台灣位於歐亞板塊與菲律賓海板塊間構造交界上，其花東縱谷與海岸山脈為擠壓呂宋島弧的增生部分和呂宋弧前所形成。花東縱谷是一構造板塊碎屑和火山入侵的複雜集合地區。此外，花東地區附近有明顯正的重力異常分布，並顯示出台灣陸地的最大布蓋重力異常值。本研究中，我們使用重力正推及濾波的方法在花東縱谷南段調查地下構造。 為了獲得可信的密度構造，利用布蓋異常值檢測幾個地球物理和地質剖面，將可靠的地質圖、速度構造等參考資料當作控制，建立密度構造。使用 Talwani 方法來計算二維密度模型的效應，所得出的計算值與觀測值相比對。然後，利用嘗試錯誤法進而修改密度模型，最後獲得與觀測重力值符合的密度構造，進一步解釋地下構造形貌。 經由地下構造模擬，結果趨勢皆與觀測值相符，但在細節上皆比觀測值有較高頻的微小起伏，原因可能是震測剖面的解析度遠高於重力資料，所以能反映出比較高頻的訊號，另外，從分層重力異常與剩餘重力異常中可以看到，至少到4公里為止，垂直方向上構造變化不大，且花東縱谷皆為相對低密度的物質組成。 ;Taiwan is located on the tectonic boundary between Eurasian Plate and Philippine Sea Plate. There are many regional structures such as folds and thrust faults. Among them, Chihshang Fault is the boundary between the Central Mountains strata and the Coastal Range region. Due to the difference of the density between the two geological environment, Gravity survey becomes one of the important methods to understanding the structure beneath the area. We collected the data from land and marine, and plotted the Bouguer anomaly of the southern part of Huatung Longitudinal Valley in this study. After obtaining the Bouguer anomaly map, we used the filtering method to separate the gravity anomaly from different depth. And then, we built reliable 2-D forward modeling profiles based on the seismic data and geological stratigraphic profiles. These 2-D models and the separated gravity anomaly are used to realized the density structure. The trend of the results is consistent with the observed value in 2-D models, but the results has some higher frequency ripples. In addition, it can be seen from the separated gravity anomaly and the residual gravity anomaly that the structural change in the vertical direction is small at least to 4 km, and the longitudinal valley has a relatively low-density material composition.
