| 摘要: | 台灣為歐亞板塊和菲律賓海板塊聚合的產物，在強烈的聚合作用影響下地殼持續變形和造山作用快速。根據不同時間尺度的抬升速率研究，均顯示中央山脈東翼有高抬升速率，但其抬升的機制與地下構造的關係至今仍多所爭論。從前人研究中推斷中央山脈東翼和縱谷的交界處應存在中央山脈斷層，但因中央山脈斷層尚未能透過野外直接觀察，故仍侷限於區域性的探討，且對於中央山脈斷層的空間型態分佈尚存在許多問題值得加以討論。故本研究透過中央山脈東翼的野外地質資料，花蓮地震同震地表破裂的調查分析，及地形分析，並配合地震資訊，綜合多方面的觀察資訊，來研究中央山脈斷層的活動特性。從地表地形分析結果顯示，中央山脈東翼與縱谷交界有許多正斷層活動的三角崖切面地形特徵，藉由三角崖切面的坡度變化可以得知中央山脈東緣於千年-十萬年尺度的抬升速率在空間中之變化，其中以立霧溪-木瓜溪區間和樂樂溪-新武呂溪區間的抬升速率最快，而豐坪溪北側的抬升速率最慢。另外，從地質調查可以觀察到正斷層的分布，並可以藉由斷層力學分析得知地表的主應力分布狀態，以及可觀察東西剖面上劈理位態的改變。本研究亦利用雙差分定位法重訂位後的地震資料以及其震源機制解，了解中央山脈東翼下方的地震構造為高角度向西傾斜的逆斷層。此外，2018年花蓮地震的餘震分布和同震地表破裂分析結果得知米崙斷層是由中央山脈斷層發育的反向逆斷層系統，且米崙斷層和鄰近的次生斷層(北埔斷層)可視為一區域性的雷氏剪切模型。綜合地形資料及地質調查結果並配合地震分布位置，推論中央山脈斷層在深部為高角度的逆斷層系統，此為造成中央山脈東翼快速抬升的主要原因之一，材料快速抬升造成淺層產生正斷層活動。於近地表時，中央山脈斷層會受不同地體構造作用或地質環境的影響，在淺層材料會往前發育新的逆斷層系統或沿著向東傾斜的正斷層系統運動，使得中央山脈東翼在各區段會有不同的活動特徵，進而影響地形特徵和抬升速率，並產生不同的地表構造和不同的地震活動特徵。;Taiwan Island is an active orogenic belt, as a result of the Philippine Sea plate colliding with the Eurasian plate with a speed of around 8.2 cm/yr. The rapidly convergent rate results in significant crustal deformation and ongoing mountain building process. Both long-term and short-term observations show a rapid uplift (~6-20 mm/yr) in the Central Range, but the uplift mechanism is still under debate. According to previous studies, the Central Range Fault could exist and serve as a boundary between the eastern flank of the Central Range and the Longitudinal Valley. However, few evidences from field investigations show the traces of the Central Range Fault, and the geometry of the Central Range Fault is still a mystery and remains many questions. In order to investigate the Central Range Fault comprehensively, I used the geomorphic analysis, structure analysis, co-seismic rupture analysis, and seismic data to understand the geometry of the Central Range Fault.
According to the geomorphology analysis, I observed many triangular facets along the eastern flank of the Central Range. As a result, the spatial variability of the uplift rate can be known by the slope change of the triangular facets over 103-105 year timescales. The uplift rate of the Liwu River -Mugua River and the Lele River -Xinwulu River are the faster than other investigated areas, and the uplift rate of the northern side of Fengping River is the slowest among the study areas. Also, some of active normal faults were found in the field, which reveal a vertical principal stress direction by a kinematic analysis. The relocated earthquakes by hypoDD with the focal mechanisms of ML ≧ 3.0 events in the eastern Taiwan show that the distribution of the earthquakes generally is a steeply west-dipping reverse fault plane under the eastern flank of Central Range, which could be the Central Range Fault. The ruptures of 2018 Hualien earthquake along the Milun Fault in the north of the Longitudinal Valley show macro-scale Riedel shear structures and the Milun Fault could be a backthrust developed from the Central Range Fault.
By combining the geomorphologic analysis, field observations and the distribution of earthquakes, we suggest that the Central Range Fault at deeper depths is a high angle reverse fault system, which leads to the rapid uplift of the Central Range, and forms the normal fault in shallow. However, influenced by different tectonic process and geological environments from north to south of the eastern flank of the Central Range, the Central Range Fault could develop as a new thrust, or connect the east-dipping normal fault near the surface. |
