Weak elastic anisotropy in global seismology
Weak elastic anisotropy in global seismology
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全球地震学中的弱弹性各向异性
DOI：
10.1130/2015.2514(04
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发表时间：
2014
影响因子：
5.3
通讯作者：
D. L. Anderson
中科院分区：
文献类型：
--
作者：
L. Thomsen;D. L. Anderson
50多年来，人们已经知道，在对大多数地震数据进行实际分析时，必须考虑地震各向异性。这方面的证据包括在许多情况下观察到的依赖性（这里简要回顾）的地震速度的传播角度和角度的S波偏振。尽管有这种公认的理解，许多目前的调查继续采用不太现实的各向同性假设。结果之一是人造物的出现，这些人造物可以用地球结构的细节来解释，而不是分析中的限制性假设。
忽略各向异性的原因大概是各向异性地震学的代数复杂性更大，自由参数的数量更多。然而，地球中的地震各向异性通常很弱，弱各向异性的方程仅比各向同性的方程稍微复杂一些。此外，通常需要额外的参数来描述数据。此外，下面定义的弱各向异性的参数（各向异性弹性模量的组合）比单独的各向异性模量本身更少地受到不确定性的复合和空间分辨率问题的影响。因此，反演应该试图用这些参数来拟合数据，而不是用那些单独的模数。本文简要地回顾了弱各向异性理论，提出了弱各向异性面波速度的新方程。该分析对以往研究中发现的“Rayleigh波-Love波不一致性”等结果提供了新的见解，包括：罗利波速度不仅取决于水平SV速度，而且还取决于各向异性; Love波速度不仅取决于水平SH速度，而且还取决于各向异性。
It has been known for over 50 years that seismic anisotropy must be included in a realistic analysis of most seismic data. The evidence for this consists of the observed dependency in many contexts (reviewed briefly here) of seismic velocity upon angle of propagation and upon angle of S-wave polarization. Despite this well-established understanding, many current investigations continue to employ less realistic isotropic assumptions. One result is the appearance of artifacts which can be interpreted in terms of details of Earth structure rather than of the restrictive assumptions in the analysis.
The reason for this neglect of anisotropy is presumably the greater algebraic complexity and the larger number of free parameters of anisotropic seismics. However, the seismic anisotropy in the Earth is usually weak, and the equations for weak anisotropy are only marginally more complex than for isotropy. Further, the additional parameters are commonly required to describe the data. Moreover, the parameters of weak anisotropy defined below (combinations of the anisotropic elastic moduli) are less subject to compounding of uncertainty and to spatial resolution issues than are the individual anisotropic moduli themselves. Hence inversions should seek to fit data with these parameters, rather than with those individual moduli. We briefly review the theory for weak anisotropy and present new equations for the weakly anisotropic velocities of surface waves. The analysis offers new insights on some well-known results found by previous investigations, for example the “Rayleigh wave–Love wave inconsistency”, including the facts that Raleigh wave velocities depend not only on the horizontal SV velocity but also on the anisotropy, and Love wave velocities depend not only on the horizontal SH velocity but also on the anisotropy.