青藏高原东北缘中上地壳介质各向异性及其构造意义

青藏高原东北缘(94°E—105°E,32°N-40°N)是高原北东向扩张的前沿地带,亦是研究高原生长过程的重要区域.本文利用青海省数字地震台网(2008-2014年)共7年的地震目录和波形数据,首先使用双差定位获取精定位震源位置,在此基础上,挑选位于S波窗口内(射线入射角≤45°)的地震事件,依据S波分裂分析方法(SAM),获取研究区域内共26个台站的S波分裂参数.研究结果表明:地处多个块体交汇部位的西宁及其周缘,地壳各向异性呈现两个优势偏振方向,表明该区中上地壳应力环境由区域主压应力场和活动断层共同约束;玉树地震序列的地壳各向异性优势偏振方向与区域主压应力场一致.

Anisotropy of middle-upper crust derived from shear-wave splitting in the northeastern Tibetan plateau and tectonic implications

The northeastern Tibetan plateau(94°E—105°E,32°N—40°N) is the front edge of expansion of this vast high land, which is crucial for understanding the evolution of the plateau. We obtain the shear-wave splitting results from waveforms of local earthquakes(2008—2014)recorded by the digital seismic network of Qinghai Province. First, we apply the HypoDD to relocate the seismic events, and choose those events within the S window (defined by the incidence angle ≤45°) for further analysis. Then, we use the SAM technique to invert the shear-wave splitting parameters. Totally, we obtain splitting parameters of 26 stations. The results show that two major fast shear-wave polarizations exist in Xining and adjacent areas, where multiple blocks converge. This means that seismic anisotropy in middle-upper crust of the area is controlled by both the tectonic stress and active faults. In addition, predominant polarization direction of seismic anisotropy in the crust from Yushu aftershocks is consistent with the principal compressive stress field in this region.