利用有限频体波层析成像研究马里亚纳输入板片下方地幔速度结构

俯冲板片在地幔内的形态及其与地幔转换带（MTZ）的相互作用决定了俯冲带的多个动力学过程。马里亚纳俯冲带是太平洋板块俯冲到菲律宾海板块之下形成的洋内俯冲带，具有典型的沟弧体系、充分发育的弧后扩张中心和外缘隆起，且地震和火山活动频繁；此外，其输入板块上分布着大量海山，还可能存在一种新型的岩浆活动形式——斑点火山。因此，马里亚纳代表了典型的俯冲过程，获得这一俯冲带地幔三维精细结构对于理解更广泛的俯冲动力过程具有重要意义。

基于在马里亚纳俯冲带中部海区最新布设海底地震仪（OBS）台阵，并结合这一区域多期次布设的海底及海岛上多达99个地震台站，本论文利用有限频层析成像的方法来对于马里亚纳中部俯冲带下方的上地幔及MTZ的速度结构进行成像。

相对于传统的射线理论，有限频地震层析方法利用三维敏感核来描述实际地震波在不均匀介质中的传播，不但有效地考虑散射和波前愈合等效应，而且在反演中提供了一种“物理”正则化方法，使得该方法能提高成像的分辨率和精度。我们选取了震级大于5.5级、震中距在30-90°范围内的321个全球地震，利用波形互相关方法拾取了高质量P波相对到时数据；针对OBS数据高噪音的特点，我们通过分频的方法来提高数据的有效率，在三个频道上，总共得到了4755个相对走时；为了消除巨大的海底地形起伏影响，对走时数据进行了地壳校正。并最终利用有限频层析成像方法获得了马里亚纳俯冲带下方的P波速度结构，分辨率测试显示，这一模型对于500 km深度以上的上地幔及部分MTZ的成像结果较为可靠。

这一速度模型显示出三个主要的速度异常区域：地幔楔中的低速异常，俯冲板块的高速异常以及输入板块下方的低速异常。其中，地幔楔中低速异常应与是俯冲板片在深部地幔脱水导致的熔融相关；而高速异常体给出了俯冲板片在上地幔和MTZ内的几何形态及其沿海沟方向的变化特征：大约以北纬16°为界，北边板片以近乎垂直的俯冲角度下行，但南部则显示出明显的板片后折趋势，这一特征与地震活动性沿海沟走向的变化特征相对应；此外，输入板块下方呈现明显的低速异常且最深处持续到在410公里间断面附近。俯冲板片在MTZ内的后折及其的低速异常可能反映了该区域斑点火山的一种形成机制，即，因俯冲板片回摆挤压其后方MTZ，致使高含水量的MTZ物质进入上地幔、脱水并熔融。

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