中文摘要：

上地幔结构研究是认识地球内部温度变化和化学结构的重要手段,并对理解地球内部对流等动力学过程具有重要意义.本文所收集的数据是中国数字测震台网（CDSN）和日本Hi-net台网记录到的2010年以来发生在汤加-斐济地区、震源深度介于270-600km且体波震级Mb介于5-7之间的3个地震的宽频带/短周期波形资料.对这些波形资料按照震中距和反方位角两个标准分区,并分别进行N次根倾斜叠加处理,最后绘制了分辨率较高的叠加灰度图.通过慢度差和时差对比,提取来自上地幔间断面反射转换的sdP、pdP震相和透射转换的SdP震相,最终得到上地幔不同间断面转换点的深度以及位置.据此,发现位于汤加-斐济地区东南部的410km间断面深度上升了约20km,在俯冲拐弯的北部区域,该间断面靠俯冲板块一侧也出现了上升现象,上升约13-27km,而离俯冲板块较远区域该间断面却出现了下降现象,最大下降深度约12km,表明汤加地区的410km间断面非常复杂,不仅受到冷的俯冲板片的影响,同时还受到自俯冲板片脱水或其他挥发分的影响.总体而言,660km间断面深度在汤加-斐济地区北部呈现出下降的趋势,最深可达47km,并存在双重下陷现象,且越靠近汤加海沟,其下降的深度越大,表明汤加地区660km间断面确实受到太平洋板片深俯冲的影响;本文还发现,在汤加地区的东南部存在300km间断面,其深度变化为280-297km,其可能由来自于洋中脊玄武岩组成的洋壳残余中的榴辉岩物质,在板片向下俯冲过程中滞留所形成的残余,而且认为其为斜方辉石到高压相单斜辉石的相变面.

英文摘要：

The upper mantle discontinuities and their characteristics are significant for understanding the material transportation in the Earth＇s interior and to provide constrains on geochemistry and mineral physics of Earth＇s interior.Vertical broadband/short period waveform data of three earthquakes with focal depths between 270 and 600km and Mb5.0-7.0since 2010 in the Tonga-Fiji subduction zone recorded by the China Digital Seismograph Network（CDSN）and the Hi-net seismic network,Japan.Then we used the N-th root slant stack method to process the data and related vespegrams were used to retrieve converted reflection phases sdP,pdP and transmission phases SdP from the discontinuities in the upper mantle based on the relations of travel-times and horizontal slownesses between target seismic phases and direct P ones.The depths and locations of conversion points related to different phases were inverted based on the Earth model IASP91.Accordingly,we found the depth of the 410 km discontinuity in the southeast of Tonga-Fiji increased by about22 to 28km,while in the northern part,the depth not only rises from 13 to 27 km near the subduction slab,but also depresses about 12 km.This phenomenon means that the 410 km discontinuity in Tonga-Fiji is affected by the cold subduction slab and the dehydration process.The 660 km discontinuity generally presents a depress to 45 km.It is also characterized by two-layer depression in the northern part.This confirms that the 660 km discontinuity is affected by the deep subduction of western Pacific plate. We also found that 300 km discontinuity exists in the southeast area of Tonga-Fiji with the depth variation of 280-297 km.It may be the stranded eclogite material which is likely remnant crust in the upper mantle.The 300 km discontinuity is preferred as the phase transition of orthopyroxene to high-clinoenstatite.