Lowermost mantle anisotropy beneath the north Pacific from differential S—ScS splitting
Lowermost mantle anisotropy beneath the north Pacific from differential S—ScS splitting
复制标题
北太平洋下方地幔各向异性的差异 S-ScS 分裂
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
G. Rümpker
中科院分区：
文献类型：
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作者：
J. Wookey;J;G. Rümpker
摘要
地震各向异性是研究最低地幔（D”）的性质、起源和动力学的重要工具。我们引入差分 S-ScS 分裂作为消除近源和近接收器各向异性影响的工具，以估计 D” 区域中产生的分裂。这适用于震中距离在 60° 到 85° 之间记录的事件。在以前对下地幔各向异性的研究中，近源各向异性常常被忽视。我们将差分 S-ScS 分裂应用于加拿大国家地震台网西太平洋地震台站的记录；这些样本来自北太平洋下方的最低地幔。 ScS 中的残余分裂（我们将其归因于 D''）显示出 1.0 至 3.9 s 之间的滞后时间。给定 D'' 中 ScS 的近水平射线路径，我们将恢复的快方向解释为快剪切波在由垂直和横向方向定义的平面中的方向，并观察到明显的非 VTI（具有垂直对称轴的横向各向同性）风格的各向异性。最大数量的结果显示了大约向东南倾斜的对称轴，我们推测这可能是由于下降的古板物质水平穿过核-地幔边界向中太平洋下方的上升流区域移动来解释的。非 VTI 对称性以及下地幔矿物、熔体和俯冲物质对 D'' 各向异性的许多可能贡献表明，通过尝试解决更一般类型的各向异性，我们对最下地幔的理解可以大大提高。
SUMMARY
Seismic anisotropy is an important tool for studying the nature, origin and dynamics of the lowermost mantle (D″). We introduce differential S–ScS splitting as a tool for removing the effect of near-source and near-receiver anisotropy to estimate splitting accrued in the D″ region. This is applicable to events recorded at epicentral distances between 60° and 85°. Near-source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy. We apply differential S–ScS splitting to records from Canadian National Seismic Network stations of western Pacific earthquakes; these sample the lowermost mantle beneath the north Pacific. The residual splitting in ScS, which we attribute to D″, shows lag times between 1.0 and 3.9 s. Given the near horizontal ray path of ScS in D″, we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non-VTI (transverse isotropy with a vertical axis of symmetry) style of anisotropy. The largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core–mantle boundary towards an upwelling region beneath the central Pacific. Non-VTI symmetry and the many possible contributions to D″ anisotropy from lower-mantle minerals, melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy.