為了了解台灣中部的地下構造，本研究配合日本東京大學地震研究所於台灣中部所設置的東西向線形地震網，於該區進行實地的重力測勘，希望藉由地震資料與重力資料的結合，獲得較可靠的地下構造。這些實測的重力資料經過各項的重力修正步驟，獲得了該測線的布蓋異常值起伏。 模擬過程中，首先假設岩層密度與震波速度間為一簡單的關係，將速度構造轉換為密度構造後，再計算該構造所對應的理論布蓋異常值，並與實測布蓋異常值做比對。很可惜的，日本方面利用地震資料所推求之地下構造，與實測布蓋異常值間有不小的差異，僅台灣西部有較好的結果。 本研究亦嘗試對數個其他的二維速度構造進行布蓋異常值的計算，並同時考慮另外兩個不同位置的剖面一併做比較。大致上來看，台灣西半部的地下岩層所受擾動較小，構造模式較單純；台灣東半部則因板塊碰撞的影響，造成較複雜的地下構造。 根據模擬的結果，莫荷不連續面的最深處出現在中央山脈下方，該處的上部地殼也有明顯的增厚現象，應為菲律賓海板塊向西擠壓造成地殼變形的結果。過了中央山脈以東，大陸性地殼轉變為海洋性地殼，造成了莫荷不連續面的變淺。在花蓮北部的剖面中，由於板塊間形態的轉變，造成上部地殼嚴重的撓曲，並於深部產生了一低密度區域。 本研究結果顯示，速度構造對於深部的控制較佳。因此，若能利用速度構造控制深部的構造形貌，配合重力資料與地表地質控制淺部構造，應可獲得較可靠之地下構造。 A more reliable subsurface structure can be obtained by combining both of the seismic and gravity data. To gain a better understanding in the central Taiwan, the seismic and gravity data were collected in summer, 2000. The seismic profile approximately perpendicular to the geological strike of Taiwan, were deployed 60 seismic instruments with an average spacing of 2 km. In the mean time, gravity survey was also made along this seismic profile to obtain the Bouguer anomaly distribution. In the modeling of subsurface density structure, available seismic data are used wherever possible to constrain the geometric and/or densities. We simply assumed that there is a direct relationship between density and velocity without any further consideration. Unfortunately, the calculated Bouguer anomaly from the velocity structure doesn’t match well with the observed Bouguer anomaly except the western segment. We also try to calculate Bouguer anomaly from other 2-D velocity structures taken from the previous 3-D velocity models. To study the compatibility between the density and velocity structures should be as a more important research topic in Taiwan. Finally, the reliable density structure across central Taiwan is obtained from the gravity data. In substance, the structure is simply in the western segment but complex in the eastern segment. According to the simulated result, the crustal deformation due to the collision between Eurasia Plate and Philippine Sea Plate caused the result that the depth of the upper crust obviously increases and the deepest position of the Moho discontinuity appears below the Central Range. Across the eastern Central Range, the crustal type transfers from the continental crust to the oceanic crust so that the Moho discontinuity becomes shallow. In the profile near Hualien, with the transition of plates, there are a heavy deformation at the upper crust and a lower density region in the depths. The results show that the velocity structure obtained by seismic investigation had a good constrain in depth. Therefore, we can obtain a more reliable subsurface structure by using the velocity structure to control the deep conformation and using the gravity and geological data to control the shallow conformation.
