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

利用谱元法计算的合成地震图研究地球内核

基本信息

批准号：
16204035
负责人：
TSUBOI Seiji
金额：
$ 30.7万
依托单位：
Independent Administrative Institution, Japan Agency for Marine-Earth Science and Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
2004
资助国家：
日本
起止时间：
2004 至 2006
项目状态：
已结题

项目摘要

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.

我们使用记录在对极台站的宽频带地震图，其中震中距离超过179度，并表明PKIIKP相位的振幅（在内核内的内核边界处的单次反射）可以通过外核底部的薄（20 km）慢P波速度（从PREM在内核边界处为-10%）层的存在来模拟。我们在地球模拟器上用谱元法计算了全三维地球模型的理论地震图。我们发现，目前的地球模型不能再现PKIIKP相位的振幅。由于PKIIKP相的幅值对地震波阻抗敏感，（密度和地震波速度的乘积）对比，建议降低P-本文还计算了几次地震的各向异性内核模型合成地震图，并与各向同性内核模型合成地震图进行了比较内核模型初步比较表明，各向异性内核模型和各向同性内核模型对PKPab相的走时相差不超过几秒。PKP各时相波形无明显差异。这些走时的差异可以帮助我们通过与观测结果的比较来改善内核精细结构。目前，我们可以用地球模拟器的507个节点（4056个CPU）计算精度达3.5秒的合成地震图。这是可取的，以便我们可以计算高达1秒的精度，这代表了主导周期的体波。我们可以估计，我们需要至少16倍于目前的地球模拟器的机器来实现1秒精度的合成计算。