地震長久以來是自然災害中的主角之一，大地震的發生對於人類生命財產所造成的損失更是不計其數。全球的大地震大多發生在隱沒帶，而隱沒帶大地震的發生有時會引發海嘯發生造成更大的天然災害。從能量累積與釋放的角度來看，在隱沒帶假若兩個版塊相互推擠愈激烈，則地震發生次數會較多，且較有機會產生大地震。因此，了解隱沒帶板塊耦合狀態是評估大地震及海嘯發生可能的重要依據之一。藉由上覆及隱沒板塊間相互作用的特性，地幔岩石圈浮力的變化可用來描述隱沒帶板塊耦合的狀態。琉球隱沒帶將近約1400公里長，位於環太平洋地震帶上，在此菲律賓海板塊隱沒到歐亞板塊下。過去研究將琉球隱沒帶分為三段，並認為其地下構造特性有明顯不同。此外，對於板塊耦合狀態及大地震發生的可能性尚存有疑慮。本研究想利用計算地幔岩石圈浮力變化的方式來評估隱沒帶板塊耦合的狀態，並了解其孕震構造的差異。 ;Earthquakes play a major role in the natural hazard. Large earthquakes could cause huge damage and lose of life and money. Great earthquakes mostly generate on plate interface of subduction zones. Furthermore, great subduction zone earthquakes sometimes generate tsunamis and cause nature hazards. Intuitively, stress is more easily accumulated, and greater amounts of seismic energy can be released to produce earthquakes, where the subducting and overriding plates are strongly coupled in a subduction zone. Therefore, understanding the strength of a plate coupling is critical for assessing potential seismic and tsunamic hazards in subduction zones. The interaction between an overriding plate and the associated subducting plate can be used to evaluate the strength of plate coupling by examining the mantle lithospheric buoyancy. The Ryukyu Trench, around 1400 km long, is part of the circum-Pacific seismic belt, where the Philippine Sea Plate is sudducted beneath the Eurasian Plate. Previous studies suggested that the Ryukyu arc can divide into three segments, and the structural characteristics of each segment were different. Besides, the strength of the plate coupling and the potential of great earthquake occurrence in the Ryukyu subduction zone has been a controversial issue. In this study, we try to calculate the mantle lithosphere buoyancy across subduction zones. Then, we can evaluate the strength of the plate coupling in subduction zones to further understand the relationship between the plate coupling status and large earthquakes.