Irregular Transition Layer Beneath the Earth's Inner Core Boundary From Observations of Antipodal PKIKP and PKIIKP Waves
Irregular Transition Layer Beneath the Earth's Inner Core Boundary From Observations of Antipodal PKIKP and PKIIKP Waves
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DOI：
10.1029/2018gc007562
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发表时间：
2018-10-01
影响因子：
3.5
通讯作者：
Koper, Keith D.
中科院分区：
文献类型：
--
作者：
Attanayake, Januka;Thomas, Christine;Koper, Keith D.
标准地球模型假定有一个简单的均匀的内核边界（ICB），将液态铁的外核与固体铁的内核分开。然而，冶金学和地球动力学实验预测了凝固过程中热化学和地球动力学不稳定性引起的沿该边界的横向变化。我们通过利用对映体PKIIKP波形对ICB底部反射点下方最上层内核剪切波速结构的敏感性来寻找这种横向非均质性的证据。在距离为178(o)至180(o)的33个罕见对向地震图中测量pkikkp /PKIKP能量比，我们发现该比值在0.1至1.1之间变化。合成地震记录表明，横向均匀的液-固ICB不能解释这种变化。相反，观测结果支持一个空间可变的ICB过渡，包括：(1)地震速度和密度的梯度，在最大深度为10km时，它们从外核平滑地增加到内核的大部分，或者(2)速度和密度的局部双不连续的层状过渡，间隔4-10km。如果剪切速度很小，层状跃迁可以在PKIKP之后产生尾波(
Plain Language Summary Standard Earth models assume a simple uniform inner core boundary (ICB) separating the liquid iron outer core from the solid iron inner core. Metallurgical and geodynamic experiments, however, predict lateral variations along this boundary originating from thermochemical and geodynamic instabilities during solidification. We search for evidence of this lateral heterogeneity by exploiting the sensitivity of antipodal PKIIKP waveforms to the shear wave velocity structure of the uppermost inner core beneath their reflection points on the underside of the ICB. Measuring PKIIKP/PKIKP energy ratios from 33 rare antipodal seismograms in the 178(o) to 180(o) distance range, we find this ratio varying between 0.1 and 1.1. Synthetic seismograms demonstrate that a laterally homogeneous liquid-solid ICB cannot account for this variability. Observations instead support a spatially variable ICB transition consisting of either (1) gradients in seismic velocities and density in which they smoothly increase from those at the outer core to those in the bulk of the inner core over a maximum depth of 10km or (2) a layered transition with localized double discontinuities in velocities and densities separated by 4-10km. A layered transition can generate a coda following PKIKP if shear velocity is small (