An attempt to estimate isotropic and anisotropic lateral structure of the Earth by spectral inversion incorporating mixed coupling
An attempt to estimate isotropic and anisotropic lateral structure of the Earth by spectral inversion incorporating mixed coupling
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DOI：
10.1111/j.1365-246x.2005.02504.x
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发表时间：
2005-02
影响因子：
2.8
通讯作者：
H. Oda
中科院分区：
文献类型：
--
作者：
H. Oda
本文提出了一种计算非球面地球模型自由振荡谱的方法，该模型是通过在参考地球模型的地震速度参数中加入各向同性和各向异性速度扰动而构造的，并考察了速度扰动对自由振荡谱的影响。速度扰动的横向变化被参数化为广义球谐函数的展开。我们假设上地幔的地震波各向异性为弱的六方各向异性，其中六方对称轴是水平分布的。合成谱表明，速度扰动不仅引起多重态单重态之间的强自耦合，而且还引起环形多重态和球形多重态之间的混合耦合。这两种耦合都引起垂直分量谱上的振幅异常。在这项研究中，我们确定的振幅异常所造成的混合耦合准环形模式。垂直走滑断层激发的准环形模式在瑞利波节线上最大，随方位角增大而减小，在环线上最小。这种光谱振幅的方位角依赖性与Love波辐射模式非常相似。此外，准环形模的振幅谱对各向异性速度扰动比各向同性速度扰动更敏感。这意味着允许混合耦合效应的模式谱可以对各向异性横向结构以及各向同性横向结构提供约束。本文提出了一种混合耦合谱反演方法，用于从包含准环形模式的自由振荡谱中反演各向同性和各向异性的速度扰动。我们确认，频谱反演方法正确地恢复各向同性和各向异性的横向结构。此外，在谱反演中引入混合耦合效应，使得有可能估计奇数阶横向结构，这是不能确定的常规谱反演，它不考虑混合耦合。当对含有混合耦合的振幅谱进行常规谱反演时，混合耦合会使高阶结构产生偏置。
SUMMARY We present a way to calculate free oscillation spectra for an aspherical earth model, which is constructed by adding isotropic and anisotropic velocity perturbations to the seismic velocity parameters of a reference earth model, and examine the effect of the velocity perturbations on the free oscillation spectrum. Lateral variations of the velocity perturbations are parametrized as an expansion in generalized spherical harmonics. We assume weak hexagonal anisotropy for the seismic wave anisotropy in the upper mantle, where the hexagonal symmetry axes are horizontally distributed. The synthetic spectra show that the velocity perturbations cause not only strong self-coupling among singlets of a multiplet but also mixed coupling between toroidal and spheroidal multiplets. Both the couplings give rise to an amplitude anomaly on the vertical component spectrum. In this study, we identify the amplitude anomaly resulting from the mixed coupling as quasi-toroidal mode. Excitation of the quasi-toroidal mode by a vertical strike-slip fault is largest on nodal lines of the Rayleigh wave, decreases with increasing azimuth angle and becomes smallest on loop lines. This azimuthal dependence of the spectral amplitude is quite similar to the Love wave radiation pattern. In addition, the amplitude spectrum of the quasi-toroidal mode is more sensitive to the anisotropic velocity perturbation than to the isotropic velocity perturbation. This means that the mode spectrum allowing for the mixed-coupling effect may provide constraints on the anisotropic lateral structure as well as the isotropic lateral structure. An inversion method, called mixed-coupling spectral inversion, is devised to retrieve the isotropic and anisotropic velocity perturbations from the free oscillation spectra incorporating the quasi-toroidal mode. We confirm that the spectral inversion method correctly recovers the isotropic and anisotropic lateral structure. Moreover introducing the mixed-coupling effect in the spectral inversion makes it possible to estimate the odd-order lateral structure, which cannot be determined by the conventional spectral inversion, which takes no account of the mixed coupling. Higher order structure is biased by the mixed coupling when the conventional spectral inversion is applied to the amplitude spectra incorporating the mixed coupling.