西元1999年發生規模7.3的集集大地震，位於車籠埔斷層地表破裂跡北端的東勢背斜大安溪段河床面受到同震抬升，當時最大垂直抬升量達10公尺，背斜東翼河道抬升量約為6.5公尺，西翼河道垂直抬升量約為4公尺，河道的平衡基準面因此而改變，伴隨著多次的清淤與颱風加速河道下切，漸而形成如今的大安溪峽谷，峽谷兩側的谷壁與大安溪南岸的吊神山山壁，使我有機會清晰描繪東勢背斜出露地表的剖面。根據當時地調所調查集集地震地表破裂跡結果顯示，東勢背斜東翼有明顯的地表破裂跡，而西翼並無明顯的地表破裂跡，推測經由大安溪的下切而出露的較深部岩層中能觀察到過去被深掩地構造。有關此區域之同震抬升，前人提出斷層作用、褶皺作用以及複合型等三種不同的機制。本研究使用無人飛行載具 (UAV) 建構大安溪峽谷的三維點雲模型，並製作空照正射圖與西北-東南向的大安溪峽谷垂直剖面正射影像與空照正射圖，結合詳細的野外調查結果繪製出現今大安溪峽谷的區域岩性構造分布圖與東勢背斜出露地表部分的剖面。利用無人機繪製出的剖面、深井井測及震測剖面，建構出東勢背斜的完整形貌，分析後認為其為一基底滑脫褶皺，主要以共軛褶皺的機制形成，而河床同震地變的型貌推測是受到既存的地質構造影響，主要是以共軛褶皺作用為主，輔以斷層作用，載荷為地下約1公里的基底滑脫斷層的瞬間錯移，此次集集地震的同震地變的確促使了東勢背斜的增長。;The Chi-Chi earthquake was an ML 7.3 earthquake resulted in the Chelungpu fault in central Taiwan in 1999. The Taan River valley crosses the Tungshih anticline, which is located at the northern end of the Chelungpu fault rupture. The river reach has been uplifted during the Chi-Chi earthquake. The coseismic deform of the river profile is gently decrease in the west, a sharp decrease in the east. The maximum vertical displacement of the river bed was 10 m. The vertical displacements were 6.5 m on the eastern fold limb and 4 m on the western fold limb. This coseismic uplift disturbed the dynamic equilibrium of the fluvial system. Typhoons and river dredging accelerated the incision. After two decades of rapid incision, the river bed became the Taan gorge. The cliffs of the gorge and the Diaoshan Mountain, on the south riverbank, allow me to sketch the exposed geological profile of the Tungshih anticline. According to the survey report of the Chi-Chi earthquake surface rupture from Taiwan Central Geological Survey, there was an obvious surface rupture on the eastern limb, but there was no obvious surface rupture or flexure on the western limb. Three mechanisms were proposed to explain the Chi-Chi earthquake-induced uplift in this area: which is faulting, folding, and combination of both. The analysis of the newly exposed outcrops of the Taan Gorge should allow me to discriminate between these proposed models. I used UAV (unmanned aerial vehicle) to construct the 3D point cloud model of the Taan gorge. And export the aerial image and NE-SW profile of the Taan gorge and Diaoshan Hill cliff. I combined field data and images from UAV to construct the regional lithology map. Then, I built a geological profile across the Tungshih anticline integrating structural data based on field measurements and the UAV 3D point cloud, a deep well log, and a seismic image profile. My result indicates that the Tungshih anticline is an asymmetric detachment fold, and mainly resulting from the mechanism of (conjugate) folding and the formation reach uplift were strongly influenced by the existing structures and formed via mainly conjugate folding and partly faulting under the loading of sudden slip along the detachment fault at a depth around 1 km. The earthquake-induced uplift participated in the growth of the Tungshih anticline.
