青藏高原东北缘壳幔各向异性研究:基于SKS和Pms震相分析

Seismic anisotropy of the northeastern margin of the Tibetan Plateau derived from analysis of SKS and Pms seismic phases

利用甘肃和青海两省固定宽频带地震台记录的远震波形资料,挑选高质量SKS震相,联合使用最小切向能量方法和旋转互相关方法获得230对高信噪比分裂参数;同时对接收函数中Pms震相随方位角的变化进行拟合,得到了24个台站的地壳各向异性分裂参数.整个区域SKS分裂快波方向均值为123°,Pms分裂快波方向均值为132°,且大部分区域SKS、Pms快波方向与地表构造走向相一致,说明青藏高原东北缘以岩石圈垂直连贯变形为主,地壳上地幔相互耦合.SKS、Pms分裂时差均值分别为1.0s和0.6s,显示地壳各向异性对于SKS分裂时差有较大贡献.昆仑断裂附近Pms、SKS分裂快波方向与昆仑断裂走向基本一致,说明昆仑断裂可能是岩石圈尺度深大断裂;而阿尔金断裂东缘二者快波方向显著差异意味着阿尔金断裂在东缘可能仅为地壳尺度的断裂.

Rock deformation may produce anisotropy, thus, seismic anisotropy contains deformation of the crust and upper mantle. Based on teleseismic SKS and Pms phases recorded by permanent broadband stations in Gansu and Qinghai provinces in the northeastern margin of the Tibetan Plateau, shear-wave splitting analysis is made to investigate the seismic anisotropy of crust and upper mantle in this region, which permits to infer the related deformation mechanism. This study obtains 230 pairs of splitting parameters at 56 stations using the minimum transverse energy method and rotation correlation method jointly from teleseismic SKS phase. The splitting parameters of crustal anisotropy were also revealed by the data from 24 stations using the sinusoidal moveout of the P-to-S converted phase from the Moho (Pms) in receiver functions. The matches and mismatches between observations at the surface and those from deep structures provide critical clues regarding the nature of the geodynamic process. Our shear-wave splitting results show that the average fast polarization directions of SKS and Pms are 123° and 132°, respectively. The NW-SE directed fast directions of crust and upper mantle are generally consistent with the main active structures on the surface, suggesting a vertically coherent deformation within the lithosphere. The average splitting times of SKS and Pms are 1.0 s and 0.6 s, respectively, indicating that crustal anisotropy appears to play an important role in explaining the amount of SKS splitting times. The fast directions of SKS and Pms near the Kunlun fault are approximately parallel to its strike, which suggests that the Kunlun fault cut through the lithospheric mantle. However, the angle difference between the fast directions of SKS and Pms at the eastern Altyn Tagh fault implies the fault is a crustal-scale structure at its eastern end.