基于三重震相方法探测日本海俯冲区地幔转换带的速度结构

Detecting the structure of the mantle transition zone in Japan subduction zone from the waveform triplications

本文基于中国数字地震台网记录的发生于日本北海道地区的一次中源地震的三重震相资料研究了日本海俯冲区地幔转换带的速度结构.结果表明,该区域P波速度结构与S波速度结构的一致性整体上较强.冷的西太平洋俯冲板块导致410km间断面出现了10km的抬升,660km间断面出现了25km的下沉;410km和660km间断面之上均存在与俯冲板块相关的高速层;660km间断面下方存在厚度为65km的低速异常.纵横波波速比vP/vS值在210—400km深度范围内偏低,约为1.827,体现出海洋板块低泊松比的特征;在560—685km深度范围内,该值偏高,约为1.831,可能预示地幔转换带底部含有一定量的水.

This paper applies the triplicated waveforms of an intermediate-depth earthquake at the Hokkaido of Japan,retrieved from the China Digital Seismograph Network,to mapping the structure of the mantle transition zone in Japan subduction zone.The results show that the P-wave velocity structure is consistent to S-wave velocity structure for the region as a whole.The cold subduction slab of the western Pacific Plate causes a 10 km uplift of the 410 km discontinuity and a 25 km depression of the 660 km discontinuity;atop the two discontinuities,there are high-velocity layers associated with the slab;below the 660 km discontinuity,there is a low-velocity anomaly with the thickness of 65 km.The seismic velocity ratio（vP/vS）shows a lower value（～1.827）zone at the depthrange of 210—400km,indicating the low Poisson’s ratio signature of the oceanic plate;and the velocity ratio shows a higher value（～1.831）zone at the depth range of 560—685km,possibly implying the hydrous environment at the base of mantle transition zone.