Seismic tomography for an azimuthally anisotropic oceanic upper mantle

地震波在海洋地幔中传播速度的方位各向异性被认为是古板块扩张方向的化石示踪物和上地幔低速带机制的指示（川崎et al.，1989年）。本文通过对地震面波的分析，对这一问题作了如下的研究：在正交曲线坐标系下改写了弹性体的运动方程和虎克定律。有了这种表示，我们已经能够获得的特征值（相速度）的面波在方位各向异性介质中的常规积分方法（称为Y-方法在地震学）。基于这种新方法，我们首次证明了由于各向异性，在东太平洋传播的面波的GDSN数字长周期地震记录上如何发生Rayleigh-Love波耦合。Sakai和川崎（1990）研究了地震各向异性如何影响体波的走时和振幅，以及面波速度与各向异性介质中体波速度的关系。Yoshida和川崎（1991）在面波反演中检验了地震波速度和密度之间的权衡关系。在川崎et al.（1989）和川崎（1991）中，我们介绍了我们的结果，但不包括关于波传播的研究项目的理论部分，并讨论了沿着这条研究路线的未来前景。

Azimuthal anisotropy of seismic wave propagation velocity in the oceanic mantle has been supposed to be a fossil tracer of paleo-plate spreading direction and an indication of mechansim of the upper mantle low velocity zone (Kawasaki et al., 1989). We have taken following steps to investigate this issue by an analysis of seismic surface waves.We have rewritten the equations of motion and Hookean low of an elastic body in an orthogonal curvilinear coordinates. With this representation, we have become to be able to obtain eigenvalues (phase velocities) of surface waves in an azimuthally anisotropic medium by conventional integration method (called y-method in seismology). Based on this new method, we have first demonstrated how Rayleigh-Love wave coupling takes place on the GDSN degital long period seismic records of surface waves propagating in the eastern Pacific ocean due to the anisotropy. This seems to be the first direct observation of seismic nature of upper mantle anisotropy.Sakai and Kawasaki (1990) investigated how seismic anisotropy effect travel times and amplitudes of body waves and how surface wave velocities are related with those of body waves propagating in an anisotropic media. Yoshida and Kawasaki (1991) checked the trade-off relation between seismic wave velocity and density in the surface wave inversions. They suggested a possibility that density in the middle part of the low velocity zone around 100 km depth could be larger than that of oceanic plate.In Kawasaki et al. (1989) and Kawasaki (1991), we presented our results excluding theoretical part of this research project on wave propagation and discussed future perspectives along the line of this study.