Seismic tomography for an azimuthally anisotropic oceanic upper mantle

大洋地幔地震波传播速度的方位各向异性被认为是古板块扩展方向的化石示踪剂，也是上地幔低速带形成机制的指示(Kawasaki等，1989)。通过对地震面波的分析，我们采取了以下步骤来研究这一问题。我们改写了在正交曲线坐标下的弹性体的运动方程和Hookean定律。有了这个表示，我们就可以用传统的积分方法(地震学中的y方法)得到方位各向异性介质中面波的特征值(相速度)。基于这种新方法，我们首次演示了由于各向异性，在东太平洋传播面波的GDSN数字长周期地震记录上，瑞利-勒夫波耦合是如何发生的。这似乎是第一次直接观测到上地幔各向异性的地震性质。Sakai和Kawasaki(1990)研究了地震各向异性如何影响体波的走时和振幅，以及面波速度与体波在各向异性介质中传播的速度之间的关系。Yoshida和Kawasaki(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。他们提出了一种可能性，即大约100公里深的低速带中部的密度可能大于大洋板块的密度。(1989)和Kawasaki(1991)，我们介绍了我们的结果，排除了这一研究项目的理论部分，并讨论了沿着这一研究方向的未来前景。

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.