SUMMARY

The causes of seismic anisotropy are still under debate. In particular, it is important to understand the extent to which seismic anisotropy is due to more recent geodynamic activities in the asthenosphere, or to frozen-in deformation in the lithosphere. We show that these two endmember cases can in principle be distinguished using shear-wave splitting observations from SKS waves. This is illustrated by the simple example of pure olivine with horizontal a-axis, and differing orientations of the other two axes, namely, vertical b and vertical c. The azimuthal dependence of shear-wave splitting measurements is described by two parameters, which can provide additional information about subsurface deformation. In particular, the oscillation parameter d₁ constrains the orientation of foliation. We demonstrate that shear-wave splitting in the Western and Central United States indeed shows the predicted azimuthal dependence, related to a mainly subhorizontally oriented flow plane of deformation in the upper mantle. This has important implications for asthenospheric flow.

中文翻译：

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从SKS剪切波分裂观测看美国西部/中部以下上地幔的流平面方向

概要

地震各向异性的原因仍在争论中。特别是，重要的是要了解地震各向异性的程度是由于软流圈中最近的地球动力学活动或岩石圈中的冻结变形引起的。我们表明，从SKS波的切变波分裂观测中，这两个端构件情况在原则上可以区分。这是通过纯橄榄石的简单示例进行说明的，该纯橄榄石的水平轴为a轴，而其他两个轴的方向也不同，即垂直b和垂直c。剪切波分裂测量的方位角依赖性由两个参数来描述，这两个参数可以提供有关地下变形的附加信息。尤其是振荡参数d ₁限制了叶子的方向。我们证明了在美国西部和中部的剪切波分裂确实显示了预测的方位角依赖性，与上地幔变形的主要是水平取向的流平面有关。这对软流圈流动具有重要意义。