新疆地区地壳S波速度结构及径向各向异性研究

Crustal S-wave Velocity Structure and Radial Anisotropy in Xinjiang Region

摘要:

应用背景噪声层析成像方法对新疆地区地壳S波速度结构以及径向各向异性进行了研究。利用中国地震局数字地震台网和吉尔吉斯斯坦及哈萨克斯坦台网记录的2009年1月至9月的连续三分量背景噪声数据，对所有台站对之间进行互相关计算，通过时频分析和相位匹配技术得到了每个台站对8 s到50 s的群速度和相速度频散曲线，显示出明显的横向不均匀性，与研究区主要地质结构和构造单元具有良好的相关性。通过反演纯路径频散数据得到了地壳和上地幔顶部的S波速度结构和径向各向异性的结果；根据SH波和SV波速度结构差异得到研究区内的径向各向异性的分布特征；讨论了各向异性产生的机理。

Abstract:

Northwest China is an ideal site for studying the effects of mountain basin coupling due to its complex topography and tectonic structure.In this study，we construct crustal shear wave velocity structure and radial anisotropy in the crust and uppermost mantle in the Xinjiang region by ambient noise seismic tomography.The data include 9 months （2009 January to 2009 September）of three component continuous data recorded at 74 seismic stations of the China Provincial Digital Seismic Networks and regional Kyrgyzstan and Kazakhstan Networks.Empirical Rayleigh and Love wave Green’s functions are obtained from interstation cross correlations.Group velocity and phase velocity dispersion curves between 8 s and 50 s periods are measured for each interstation path by applying the time frequency analysis method with phase matched processing.The group velocity and phase velocity maps show clear lateral variations that correlate well with major geological structures and tectonic units in the study regions. At short periods（<20 s）， the basins show low group velocity and phase velocity，and the eastern part of Tarim Basin displays relatively lower velocities than the western part.A small patch of high velocity is observed in the northwest part of Tarim Basin，indicating that the structure beneath Tarim Basin is not horizontally homogeneous.The orogenic belt shows high velocity.As the period increases，the imprint of the sedimentary layers diminish. At long periods （>30 s），surface wave velocities are strongly influenced by crust thickness and shear velocities in the lower crust and uppermost mantle.High velocity is observed beneath the basins.In contrast，the Tianshan orogenic belt stands out as a low velocity region.SV and SH wave velocity structures of the crust and upper mantle are inverted from Rayleigh and Love wave dispersion maps.Because of thick sedimentation in the basins，S wave velocities of the Tarim and Junggar basins are relatively low at shallow depth，whereas S wave velocities of the mountain area are high due to the widely appearance of magmatic rocks.In the middle and lower crust，the basins show clear features with high velocities，whereas the Tianshan orogenic belt shows those with low velocity.In addition，we compute the radial anisotropy by measuring the differences between SH wave and SV wave velocity.The Tianshan orogenic belt shows negative radial anisotropy，which reflects dominant vertical movement of the deep crustal materials during the process of crustal shortening and thickening beneath this belt.Western Tianshan displays stronger anisotropy than eastern Tianshan，which may be related to different blocking effects due to the Tarim Block.Positive radial anisotropy is a major characteristic of the Tarim Basin in the upper crust，and the velocity of the SH wave of horizontal polarization is faster than the SV wave of vertical polarization mainly due to the effects of the sedimentary layer.In the stable block，the radial anisotropy represents fossil anisotropy left in the latest large scale tectonic movement.Because movement in the entire Tarim block resulted in mineral crystal arrangement dominated in the horizontal direction，the middle crust of the Tarim Basin shows positive radial anisotropy.