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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.
The study of the structure of the upper mantle is an important means to recognize the temperature change and chemical structure inside the Earth and is of great significance for understanding the convection and other dynamic processes in the Earth.The data collected in this paper are the data from China Digital Seismograph Network (CDSN) and Japan Hi- The net recorded broadband / short-period waveform data of three earthquakes that have occurred in the Tonga-Fiji region since FY 2010, focal depths between 270 and 600 km, and body-wave magnitude Mb between 5 and 7. For these The waveform data is divided into two areas according to the epicentral distance and the anti-azimuth angle, respectively, and N times of root tilt-superposition are respectively processed, and finally, a superimposed grayscale image with high resolution is drawn. By comparing the slowness and the time difference, Surface sdP, pdP facies and SdP facies of the transconducted transition, and finally the depth and location of the transition points of different discontinuities in the upper mantle were obtained.Accordingly, the depth of the 410 km intercept located in the southeastern part of the Tonga-Fiji region was increased by about 20km. In the northern part of the subducted bend, the ascending part also appears on the subducting plate side, rising about 13 ~ 27km. However, the subducting area has a descending phenomenon in the area far away from the subducting plate, The large descent depth is about 12 km, indicating that the 410 km section in the Tonga region is very complex not only affected by cold subducting slabs, but also by dewatering or other volatiles from the subducting slab. Overall, Tonga - The northern Fijian region shows a declining trend up to 47 km with double subsidence, and the closer it is to the Tonga trench, the greater its descending depth indicates that the 660 km section in the Tonga area is indeed affected by the deep subduction of the Pacific plate It is also found that there is a 300 km section in southeastern Tonga with a depth variation of 280-297 km. It may be caused by the eclogite material from the oceanic crust basalt consisting of mid-oceanic ridges, During the process, the remnant formed was retained, and it was considered as the phase transformation from orthopyroxene to high-pressure phase clinopyroxene.