tibetan plateau as one of main regions where the plateau grows toward outside is an ideal place to study the crustal deformation and the geodynamics. It has been still being under controversy that the mid-to-lower crustal flow model was used to explain the growing mechanisms of the northeastern Tibetan plateau. S-wave velocity structure of the crust inverted by the joint analysis of Rayleigh wave dispersion and receiver functions and the crustal seismic anisotropy estimated with Ps phase of receiver functions are the reliably seismological proofs for testing the mid-to-lower crustal flow model. We plan to collect the seismic data that recorded by the array of ChinArray-phase 2, previous temporary seismic array and permanent stations from provincial networks in this study area, and then measure the dispersion curves of teleseismic surface wave and compute receiver functions of each station. The three dimension S-wave velocity model will be obtain by joint inversion of surface wave dispersion and receiver function to constrain the crustal structure of the northeastern Tibetan plateau and the crustal seismic anisotropy will be estimated with receiver functions, which will used to provide the basis for constraining the mid-to-lower crustal flow model and discuss the geodynamical problem of the crustal thickening and deformation mechanism of the northeastern Tibetan plateau.

青藏高原东北缘是高原向外生长的主要地区之一，是开展地壳变形和地球动力学研究的理想场所。地壳流模型是否适用于解释青藏高原东北缘的生长机制一直存在着争议，面波频散与接收函数联合反演的地壳S波速度结构和接收函数Ps转换波的各向异性特征是检验地壳流模型的有效地震学依据。本研究拟收集中国地震科学探测台阵项目—“南北地震带北段”在青藏高原东北缘布设的密集台阵，已有的流动台站和省局区域台网的固定台站观测数据，测量面波频散曲线和提取每个台站的接收函数。利用面波频散与接收函数联合反演方法获取约束青藏高原东北缘地壳结构的三维S波速度模型，采用接收函数各向异性方法计算地壳各向异性特征，为约束地壳流模型提供依据，以探讨青藏高原东北缘地壳增厚与变形机制的地球动力学问题。

本研究通过收集青藏高原东北缘地区1048个台站的数据资料，采用接收函数、面波频散和联合反演方法，获取研究区的地壳厚度和波速比、三维S波速度和地壳各向异性特征，来加强我们对青藏高原东北缘地壳增厚和变形等地球动力学问题的认识。我们研究发现：青藏高原东北缘莫霍面深度变化强烈，与地表地形有一定相关性，在一定程度上也支持地壳均衡理论，呈“锯齿形态”（zigzag geometry）下陷的莫霍面前缘表明青藏高原东北缘向外围的扩展和生长并不是均匀的；波速比分布具有很强的横向不均匀性，祁连造山带、松潘-甘孜地体和秦岭-大别造山带的平均波速比最低，而柴达木盆地、西祁连造山带、西鄂尔多斯块体、西河套盆地和银川地堑西部具有最大的波速比值，其平均值大于1.8，波速比与地壳厚度的变化成反相关的特征；地壳三维S波速度结构分布具有明显的分区特征，祁连造山带及其以南的高原地区S波速度相对较低，而阿拉善块体、鄂尔多斯块体和四川盆地则分布着相对高速异常，但未观测到青藏高原东北缘中下地壳的S波低速层分布；大部分台站簇的Ps震相快轴极化方向均呈北西-南东方向，大体平行于地表构造走向和XKS快轴方向，而河套盆地和银川地堑地区的极化方向为北东-南西向，几乎与XKS的结果相垂直。经过分析和讨论，我们推断青藏高原东北缘地区可能是地壳、上地幔垂直连贯的耦合变形，而河套盆地和银川地堑地区为壳、幔解耦的变形方式。