The lowermost mantle beneath northern Asia—II. Evidence for lower-mantle anisotropy
The lowermost mantle beneath northern Asia—II. Evidence for lower-mantle anisotropy
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亚洲北部的最下地幔——下地幔各向异性的证据
DOI：
10.1046/j.1365-246x.2002.01760.x
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发表时间：
2002
影响因子：
2.8
通讯作者：
J. Kendall
中科院分区：
文献类型：
--
作者：
Ch. Thomas;J. Kendall
我们在研究最低地幔各向异性时，分析了西伯利亚北方地下具有转折点的预衍射S波。使用了西欧台站记录的马里亚纳、伊豆小笠原和日本海下的深源地震。上地幔各向异性的校正适用于数据。对有和没有高速D层的模型的数据与合成数据的比较表明，在D层区域的顶部存在速度不连续性，并且该区域的各向异性类型是横向各向同性的。径向分量和横向分量上的S波之间的时间分离显示出较弱的趋势，分离随震中距离的增加而增加。将这种分离与D ′（该地区厚度为300 km）内的旅行距离归一化表明，各向异性在该层内均匀分布，平均值为0.5%。对欧洲和北方西伯利亚下地幔最低层结构的不同研究的结合揭示了一幅复杂的画面。来自该地区的层析成像模型和D ′各向异性、下地幔散射体、D ′不连续面反射和超低速带的证据表明存在两个不同的区域。一个表现出高速度，D ′各向异性，D ′不连续性，没有散射体或超低速区的证据。这些特征很可能与伊那木板块向地幔底部的古俯冲有关。另一个区域具有较低的整体速度，并显示出散射体和超低速区的证据，这可能表明存在部分熔融。这些结果表明，在北方亚洲之下的最低地幔的性质在大约30度的长度尺度上存在着巨大的横向变化。
SUMMARY We have analysed prediffracted S-waves with turning points beneath northern Siberia in a study of anisotropy in the lowermost mantle. Deep-focus earthquakes beneath the Marianas, Izu Bonin and the Sea of Japan recorded at stations in western Europe are used. A correction for upper-mantle anisotropy is applied to the data. Comparisons of the data with synthetic data for models with and without a high velocity D �� layer suggest that there is a velocity discontinuity at the top of the D �� region and that the style of anisotropy is transversely isotropic in this region. Time separations between S-waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance. A normalization of this separation with the travel distance within D �� (300 km thick in this region) suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0.5 per cent. A combination of different studies which investigate the structure of the lowermost mantle beneath Europe and northern Siberia reveals a complicated picture. Tomographic models from this area and evidence of D �� anisotropy, lower mantle scatterers, reflections from a D �� discontinuity and ultralow-velocity zones suggest two distinct regions. One exhibits high velocities, D �� anisotropy, a D �� discontinuity and no evidence of scatterers or ultra-low-velocity zones. These features are likely associated with the palaeosubduction of the Izanagi plate well into the lowermost mantle. The other region has a lower overall velocity and shows evidence of scatterers and ultra-lowvelocity zones, perhaps suggesting the presence of partial melt. These results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern Asia over a length scale of roughly 30 degrees.