Determination of the structure of the Earth's core using normal mode of the Earth

我们计算了地球模型的简正波，该模型在液体外核的底部有一个轻微的刚性层。结果表明，在厚度约40公里、横波速度为0.017公里/秒的情况下，可以产生与液核模型的本征频率相近的简正波，而不影响基模和大部分高阶振型。我们的结果表明，外核底部的薄层有限刚性层可能解释了地球简正波的反常分裂，而这种分裂并不能完全用内核的各向异性来解释。我们还发现，出现在_3S_2处的软芯模可能是由于外芯底部的薄层有限刚性层中的陷阱模与普通简正模的耦合而产生的。为了利用体波证实外核底部存在这一薄的有限刚性层，我们编制了计算全三维地球模型合成地震波形的计算机程序。我们在地球模拟器上使用谱元素方法。我们使用地球模拟器的243个节点(1944个CPU)来计算合成地震波，其精度高达5秒。这种合成波形考虑了地球内部的所有非均质性及其对地震波传播的影响。因此，我们应该能够利用这些波形来研究地核的精细结构。

We calculate normal modes for the Earth model, which has a slight rigidity layer at the base of the liquid outer core. We show that such a layer with thickness about 40 km and the shear wave velocity of 0.017 km/sec can produce a normal mode, which has a close eigenfrequency to that of liquid core model, without affecting fundamental modes and most of the higher modes. Our results indicate that the thin finite rigidity layer at the base of the outer core might explain the anomalous splitting of the Earth's normal modes, which has not been fully explained by the anisotropy in the inner core. We also have shown that we may interpret that the soft core mode, which appeared at _3S_2 , may arise as a result of coupling between those trapped modes within thin finite rigidity layer at the bottom of the oute core and the ordinary normal modes. For a preparation of confirmation of the existence of this thin finite rigidity layer at the bottom of the outer core by using body wave, we have set up a computer program to calculate synthetic seismic waveform for fully three dimensional Earth model. We use a spectral element method on the Earth Simulator. We use 243 nodes(1944 CPUs) of the Earth simulator and calculate synthetic seismic wave which is accurate up to 5 seconds. This synthetic waveforms consider all of the heterogeneity inside the Earth and its effect to the seismic wave propagation. Thus we should be able to use these waveforms to study fine structure of Earth's core.