西北太平洋俯冲板块形态及其内部结构的精细地震学研究

The existent morphology of the subducted slab in the Earth’s interior have the significant effects on the plate movement, the mantle convection and the regional tectonic evolution. The fine interior velocity structure of the slab can provide the direct and effective seismic constraints on the relevant physical and chemical properties of the subducted/stagnated slab. The northwestern Pacific subduction zone shows the multiple slab morphologies, and has the abundant earthquake events with different focal depths from the shallow to the deep in the whole world. The dense seismic networks, including the China Digital Seismograph Network (CDSN), the Northeast China Extended Seismic Array (NECESSArray) and the Japan High Sensitivity Seismograph Network (Hi-net), have continually accumulated the huge high-quality waveform data sets in the proper azimuth and epicentral distance ranges. This proposal will mainly consider the variations of the seismic wave field related to the existence of the slab, including the multipathing features of different phases and the secondary reflected/converted phases, and employ the combination of the ray-tracing and seismic waveform fitting methods to detect the morphology and the fine velocity structure of the northwestern Pacific slab in the Earth’s deep interior, to analyze the dehydration and degassing process of the slab and the relevant temperature anomaly; based on the results of geochemistry and other subjects, we will analyze the influence of the slab subduction to the relevant tectonic evolution activity and geodynamic process in the northwestern Pacific area.

俯冲板块在地球内部的赋存形态对板块运动、地幔对流和区域构造演化具有重要影响。板块内部精细速度结构可以为俯冲/滞留板块相关的物理和化学性质的确定提供直接而有效的地震学约束。西北太平洋俯冲带具有多样化的板块形态，并发育有全球最丰富的各个深度上的地震事件。包括中国数字地震台网(CDSN)、中国东北延伸地震台阵(NECESSArray)和日本高敏感度地震台网(Hi-net)在内的密集台网在合适的方位和震中距范围内持续积累了大量的高质量波形资料。本项目将主要考虑与板块存在相关的地震波场信息，包括不同震相的多路径现象和次生反射/转换震相，综合运用射线追踪和地震波形拟合方法来探测西北太平洋板块在地球深部的形态和精细速度结构，以此分析板块脱水、脱气过程以及有关的温度异常；结合地球化学等学科成果，分析板块俯冲对西北太平洋地区相关构造演化活动和地球动力学过程的影响。

本项目主要基于中国数字地震台网(CDSN)、中国东北延伸地震台阵(NECESSArray)、美国USArray以及欧洲地区部分地震台网，收集了西太平洋俯冲区中源和深源地震事件的宽频带波形资料，综合利用波形拟合方法和N次根倾斜叠加方法研究了西北太平洋俯冲板片精细结构和俯冲背景下的地幔深部结构，为认识俯冲板片与周围地幔的相互作用机制提供了重要基础资料。通过本项目研究工作，我们发现西太平洋俯冲板片在中国东海地区地幔转换带内形成了滞留形态，410 km间断面上存在与部分熔融有关的低速层；古南海北部俯冲板片滞留于南海北部地区地幔转换带内，地幔转换带厚度变大，410 km间断面上存在与部分熔融有关的低速层；伊豆-小笠原俯冲区和南美俯冲区内活跃地幔对流对LAB存在一定的改造作用；首次发现琉球俯冲区存在与拆沉洋壳碎片内柯石英到斯石英相变相关的X间断面。此外我们还发现了兴都库什和帕米尔大陆俯冲区410 km间断面起伏差异，这与印度板片和亚洲板片的双向俯冲相关。