基于密集台阵资料的背景噪声研究青藏高原东南缘地震各向异性

Seismic anisotropy in the southeastern margin of the Tibetan Plateau revealed by ambient noise tomography based on high-density array

新生代以来,青藏高原快速隆升、地壳缩短和东向挤出.受到稳定的扬子地块阻挡,青藏高原东南缘地壳发生强烈变形.地震各向异性研究有助于认识地壳内部精细结构及内部运动学过程.通过收集密集地震台阵的观测资料,利用环境噪声提取Rayleigh波频散曲线,采用多角度频散曲线反演方法,获得地壳和上地幔顶部高分辨率的地震S波速度和各向异性图像.青藏高原东南缘地区上地壳的地震快波方向与其相邻的走滑断裂带走向、GPS水平速度场方向基本一致,围绕喜马拉雅东部构造结顺时针旋转.然而,中、下地壳的各向异性与上地壳存在明显差异,例如,在木里盐源盆地和滇中地块等各向异性方向发生大幅度转向,从上地壳的NE方向转为中、下地壳的NW方向.中、下地壳的各向异性方向与其低速层的延伸方向吻合.在下地壳底部和上地幔顶部的范围内,地震快波方向再次发生改变,与上地壳的各向异性分布一致,可能说明在较早的历史时期上地壳与下地壳是耦合在一起的,在中新世时期低速黏滞性流体挤入青藏高原东南缘中下地壳,使原有的上地壳与中下地壳发生解耦.因此,新生代以来高原物质挤出可能导致青藏高原东南缘地壳发生强烈变形.

The Tibetan Plateau uplifted rapidly with that the crust shortened and eastward extruded since the Cenozoic.Blocked by the stable Yangtze block,the crust has been strongly deformed in the southeastern margin of the Tibetan Plateau.Seismic anisotropy research is helpful to understand the fine crust structure and the inner dynamic process.We therefore collected seismic data on a high-density array,extracted the Rayleigh wave dispersion based on the ambient noise,and adopted an azimuth-dependent dispersion curve inversion to obtain high-resolution S-wave velocity and anisotropy image in the crust and uppermost mantle in this region.The seismic fast wave directions in the upper crust are basically consistent with the strikes of the adjacent faults and the GPS horizontal velocities,rotating clockwise wrap around the Eastern Himalayan Syntaxis(EHS).However,the anisotropy of the mid-lower crust is obviously different from the upper crust.For example,in both the Muli-Yanyuan basin and Central Yunnan block,the seismic fast wave directions have been changed from the NE direction in the upper crust to the NW direction in the mid-lower crust.The anisotropy directions in the middle and lower crust are consistent with the distribution trends of the low velocity zone.In the range of the bottom of lower crust and the top of the upper mantle,the direction of the seismic fast wave changed again,which is consistent with the anisotropic distribution of the upper crust.This may indicate that the upper crust and lower crust were coupled in the early historical period.The low velocity viscous fluid squeezed into the mid-lower crust in the Miocene,leading to decoupling between the original upper and lower crust in the southeastern margin of the Tibetan Plateau.Material extrution may lead to the crustal deformation in the southeastern margin of the Tibetan Plateau since the Cenozoic.