Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth
Seismological Evidence for Laterally Heterogeneous Lowermost Outer Core of the Earth
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DOI：
10.1029/2018jb015857
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发表时间：
2018-12-01
影响因子：
3.9
通讯作者：
Tsuchiya, Taku
中科院分区：
文献类型：
--
作者：
Ohtaki, Toshiki;Kaneshima, Satoshi;Tsuchiya, Taku
地球的外核被认为是横向均匀的，因为它的低粘度。然而，由于其不均匀的生长，内核可能存在半球差异，这可能伴随着局部的轻元素向外核释放。一些地震学研究提出了非均匀的最下层外核（称为F层），但使用的方法对F层结构不太敏感。在之前的研究中，我们开发了一种对F层结构敏感而对其他结构不敏感的新方法。该方法分析了在内核边界上反射的P波走时与在边界上旋转的P波走时的差异，以及F层中波底或衍射的频散，并应用该方法获得了东北太平洋P波速度的F层模型。在本文中，我们用同样的方法研究了澳大利亚地下的F层结构。观测到的弥散要求澳大利亚下面的速度梯度比东北太平洋下面的速度梯度低，而观测到的走时差要求澳大利亚下面的平均速度更高。两个区域的结果表明，F层具有横向不均匀性，澳大利亚下方的F层比东北太平洋下方的F层具有更高的速度和速度梯度，而下部的F层的速度梯度要小得多。两个区域的最大速度差为0.04km/s，根据从头计算弹性性能，对应于0.8wt%的过量氧。这些结果表明了澳大利亚地下的区域性轻元素浓度。
The Earth's outer core is believed to be laterally homogeneous because of its low viscosity. However, a hemispherical difference in the inner core likely exists due to its uneven growth, which may be accompanied by localized light-element releases to the outer core. A few seismological studies proposed heterogeneous lowermost outer core (called F layer) but using methods that are not very sensitive to F layer structures. In a previous study we developed a new method sensitive to the F layer structure and insensitive to others. The method analyzes differences in P wave traveltimes reflected on the inner core boundary and those that turn above the boundary as well as dispersion in waves bottoming or diffracting in the F layer, and was applied to obtain an F layer model of P wave velocity for the northeastern Pacific Ocean. In this paper, we examine the F layer structure beneath Australia using the same method. The observed dispersion requires a lower-velocity gradient beneath Australia than beneath the northeastern Pacific, whereas the observed traveltime differences require higher average velocities beneath Australia. The results obtained for the two regions indicate that the F layer is laterally heterogeneous and that the layer beneath Australia has a higher velocity and velocity gradient in its upper part and a much smaller gradient in its lower part than beneath northeastern Pacific. The maximum velocity difference between the two regions is 0.04km/s, which corresponds to 0.8wt% excess oxygen according to ab initio calculations of elastic properties. These results suggest regional light-element concentration beneath Australia.