云南地区地壳上地幔三维S波速度与径向各向异性结构研究

云南地区地处青藏高原东南缘，一直是地球科学研究的热点地区.目前，一些热点问题，如云南地区是否存在中下地壳低速流及其空间分布，仍有一定的争议.通过建立云南地区精细的地壳上地幔速度与各向异性结构，可为深入认识上述问题提供重要信息.本文利用天然地震波形记录，采用双台法提取了12~60 s周期的Rayleigh和Love波相速度频散，并进一步反演获得了云南地区10~100 km的三维地壳上地幔SV和SH波速度及径向各向异性结构.结果表明：S波速度与径向各向异性结构在横向和垂向均存在显著变化.在20~30 km深度，存在两个低速条带，且条带内呈现出正径向各向异性（V_SH>V_SV）特征，暗示了中下地壳低速物质的水平向运动.在80~100 km深度，云南西南和东南部显示为低速异常和正径向各向异性特征，暗示了软流圈物质的水平流动.在云南北部的丽江、攀枝花和昭通地区，岩石圈地幔中则存在明显的负径向各向异性（V_SH<V_SV），可能反映了地幔物质的上涌痕迹.历史强震多发生在地壳低速区域或高低速过渡带，且地震附近的径向各向异性为负或者较弱.一些地震震源下方存在低速层，地壳低速层可能会促进强震发生.

3D S-wave velocity and radial anisotropy structure of the crust and uppermost mantle in Yunnan,SE Tibet

Yunnan area is located to the southeast of Tibetan Plateau, which draws great attention of geo-scientists. Some hotspot problems, such as whether connected mid-lower crustal low-velocity (low-V) zones exist and what the spatial distributions are if they exist, are still not very clear. High resolution 3D velocity and anisotropy model of the crust and uppermost mantle can provide valuable information to address these problems. In this study, we measured the interstation phase velocities of Rayleigh and Love waves in the period range of 12~60 s using the seismic data recorded by permanent stations in Yunnan and adjacent areas, and then inverted for the SV, SH velocity and radial anisotropy model in the depth range of 10~100 km. The results show that the S-wave velocity and radial anisotropy have obvious variations both in lateral and vertical directions. There are two low-V belts in the depth range of 20~30 km, with positive radial anisotropy (V_SH>V_SV). These two low-V belts may hint horizontal flows in the mid-lower crust. In the depth range of 80~100 km, there are low-V anomalies in southwest and southeast Yunnan, where the radial anisotropy are positive, which may indicate the horizontal flow of the asthenosphere. In the area around Lijiang, Panzhihua and Zhaotong, negative radial anisotropy (V_SH<V_SV) exists in the lithospheric mantle, suggesting the upwelling of mantle material. Most strong earthquakes occurred in the low-V region or transition zone between high-velocity (high-V) and low-V, and the radial anisotropy around the hypocenters is negative or very weak. Low-V zones are shown beneath some strong earthquakes, which may suggest that crustal low-V zones may promote strong earthquakes.