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上地幔速度间断面的存在形态及其性质的研究对于理解地球构造运动以及地球内部物质运移过程具有重要意义.针对上地幔中可能存在的300km速度间断面,本文收集日本Hi-net地震台网所记录的2004年以来、震源深度在145～219km之间、且震级为mb5.0～6.0之间的6个汤加—斐济地区地震的波形资料.利用4次根倾斜叠加方法对收集到的波形资料进行叠加处理,获得了相应的叠加灰度图,从中提取的离源下行的SdP次生转换震相,进而据此确认了汤加—斐济地区下方的300km深度附近的速度界面.因该界面起伏较小,更倾向于该界面为辉石的斜方到高压单斜相变面;同时基于转换震相的强度差异,我们发现该界面速度跃变量要大于410km间断面,因而俯冲带的化学分异和脱水过程产生的较轻物质可能在该界面处形成一定的聚集,使得汤加—斐济地区的该界面更容易被观测到.
The research on the existing forms and properties of velocity discontinuities in the upper mantle plays an important role in understanding the tectonic movement of the earth and the process of material movement inside the Earth.For the possible 300 km velocity discontinuities in the upper mantle, The records of seismic waveforms of six Tonga-Fijian earthquakes of epicenter depth of 145 ~ 219km and magnitude of mb5.0 ~ 6.0 have been recorded since 2004. The waveform of the collected waveforms Data were superimposed to obtain the corresponding superimposed grayscale, extracted from the downstream source SdP secondary transformation phase, and thus confirm the Tonga - Fiji below the depth of 300km near the velocity interface due to the interface undulating Smaller and more inclined to the interface of pyroxene to the high pressure monoclinic phase transformation; at the same time, based on the intensity difference of converted phase, we found that the interface velocity jump is greater than 410km, so the chemical subduction Lighter substances resulting from the process of isomerization and dehydration may form some aggregates at this interface, making this interface in the Tonga-Fiji region more easily observed.