用地震波衰减成像方法研究水岩作用：以苏门答腊俯冲带为例

Water-rock interactions are widespread in subduction zones and are the key for understanding subduction dynamics. This project aims to study the water-rock interactions of the Sumatra subduction zone. Previous studies show that many detailed problems about water-rock interactions, such as fluid distribution, magma evolvement, mantle wedge serpentinization, are still not clear due to lack of geophysical evidence. Seismic attenuation is particularly sensitive to water contents and temperature variation, hence is an ideal indicator for fluids and meltings in the rock. The present project first develops a non-damping seismic attenuation tomographic method which can best calculate the absolute Q values by using the L-BFGS-B algorithm, and obtains the 3-D attenuation model of the Sumatra subduction zone by the developed method. Then, the water contents of the study region are estimated in a quantitative sense considering previous rock mechanical and geophysical results. Finally, the types and generating mechanism of water-rock interactions, as well as their relationship with subduction dynamics are analyzed. The results of this project can provide methodological experience for understanding water-rock interactions and subduction dynamics of other subduction zones.

水岩作用在俯冲带普遍存在，是研究俯冲动力学的关键。本项目以苏门答腊俯冲带为目标进行针对性研究。前期研究表明苏门答腊俯冲带水岩作用的若干问题（例如含水流体分布、岩浆的演化、是否发生蛇纹石化等）仍缺乏详细的地球物理学证据，尚待研究。地震波衰减特征对岩体介质的温度和含水量变化特别敏感，因此适合表征含水流体和部分熔融的岩石介质。本项目拟通过地震波衰减成像方法，在现有的衰减成像基础上引入L-BFGS-B求解算法，实现无阻尼反演，获得能够更准确求解绝对Q值的衰减成像方法，从而得到苏门答腊俯冲带的三维衰减结构；其次根据衰减结构和岩石力学关系，并结合岩石实验与其它地球物理结果定量估计该地区含水量变化，进而判断该地区地下结构中存在的水岩作用。最后对比其他俯冲带，分析这些水岩作用的产生条件及其与俯冲动力过程的关系。本项目研究结果将为其它俯冲带水岩作用和俯冲动力学过程的研究提供重要的方法支持。

本项目旨在发展地震波衰减层析成像，并研究俯冲带水岩作用。本项目首先改进了地震波衰减层析成像技术，比如发展了单事件频谱比法用于确定t*值，和1/Q格式的衰减层析成像反演方法。然后，本项目收集了苏门答腊俯冲带前弧地区151个地震台站记录到的波形数据，从中提取了近30000个t*数据，建立了苏门答腊地区高分辨率三维地震波衰减层析成像模型。最后，综合苏门答腊俯冲带和其它地区俯冲带的衰减结构、速度结构和各向异性结构，确认了海沟外侧正断层运输水分参与水循环的机制。海水通过这些正断层进入海洋板块，与海洋地幔物质发生蛇纹石化，形成水合断层。水合断层随着海洋板块俯冲进入地幔深部是水循环的重要环节。