台灣位在歐亞大陸板塊與菲律賓海板塊的交界，存在於兩個隱沒系統間。在台灣東北部由菲律賓海板塊沿琉球海溝向北隱沒至歐亞大陸板塊之下，形成了沖繩海槽。沖繩海槽為弧後擴張盆地，其東邊與南邊為琉球海溝，西邊與北邊為東中國海大陸棚，其擴張延伸進入台灣造山帶，而宜蘭平原正位在其西南端。這個構造區域內地震活動頻繁，地下構造複雜，仍然有許多未知的構造問題待釐清。本研究利用區域內密集的重力測點，繪製布蓋重力異常圖，並進行地下構造之推演。
本研究所使用之資料包含陸地、海域及空中的重力觀測資料。陸上重力資料來源為中油、內政部地政司及中研院地球所，海上重力資料為1998年所發表之海上重力異常圖(Hsu, 1998)。另外，由於本研究擴及的區域涵蓋廣大，相較於平地區域的重力測點，在山區與海上的測點較少。為了補足測點分布不均的問題，本研究考慮加入空載重力測量(Hwang, 2007)的結果來進行地下構造模擬。台灣於2005 年實施空載重力測量工作，使用最新的空載重力儀，並配合內政部與林務局的GPS 地面固定站定位並計算重力，施測範圍涵蓋台灣全島與附近海域，航高約為5000公尺，飛機時速306 公里，約43 個工作天。本研究主要處理、整合所有重力觀測資料，而其中一項重要的部分為空載重力測量與地面重力測量的結果分析比較。初步結果發現在海域的船測重力與空載重力結果較接近，而在山區的部分，與陸地重力測量差異甚大。
為了瞭解台灣東北部的地下構造形貌，本研究藉由重力資料的處理和分析，進而了解地下密度構造的分布。在進行密度構造推演時，需要先給定一個初始模型(Hsieh, 2009)，將此模型所得到的理論重力值和觀測重力值做比對，並配合地質資料反覆修改，最後得到一合理的三維地下密度模型。由本研究三維密度模型可知，Moho深度在宜蘭平原與中央山脈下方較深，可達45公里；台灣北部及西部麓山帶較淺約30至35公里。

The Okinawa Trough, a backarc basin, is located at the Eurasian plate and the Ryukyu Trench boundary and extends to north-eastern Taiwan. As a subduction zone, there are some complex unknown structures unsolved. In this study, we use the detail gravity data around this area to investigate the reliable subsurface density structure. First, we combine with land and marine gravity data to obtain gravity anomaly. In this study we utilize the data including land, marine and airborne gravity data. The land gravity data was measured by CPC, MOI and IES. The marine data was published by Hsu (1998). Furthermore, the observation data is sparse in mountain and marine. In order to solve the uneven distribution of measuring points, we add the airborne gravity data by Hwang (2007) to achieve observation covering better. The main processing in this study are integrating and comparing all the gravity observations included the airborne gravity measurements and surface gravity survey. Preliminary results show that the ship-borne gravity and airborne gravity are closer, but in mountains with large differences. Second, considering the geology, tomography and other constrains, we simulate the 3D density structure. According to the three-dimensional density model in this study, the Moho depth is deeper at the bottom of Ilan plain and Central Range, achieve to 45 km; It is only 35 km in northern Taiwan and western foothills.

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