核幔边界超低速区（ULVZ）的分布和横向变化

核幔边界是地球内部物质和能量交换的重要场所，影响和控制着地球内部很多动力学过程。地震学研究发现核幔边界分布着很多块状区域，其厚度约为5-60公里，宽度一般在上百甚至到上千公里，称为超低速区（ULVZ），其中P波和S波的速度降低可以达到10%和30%，密度增加也可以达到10%。目前研究发现的超低速区大多分布在下地幔大剪切波低速体（LLSVP）边界附近，表明ULVZ与LLSVP有关，并得到地球动力学模拟的证实。但是，ULVZ的研究区域还有很多空白，即并不是所有的LLSVP边界都被研究过；另外，最近新发现的范围在上千公里的ULVZ区，其内部结构也有一定变化，表明LLSVP边界结构或动力学过程的不均匀性。为此，本项目将聚焦ULVZ研究的空白区和大ULVZ区的内部结构变化，采用PKP前导波及 ScS, ScP等震相，通过台阵分析，波形拟合等方法，探讨ULVZ的分布和形态变化等特征和LLSVP的关系，以便更好地理解核幔边界的动力学过程。

As the boundary of iron core and silicate mantle, the core mantle boundary (CMB) is one of the most important boundary layers that have rigorous thermal and compositional exchanges。 It plays important roles in dynamic processes within the Earth. Seismological studies have found that there exist anomalous patches above CMB, which are called ultra low velocity zones (ULVZs). ULVZ generally has a thickness 5-60 km, dimension of tens to hundreds of kms. The density of ULVZ can increase to about 10%, and P and S velocity can reduce up to 10% and 30%, respectively. Most of the detected ULVZs are loated near the boundary of large shear velocity provinces (LLSVPs), and geodynamic modeling results have proved this could be true for convections with large compositional reservoirs. However, the mapping of ULVZs are still not complete, with LLSVP boundaries that have not been studied; On the other hand, recently discovered large ULVZs that extend to about a thousand kms seems to have more complicated variations inside them, indicating complicated heterogeneities near the LLSVP boundary. In this proposed study, we will use unconventional seismic phases（e.g.，PKP Precursors) and other body wave phases (e.g., ScS and ScP) to study ULVZs near the east side of Pacific LLSVP and west side of Africa LLSVP boundaries, to map the distribution and geometry of ULVZs. With our results, we hope to have more constraints or evidences to better understand ULVZ mechanism, small convection patterns near LLSVP boundaries.