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回顾了与深俯冲和地幔底部结构有关的层析成像的一些新近结果。成像显示出了一些地震波速高于下地幔平均值的狭长结构,它们和上地幔地震板块相连续,其地理分布和过去120Ma 中板块敛合区出露于地表的位置相关性很强。在大约2000km 深处,这些长的线性结构分裂成更小的单元。然而,在一些地区,狭窄的下降体看起来好象是延伸到了地幔底部,在这些地方它们分散开来形成长波长结构,这样就支持了认为深俯冲和下地幔的复杂结构有关系的预测。上升体似乎没有像下降体那样更普遍地存在,而且所使用的 P 波数据也成像较差。这些结果证实了很多过去基于残差球谐分析的结果,也和放射性衰减从内部加热而产生的全地幔对流假说一致。尽管有全地幔的对流,但对流流动似乎在上地幔过渡带以及在大约1800～2300km 深处的过渡带被强烈地扰动。虽然成像未必能反映出因状态变化而引起的深部对流形态的变化,但是 P 波和 S 波数据的联合反演表明它和纵波及横波波速之比的变化是一致的。弹性模量的这种表现在其他地方也有报道,这暗示了地幔底部附近广泛存在化学非均匀性。因此地幔底部约800km 的区域包含了有助于我们理解地球构造和演化的信息。不幸的是,我们不能采集到足够多的直达 P波数据,但如果使用核震相,则数据覆盖能够得到改善。我们初步的研究认为,虽然本文提出的 P 波模型解释了大量的已被用于研究核幔边界上部非均匀性的 PKP波走时残差,但是却低估了它们的幅度。走时差和直达 P 波数据的联合反演可望减少波速模型中非均匀性程度的明显偏差,而且它还充分利用了走时残差的优点,而不必假定异常的位置。
Some recent results of tomosynthesis related to deep subduction and mantle substructure are reviewed. The images show some elongated structures with seismic velocities higher than the average of the lower mantle. They are continuous with the upper mantle seismic plate, and their geographical distribution is highly correlated with the position of the plate convergence area exposed to the earth in the past 120 Ma. At about 2000km deep, these long linear structures split into smaller units. However, in some areas, the narrow descending bodies appear to extend to the bottom of the mantle, where they disperse to form long-wavelength structures, thus supporting the prediction that deep subduction has a relationship with the complex structure of the lower mantle. The ascending body does not appear to be as prevalent as the descending body, and the P wave data used is also poorly imaged. These results confirm many of the past results based on residual spherical harmonic analysis and are also consistent with the assumption of the mantle convection produced by internal heating of radioactive decay. Despite all-mantle convection, convection seems to be strongly disturbed in the upper mantle transitional zone and in the transitional zone at depths of about 1800-2300 km. Although the imaging may not reflect the change of deep convection morphology caused by the change of state, the joint inversion of P-wave and S-wave data shows that it is consistent with the change of wave velocity ratio between longitudinal wave and transverse wave. This behavior of elastic modulus has also been reported elsewhere, suggesting the widespread chemical heterogeneity near the bottom of the mantle. Therefore, an area of about 800 km at the bottom of the mantle contains information that helps us to understand Earth’s tectonics and evolution. Unfortunately, we can not acquire enough P-wave data, but the data coverage can be improved if the core phase is used. Our preliminary study argues that although the P-wave model proposed in this paper accounts for a large number of PKP wave travel residuals that have been used to study the upper non-uniformity of the core-mantle boundary, they underestimate their amplitudes. Joint inversion of travel-time and direct P-wave data is expected to reduce the apparent bias of the non-uniformities in the wave velocity model, and it also takes full advantage of the traveltime residuals without having to assume anomalous locations.