Study of Earth's inner core using synthetic seismograms calculated by Spectral-Element method

我们使用了在震中距离超过179度的对跖站记录的宽带地震图，结果表明，PKIIKP相位（内核边界内的单反射）的振幅可以通过存在于外核底部的薄（20km）慢p波速度层（内核边界比PREM低10%）来模拟。在地球模拟器上利用谱元法计算了全三维地球模型的理论地震记录。我们发现目前的地球模式不能再现PKIIKP相位的振幅。由于PKIIKP相幅值对内核边界的地震阻抗（密度与地震波速度的乘积）对比很敏感，因此建议该层内纵波速度的降低不应伴随密度的降低。我们还计算了几次地震的各向同性内核模型合成地震记录，并与各向同性内核模型合成地震记录进行了比较。初步比较表明，各向异性内核模型与各向同性内核模型在PKPab相上的传播时间差异不超过几秒。PKP相的波形似乎没有显著差异。这些行程时间的差异可以通过与观测的比较来帮助我们改善内核的精细结构。目前我们利用地球模拟器的507个节点（4056个cpu）可以计算出精度高达3.5秒的合成地震图。这样我们就可以计算出高达1秒的精度，这代表了体波的主导周期。我们估计，要实现1秒精度的合成材料计算，至少需要比目前地球模拟器大16倍的机器。

We use broadband seismograms recorded at an antipodal station, where the epicentral distance is more than 179 degrees, and show that the amplitude of a PKIIKP phase (single reflection within the inner core at the inner core boundary) can be modeled by the existence of thin (20km) slow P-wave velocity (-10% at the inner core boundary from PREM) layer at the bottom of the outer core. We calculate theoretical seismograms for fully three dimensional Earth model using the Spectral-Element method on the Earth Simulator. We found that the current Earth model cannot reproduce the amplitude of PKIIKP phase. Because the amplitude of PKIIKP phase is sensitive to seismological impedance (product of the density and the seismic wave velocity) contrast at the inner core boundary, it is suggested that the reduction of P-wave velocity within this layer should not accompany a reduction of density.We also have calculated synthetic seismograms with aisotropic inner core model for several earthquakes and compared with the synthetics which are calculated for isotropic inner core model. Preliminary comparison shows that the travel time differences between anisotropic inner core model and isotropic core model for PKPab phases are at most a few seconds. There seems to be no significant differences in waveforms of PKP phases. These differences in travel times may help us to improve inner core fine structure by comparing these synthetics with observation.At present we could calculate synthetic seismograms that are accurate up to 3.5 sec by using the 507 nodes (4056 CPUs) of the Earth Simulator. It is desirable so that we can calculate up to 1 sec accuracy, which represents dominant period of body wave. We could estimate that we need at least 16 times larger machine than the current Earth Simulator to realize computation of synthetics which have 1 sec accuracy.