东北亚边缘地区地幔过渡带内滞留太平洋板片上界面的三重震相研究

东北亚地区下方地幔过渡带与太平洋俯冲滞留板片的相互作用对于区域深部物质运移和区域构造演化具有重要影响.基于中国国家数字台网记录的发生于鄂霍次克海与千岛群岛地区的两个深源地震的宽频带波形资料, 我们发现了与660-km间断面上方一个特殊界面有关的清晰的新P波三重震相波形; 通过对观测波形进行波形拟合以及搜索分析, 本文获得了东北亚边缘地区下方660-km间断面附近的精细速度结构.结果表明: 东北亚边缘下方的660-km间断面之上存在一高速异常层, 其具有尖锐的速度异常上界面, 深度介于455~510 km之间, P波速度异常达2%~4.5%;与此同时, 该地区下方660-km间断面整体速度跃变量较小, 且存在0~15 km的下沉; 660-km间断面下沉与高速层上界面的形态具有很强的相关性, 均表现为西浅东深、南浅北深的特征.结合前人研究结果, 我们推测受日本—千岛海沟回撤速率差异的影响, 西北太平洋俯冲板片对上地幔底部间断面作用的差异是造成660-km间断面整体形态南北差异的主要原因; 而俯冲板片在地幔过渡带内的滞留并向西展平堆积使得板片上界面西侧较东侧更浅; 滞留板片上界面的速度异常特征显示, 俯冲沉积物可能到达地幔过渡带且经脱水相变形成了含黄玉的矿物.

Upper interface of the Pacific slab stagnant in the mantle transition zone beneath Northeast Asia edge derived from triplicated waveforms

The interaction between the mantle transition zone and the Pacific subduction stagnant slab beneath Northeast Asia has an important influence on the regional deep material migration and tectonic evolution. Based on the broadband waveform data of two deep earthquakes in the sea of Okhotsk and Kuril Islands recorded by the China National Seismic Network (CNSN), we find a new clear P-wave triplication related to a special interface above the 660-km discontinuity. Through the waveform fitting and grid searching of the observed triplicated waveforms, we obtain the fine velocity structure near the 660-km discontinuity beneath the edge of Northeast Asia. Our results reveal that: (1) there is a high-velocity anomaly layer above the 660-km discontinuity beneath Northeast Asia edge, which has a sharp upper interface at depth of 455~510 km with a P-wave velocity anomaly of 2%~4.5%; (2) the 660-km discontinuity shows small velocity increment and slight depressions of 0~15 km; (3) there exists good consistency between the depression of the 660-km discontinuity and the upper interface of the high-velocity layer, showing the morphological characteristics deepening trend from west to east and also from south to north. Combining with the results of previous studies, we speculate that the uneven impact of the Northwest Pacific subduction slab on the lowermost upper-mantle due to the heterogeneity of the Japan-Kuril trench retraction rate is the main cause for the overall north-south difference; possible flattening and stacking of the stagnant slab in the mantle transition zone may induce a shallower upper interface of the slab in the west side than that in the east side; the velocity anomaly characteristics at the upper interface of the stagnant slab indicate that subducted sediments may reach the mantle transition zone and then form topaz-bearing mineralogical assemblage through dehydration and phase transformation.