The Ryukyu trench-arc-back arc system is part of the subduction margins of the Philippine Sea plate. Previous studies have indicated that several geophysical and geological characteristics reveal significant variations (including convergent rate, topography, subducting slab angle etc.) along this subduction system. In addition, the strength of plate coupling and the potential of large earthquake occurrence in the Ryukyu subduction zone have been major subjects of debate for decades. To gain new insights into the spatial variations in the crustal structure and strength of plate coupling along the Ryukyu subduction zone, in the present study, based on three P-wave seismic velocity profiles, we construct density models for 2-D gravity modeling. Then, we estimate the mantle lithosphere buoyancy (Hm) using these three density models to determine the strength of plate coupling between the subducting Philippine Sea plate and the overriding Eurasian plate, which could provide information for evaluating large earthquakes potential. 2-D gravity modeling results reveal that oceanic plateaus and/or submarine ridges with obviously less dense and thick oceanic crust are subducting in the northern and central parts of the Ryukyu Trench, which could increase the slab buoyancy in these regions. The Hm results indicate that the strength of plate coupling is almost weak in the north and is relatively strong in the central Ryukyu subduction zone.

中文翻译：

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琉球俯冲带地壳结构及板块耦合强度变化

琉球海沟-弧背弧系统是菲律宾海板块俯冲边缘的一部分。先前的研究表明，一些地球物理和地质特征揭示了沿着该俯冲系统的显着变化（包括收敛速度、地形、俯冲板片角度等）。此外，板块耦合的强度和琉球俯冲带发生大地震的可能性几十年来一直是争论的主要话题。为了获得对沿琉球俯冲带的地壳结构和板块耦合强度的空间变化的新见解，在本研究中，基于三个纵波地震速度剖面，我们构建了用于二维重力建模的密度模型。然后，我们利用这三种密度模型估算地幔岩石圈浮力（Hm），以确定俯冲菲律宾海板块与上覆欧亚板块之间的板块耦合强度，为评估大地震的可能性提供信息。二维重力模拟结果表明，在琉球海沟的北部和中部，明显密度较低且较厚的洋壳的海洋高原和/或海底脊正在俯冲，这可能会增加这些地区的板片浮力。Hm结果表明，板块耦合强度在北部几乎较弱，而在琉球俯冲带中部相对较强。