Modeling Body and Surface Wave Anisotropy in 3-D Media with a Case Study in Iceland

地震各向异性对岩石圈的变形和软流圈的地幔流动模式具有重要的制约作用。径向各向异性可由瑞利波和乐夫波频散测量，方位各向异性通常由SKS相的剪切波分裂和瑞利波层析得到。然而，这两种方法的方位各向异性结果在许多地区并不一致，例如在科罗拉多州落基山脉、冰岛和澳大利亚。造成这种差异的部分原因是体波和面波的不同性质。例如，SKS分裂反映了从核幔边界到地表各向异性的积分结果，而周期小于100 S的瑞利波主要采样于200公里以上的地球。这种差异还可以归因于许多其他因素，包括横向非均质性和各向异性之间的权衡，各向异性的三维变化，以及有限的频率。本研究的目的是研究一般情况下横波分裂各向异性与瑞利波各向异性之间的差异。以上列出的可疑原因将单独进行研究。我们建议采用并行伪谱方法来计算各种三维非均匀和各向异性模型中SKS位相和面波的合成波形。该方法已成功地应用于横波分裂的模拟。在用伪谱方法模拟表面波方面，还将进行更多的实验。利用横波分裂技术和双平面波反演法对合成资料进行各向异性和非均质性的反演。这项研究将提高我们对体波和表面波方位各向异性差异的理解。

Seismic anisotropy provides important constraints on deformation in the lithosphere and mantle flow patterns in the asthenosphere. Radial anisotropy can be measured from Rayleigh and Love wave dispersions; and azimuthal anisotropy is often obtained from shear-wave splitting of SKS phases and from Rayleigh wave tomography. However, the results of azimuthal anisotropy from the two methods do not agree in many areas, such as in the Colorado Rockies, Iceland, and Australia. Part of the discrepancy is due to the different properties of body and surface waves. For example, SKS splitting reflects an integral result of anisotropy from the core mantle boundary to the surface while Rayleigh waves at periods less than 100 s primarily sample the earth above 200 km. The discrepancy can also be attributed to many other factors, including the tradeoff between lateral heterogeneity and anisotropy, 3-D variations of anisotropy, and the finite frequency. The goal of our proposed study is to investigate the discrepancy between shear-wave splitting anisotropy and Rayleigh wave anisotropy in general cases. The suspected reasons listed above will be studied individually. We propose to adopt the parallel pseudo-spectral method in computing synthetic waveforms of SKS phases and surface waves in various 3-D heterogeneous and anisotropic models. This method has been successfully applied in modeling shear-wave splitting. More experiments will be performed in modeling surface waves through the pseudo-spectral method. The shear-wave splitting technique and the two-plane-wave inversion method will be applied to the synthetic data to retrieve anisotropy and heterogeneity. The proposed study will improve our understanding about the discrepancy in azimuthal anisotropy from body and surface waves.