本研究結合地震與重力資料，探討台灣東北部(包括宜蘭平原、龜山島及外海)三維速度與密度構造。研究區域位於琉球隱沒帶周圍，受菲律賓海板塊隱沒與沖繩海槽擴張共同作用，地質構造複雜。本研究結合雙差分層析成像法與重力逆推，透過順序逆推流程得到Vp、Vs、Vp/Vs以及密度等參數，並評估模型穩定性與構造意義，最終確認模型穩定、地震走時及重力觀測殘差收斂。結果顯示，淺部於宜蘭平原北邊呈低Vp與低Vs區域，反映未固結沉積層；平原以南呈高Vp與Vs區域，反映中央山脈堅硬變質岩出露。由0至4公里深度區間發現低Vs與高Vp/Vs比值的異常帶，向外海延伸至宜蘭平原內部，推測與岩漿庫供應淺層至地表熱液活動有關。至更深處呈現由南至北的高Vp/Vs異常體，可對應至琉球隱沒帶深部含水物質與流體上升通道。本研究證實，結合地震與重力資料的順序逆推方法，能有效克服單一資料類型的限制，建立同時符合兩種觀測量且具地質意義的構造模型。此結果不僅有助於理解區域岩漿活動機制，同時希望為後續地熱潛能評估與地震風險分析提供重要依據。

This study integrates seismic and gravity data to investigate the three-dimensional velocity and density structure of northeastern Taiwan, including the Ilan Plain, Kueishan Island, and adjacent offshore areas. The study area is located at the frontal boundary of the Ryukyu subduction zone, where complex geological structures arise from the combined effects of the Philippine Sea Plate subduction and the back-arc spreading of the Okinawa Trough. By applying double-difference seismic tomography (tomoDD) and sequential gravity inversion, this study reconstructs shallow Vp and Vs velocity models and density model, and evaluates their spatial stability and structural significance. The results show that the shallow crust beneath the Ilan Plain is characterized by low Vp and low Vs, reflecting unconsolidated sediments. At depths between 7 and 15 km, a high-Vp and high-Vp/Vs anomaly extends seaward toward a high-ratio zone, which is inferred to be related to partial melting or a fluid conduit. In deeper regions, the model reveals a northward-trending high-Vp/Vs anomaly, corresponding to hydrated materials and vertical fluid migration pathways associated with the deeper Ryukyu subduction zone, consistent with previously proposed structures. The final model is highly consistent with earlier results and shows significant improvement in shallow-layer resolution. This study confirms that the joint inversion of seismic and gravity data effectively compensates for the limitations of single-dataset approaches, yielding a more stable and geologically meaningful structural model that enhances our understanding of regional magmatic and fluid processes.

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