日本海域海洋岩石圈上地幔Pn波几何扩展和Q值成像研究

The subduction of the Pacific Plate under the Eurasian Plate led to the formation of the Japan Arc. Strong tectonic activities occurred in front of the island arc. Behind the Japan Arc, the west Pacific subduction belt interacted with the margin of the Eurasian Plate, formed the back arc sea – the Japan Sea. Compared to the arc of Japan Island and the related subduction process, which has attracted numerals studies, the investigations on the back arc sea is left behind. The regional phase Pn is mainly propagating along the uppermost mantle path, and carries valuable information regarding the processes such as the material exchange between the crust and upper mantle, and the geothermal activities. The Pn wave is characterized by its low noise, long propagation distance, and is particularly suitable for investigating the oceanic areas lacking of seismic stations. Based on these considerations, we propose to use seismic data generated by explosions and natural earthquakes with known mechanisms around the Japan Sea to build the geometrical spreading and attenuation models in Northeast China, Yellow Sea, Bohai Sea, the Korean Peninsula, and Japan Sea. We propose using geostatistical methods to investigate the Pn Q for different geology units, delimit the upper mantle low Q anomalies, from which we can predict the channels for crust-mantle material exchange, existing or residual spreading centers, renew our understanding to the marginal sea and the surrounding environment. This will provide seismology constrains to dynamic processes such as plate subductions, collisions, mantle convection, and mantle intrusion, the retreat of the continental plate or oceanic trenches.

太平洋板块向西俯冲进入欧亚板块之下并导致了日本岛弧的形成。在其前缘存在极其显著的构造运动，而在其后缘西太平洋俯冲带与欧亚大陆边缘相互作用，形成作为边缘海的日本海。相对于对日本岛弧前沿研究所取得的巨大进展，对弧后海的研究却长期滞后。区域震相Pn波主要经上地幔顶部传播，它的衰减携带有关于壳幔物质交换和热运动形态的宝贵信息。该震相噪声低，传播距离长，特别适用于对观测台站不足的海域进行研究。据此，我们提出利用日本海周边的爆炸震源和已知机制解的天然地震，建立包括中国东北和黄渤海海域、朝鲜半岛、以及日本海的Pn波几何扩展和衰减模型。通过衰减分布统计不同地质单元的Q值，圈定上地幔顶部的异常区。并据此推测幔源物质的上升通道，现存或残余扩张中心的位置，提高对日本海及周边地区岩石圈碰撞和地幔对流的认识。从而对板块俯冲、碰撞、地幔对流、地幔柱注入、海洋板块或海沟后撤等动力学过程提供新的地震学约束。

为了揭示日本海及其周边地区的岩石圈地幔的热结构，我们首次在该地区开展了高分辨率（1°×1°）的 Pn波衰减层析成像研究。成像结果表明，长白山等陆内火山、日本海弧后盆地部分地区，以及日本弧火山下方的上地幔顶部均表现出不同频率依赖特征的强衰减结构。这些结果为西太平洋板块俯冲、弧后海盆形成和演化、幔源物质上涌和火山形成等地幔动力学过程提供了新的约束。中国东北和朝鲜半岛火山下方的热物质来源于深部地幔，在上地幔中分为多支，通过进一步的减压熔融作用，形成了现今观测到的长白山、镜泊湖和中加良火山。在菲律宾板块和太平洋板块差异俯冲回退的作用下，中国东北部下方的地幔上升流向东逃逸，形成了目前仍然可以观测到的地幔流的强衰减结构。因此，我们推测在大洋板片俯冲回退的拉张力和地幔流的推动力的作用下，日本海在中新世至上新世期间以不同的旋转角度打开。同时，日本海下方的强衰减异常与较厚的洋壳相对应，这说明在日本海打开期间，地幔流的加热和侵入作用使对马盆地和日本盆地西北缘的弧后地壳增厚。