Review on studies of ultra low velocity zone

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 Vs 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.