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Study of inner core differential rotation detected from earth's free oscillation

地球自由振荡探测内核差动旋转的研究

基本信息

批准号：
15540410
负责人：
ODA Hitoshi
金额：
$ 1.54万
依托单位：
Okayama University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2003
资助国家：
日本
起止时间：
2003 至 2005
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-15540410/
关键词：
Inner core of the earth Inner core differential rotation Splitting function Anisotropy Inner core modes Earth's free oscillation Free oscillation spectrum 地球中心核(内核)差動回転スプリッティング関数コアモードスペクトル地球中心核(内接)地震波異方性

项目摘要

The anisotropic structure of the inner core is retrieved from interaction coefficients of splitting functions of the inner core modes. Five elastic constants are determined assuming that the inner core has anisotropic properties of hexagonal symmetry. If the seismic anisotropy is dependent on radius, ε, which denotes fractional difference between P-wave velocities propagating along the earth's rotation axis and equatorial plane, becomes positive and its radial dependence has a minimum at 230 km depth from the inner core boundary. The effect of inner core anisotropy on free oscillations spectra is investigated using the retrieved anisotropy structure. Because the strain energy due to the elastic anisotropy is usually small in the inner core, the inner core anisotropy and inner core differential rotation are hard to have a large influence on the amplitude spectrum of core modes. Thus it is not easy to detect the inner core differential rotation and to show that the inner-core anisotropy is restricted to the east hemisphere of the inner core. But the inner core rotation is possibly detected from systematic change in amplitude spectra even if the spectral changes are small, when a lot of spectral data are used for the analysis. We try to estimate a rate of the inner core rotation from time variations of splitting functions of seven inner-core modes. The splitting functions are corrected for the effect of mantle heterogeneity to obtain splitting functions that reflect only the inner-core structure. From time variation of the corrected splitting functions, a mean rate of the inner-core rotation is estimated to be as small as 0.03 ± 0.18° per year westward.Although our estimate shows a slightly westward rotation, the rotation rate is insignificantly different from zero.Therefore we conclude that the inner core is not rotating at a significant rate relatively to the crust and mantle.

从内芯模式分裂函数的相互作用系数中反演出了内芯的各向异性结构。假定内核具有六角对称的各向异性，确定了5个弹性常数。如果地震各向异性与半径有关，则表示沿地球自转轴和赤道平面传播的P波速度之差的分数ε变为正值，其径向相关性在距内核边界230公里处最小。利用恢复的各向异性结构研究了内核各向异性对自由振荡谱的影响。由于弹性各向异性引起的内芯应变能通常很小，内芯的各向异性和内芯的差动转动很难对芯模的振幅谱产生较大的影响。因此，不容易检测到内核的差异旋转，也不容易表明内核的各向异性仅限于内核的东半球。但是，当使用大量的光谱数据进行分析时，即使光谱变化很小，也可以从幅度谱的系统变化中检测到内核的旋转。我们试图从七个内核模式分裂函数的时间变化中估计出一个内核自转速率。对分裂函数进行了地幔非均质性影响的校正，得到了只反映核内结构的分裂函数。根据修正分裂函数的时间变化，估计地核自转的平均速率为0.03±0.18°/年，虽然略向西自转，但自转速率与零相差不大，因此我们得出结论，地核相对于地壳和地幔的自转速率并不是很大。