西太平洋俯冲带地震成因与岩浆活动研究

The western Pacific subduction zone, an important part of the margin of the Asian continent, is an ocean-mainland transition zone with back-arc basin development. The strong earthquake activity and frequent volcanic magmatism in the western Pacific subduction zone have very important influence on the tectonic movement and evolution of the Asian continent. We plan to apply a new earthquake location technique for earthquakes occurred outside the seismic network and a new joint inversion method for seismic velocity and Poisson’s ratio in the crust and upper mantle in the western Pacific subduction zone. We then calculate the thermal structures for the entire-arc of the subduction zone using the inverted coherent velocity and Poisson’s ratio models. We will also determine the velocity discontinuities of the Moho and upper boundary of the Pacific slab using receiver function method. The thermal structures on the basis of the mineral composition of pyrolite for the mantle wedge and peridotite of the Pacific plate is useful for a better understanding of the magmatism and earthquake generation of the mantle wedge. .Three-dimensional velocity (Vp, Vs), Poisson’s ratio (σ), crack-density and bulk-sound velocity structures along the slab upper boundary beneath the fore-arc regions will be determined using a large number of high-quality P-wave and S-wave arrival times from both onshore and offshore earthquakes in the western Pacific subduction zone. The velocity and Poisson’s ratio images could provide a compelling evidence for a highly hydrated and serpentinized fore-arc mantle and fluid-bearing anomalous slow velocity and high Poisson’s ratio associated with slab dehydration under the fore-arc areas. Most great thrust earthquakes (M>7.5) occurred at or close to the high-velocity areas along the slab interface under the fore-arc areas, suggesting strong interplate coupling (asperities) with slab subduction. On the other hand, prominent low-velocity anomalies were revealed along the slab upper boundary, which may reflect weak coupled or decoupled patches (aseismicity) of the plates due to serpentinization of the fore-arc mantle wedge. The crack-density and bulk-sound velocity images, calculated from the corresponding velocity models, indicate that the interplate coupling in subduction zone is different from that under central and Southwestern Japan owing to differences between the tectonic backgrounds of the subduction system, such as the geological age, thermal regime and dipping angle of the oceanic plates. We consider that the fluid-bearing anomalies along the interface of the subducting slab, attributing to a vary of processes such as slab dehydration and serpentinization of the fore-arc mantle, are mainly contributed to the interplate coupling and the repeated generation of the great thrust earthquakes under the forearc regions in the subduction zone.

西太平洋俯冲带是弧后盆发育的海洋-大陆过渡带，是亚洲大陆边缘的重要组成部分。西太平洋俯冲带的强烈地震活动和频繁火山岩浆活动对亚洲大陆的构造运动和演化具有十分重要的影响。本研究拟采用台网外地震定位新技术和速度与泊松比联合反演新方法，获取西太平洋俯冲带壳幔精细结构、精细地球物理边界场刻画、上地幔岩石各向异性以及俯冲带温度场等多种地球物理参数，揭示太平洋板块与欧亚板块之间相互作用模式，从地球深部结构上认识影响西太平洋俯冲带的岛弧火山和地震形成的主控因素，探讨西太平洋俯冲带地震产生与火山岩浆活动与大洋板块俯冲、脱水和板块耦合之间的内在联系。该研究成果有助于揭示太平洋板块俯冲过程及物质输运作用对东亚大陆边缘构造运动的影响。

通通过收集沿西太平洋俯冲带的陆地上固定和临时台站2078个、海洋区域的固定和临时OBS台站306个记录的1,809,706条P-和S-波走时资料。首先采用台网外地震定位方法技术对东北日本-北海道、九洲和台湾地区的海域地震参数精确定位，然后应用三维非线性多参数联合反演新方法技术，获得了沿太平洋俯冲带的东北日本俯冲带、九州和台湾俯冲带的三维深部多参数精细结构。结合地表地质和地壳应力场分布特征，揭示了东北日本俯冲带岛弧火山岩浆作用不仅与太平洋板块俯冲、脱水和俯冲洋壳的部分熔融的密切相关，同时还与该俯冲带的温度场分布特征及上地幔的矿物岩石成分（Peridotite）紧密相关；厘定了日本九州俯冲带岛弧火山岩浆活动除了与俯冲菲律宾板块的脱水和岩石相变有关外，还与由于冲绳海槽扩张引起的上地幔热物质上升有关；揭示了台湾造山带深部岩浆作用和灾害性大地震分布规律主要受控于深部流体侵入导致部分熔融底侵岩浆作用。同时，流体侵入也导致沿地球物理参数变化边界的流体应力增加和减弱了断层面见的摩擦系数，从而容易诱发地震；揭示了马尼拉俯冲带的南海板块和菲律宾板块沿盖俯冲带的双向俯冲形态、岩浆活动及分布规律。.在本项目的支持下，与项目相关的研究成果发表在Journal of Geophysical Research-Solid Earth, Tectonophysics 国际SCI期刊和《中国球物理学报》等国内SCI期刊上8篇，国内核心期刊论文2篇。相关研究成果参加国内外学术交流17人次（包括邀请报告7人次），同时邀请日本院士来华开展合作研究2次共16天，培养博士毕业生2人，博士后出站1人，在读博士和硕士2人，指导青年研究人员1人晋升更高级别的职位。2018年，相关研究成果为项目申请人获得中国科学院“百人计划”终期评估优秀提供了重要支撑。