中文摘要：

作为固态地幔和液态外核的过渡区,D″层是地球内部最重要的边界层之一,也是物理、化学现象最丰富的区域之一.而D″层的底部零落散布着厚5～60 km,横向尺度200～300 km,剪切波（Vs）速异常可达-30%,压缩波（Vp）异常达-10%和密度异常可达＋10%的异常体,称为超低速区（ultra low velocity zone, ULVZ）.研究超低速区的地震学手段主要是分析体波（PcP, ScP, ScS, SKPdS, PKKP,PKP）波形的复杂性.本文评述了超低速区的地震学研究的历史及现状,并介绍超低速区的可能成因及其地球动力学作用,最后展望超低速区研究的未来.

英文摘要：

As the transition zone between the solid mantle and the liquid outer core, D＂ is one of the most important boundary layers in the earth, featuring complicated physical and chemical processes. And at the bottom of D＂, there are small patches about 5-60 km high and 200-300 km across, with V1 reduced up to 30%, Vp up to 10% but with density increase up to 10%, as compared to typical anomaly of a few percent in other parts of the earth. These extreme anomalies are named ultra low velocity zones （ULVZ）, and are best modeled with seismic body waves such as PcP, ScP, SKPdS, ScS, PKP and PKKPs. The history and state of the art researches on ULVZs, their probable origin and influences on geodynamics were reviewed. Finally, a discussion was given about what can be expected of studies on ULVZs in the near future.