A systematic investigation of piercing-point-dependent seismic azimuthal anisotropy
A systematic investigation of piercing-point-dependent seismic azimuthal anisotropy
复制标题
DOI：
10.1093/gji/ggab285
复制
发表时间：
2021-08-16
影响因子：
2.8
通讯作者：
Gao, Stephen S.
中科院分区：
文献类型：
--
作者：
Jia, Yan;Liu, Kelly H.;Gao, Stephen S.
绝大多数远震XKS（包括SKS、SKKS和PKS）剪切波分裂研究都是基于记录台站附近各向异性结构在空间上不变这一假设来解释观测到的分裂参数（快波方向和分裂时间）的。对于这种各向异性结构，如果地震射线路径所穿过的介质可以用具有水平对称轴的单层各向异性（即简单各向异性）来表示，那么观测到的分裂参数与地震射线路径的入射方位角无关；或者对于更复杂的各向异性结构（例如具有水平对称轴的多层结构，或具有倾斜轴的单层结构），分裂参数随入射方位角呈周期性变化。当一个记录台站位于两个或多个具有不同各向异性特征的区域边界附近时，观测到的分裂参数取决于射线穿透点的位置。在中国中部四川盆地东北边缘附近的三个台站，总共使用360对XKS分裂参数可以清楚地观察到这种穿透点依赖性。对于给定的台站，快波方向的差异可达90度，并且快波方向的方位角变化缺乏双层或倾斜轴各向异性所预期的90度或180度周期性。当把这三个台站的观测分裂参数投影到大约250公里深度时，它们在空间上最为一致，并且可以用与绝对板块运动相关的剪切应变以及被四川盆地锥形岩石圈根部偏转的地幔流来解释。
The vast majority of teleseismic XKS (including SKS, SKKS and PKS) shear wave splitting studies interpret the observed splitting parameters (fast orientation and splitting time) based on the assumption of a spatially invariant anisotropy structure in the vicinity of a recording station. For such anisotropy structures the observed splitting parameters are either independent of the arriving azimuth of the seismic ray paths if the medium traversed by the ray paths can be represented by a single layer of anisotropy with a horizontal axis of symmetry (i.e. simple anisotropy), or demonstrate a periodic variation with respect to the arriving azimuth for a more complicated structure of anisotropy (e.g. multiple layers with a horizontal axis of symmetry, or a single layer with a dipping axis). When a recording station is located near the boundary of two or more regions with different anisotropy characteristics, the observed splitting parameters are dependent on the location of the ray piercing points. Such a piercing-point dependence is clearly observed using a total of 360 pairs of XKS splitting parameters at three stations situated near the northeastern edge of the Sichuan Basin in central China. For a given station, the fast orientations differ as much as 90 degrees, and the azimuthal variation of the fast orientations lacks a 90 degrees or 180 degrees periodicity which is expected for double-layered or dipping axis anisotropy. The observed splitting parameters from the three stations are spatially most consistent when they are projected at a depth of similar to 250 km, and can be explained by shear strain associated with the absolute plate motion and mantle flow deflected by the cone-shaped lithospheric root of the Sichuan Basin.