The limits of ray theory when measuring shear wave splitting in the lowermost mantle with ScS waves
The limits of ray theory when measuring shear wave splitting in the lowermost mantle with ScS waves
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ScS波测量下地幔剪切波分裂时射线理论的局限性
DOI：
10.1093/gji/ggw358
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发表时间：
2016
影响因子：
2.8
通讯作者：
Nowacki A
中科院分区：
文献类型：
--
作者：
Nowacki A
剪切波分裂的观测提供了明确的证据，证明在地球最低的地幔中存在各向异性，该区域被称为D″。最近的许多工作试图利用这些观测结果来限制核幔边界（CMB）以上的应变，因为这可能有助于绘制整个地幔的流动图。以前，这种解释依赖于波可以被建模为无限频率射线的假设，或者地球是径向对称的。由于计算的限制，直到现在还不可能测试这些近似。我们使用全三维，一般各向异性的模拟ofScSwaves的频率的观测表明，射线方法有时是不够的，以解释所看到的信号。我们测试了简单的均匀模型，对于薄至50 km的D″层，可能会产生显著的分裂，我们发现恢复的快速取向通常反映了CMB上方施加的快速取向。射线理论在这些情况下提供了有用的结果，虽然偶尔有显着的差异之间的正演方法。各向同性模型不会产生明显的分裂。我们还测试了更复杂的模型，包括基于我们目前对D″中矿物塑性和弹性的理解的模型。结果表明，即使在几百公里范围内各向异性的变化也会导致射线理论计算和有限频率计算的差异很大。重要的是，在D″中具有极端矿物排列的模型产生的分裂时间与观测结果（δt≤ 3 s）没有什么不同，这表明最低地幔的各向异性可能比以前认为的要强得多-可能是10%或更多的剪切波各向异性。我们表明，如果地幔的基础是复杂的，因为我们相信，未来的研究最低的地幔各向异性将不得不将有限频率的影响，充分解释观测剪切波分裂。
Observations of shear wave splitting provide unambiguous evidence of the presence of anisotropy in the Earth's lowermost mantle, a region known as D″. Much recent work has attempted to use these observations to place constraints on strain above the core–mantle boundary (CMB), as this may help map flow throughout the mantle. Previously, this interpretation has relied on the assumption that waves can be modelled as infinite-frequency rays, or that the Earth is radially symmetric. Due to computational constraints it has not been possible to test these approximations until now. We use fully 3-D, generally anisotropic simulations ofScSwaves at the frequencies of the observations to show that ray methods are sometimes inadequate to interpret the signals seen. We test simple, uniform models, and for a D″layer as thin as 50 km, significant splitting may be produced, and we find that recovered fast orientations usually reflect the imposed fast orientation above the CMB. Ray theory in these cases provides useful results, though there are occasional, notable differences between forward methods. Isotropic models do not generate apparent splitting. We also test more complex models, including ones based on our current understanding of mineral plasticity and elasticity in D″. The results show that variations of anisotropy over even several hundred kilometres cause the ray-theoretical and finite-frequency calculations to differ greatly. Importantly, models with extreme mineral alignment in D″yield splitting times not dissimilar to observations (δt≤ 3 s), suggesting that anisotropy in the lowermost mantle is probably much stronger than previously thought—potentially ∼10 per cent shear wave anisotropy or more. We show that if the base of the mantle is as complicated as we believe, future studies of lowermost mantle anisotropy will have to incorporate finite-frequency effects to fully interpret observations of shear wave splitting.
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