花東縱谷北段海岸山脈西側存疑性斷層的活動性探討

Abstract

在過往的同震變形紀錄中，花東縱谷北段的活動斷層具有顯著的左移特性，如1951縱谷地震序列中米崙斷層的地表破裂，以及2018花蓮地震中最北端的嶺頂斷層。若將2018年的嶺頂斷層破裂段沿著其走向直接向南延伸，會發現此破裂段可以對應至海岸山脈西側上的一系列左移錯斷河與閉塞丘等構造地形，且可以再往南接到其他線形構造（由北往南分別為：月眉、大坪、米棧，與山興線形）－即前人透過航照與40米數值地形（DEM）所判釋的一系列左移斷層跡，暗示了此系統應具有一定的活動性。然而，受限於地形資料的解析度不足及植被過於茂密的問題，近期的研究認為這些在海岸山脈西側出現的線形構造不夠連續，且未有全新世的活動證據，因此判定這些斷層僅是活動性存疑的次要斷層。 透過中央地調所的山崩雲平台所提供的高解析度光達數值地形影像，輔以20米DEM、航照立體對，及1:5000像片基本圖，本研究重新對嶺頂斷層上盤的海岸山脈西側進行構造地形的判釋；並利用GIS軟體與回復原始地形的方式，得到河道與其周圍地形所累積的最大水平錯移量；同時配合野外調查，檢視斷層與沖積扇扇階的判釋成果，及確認是否有近期的活動證據。斷層跡的分布顯示，前人所提出的四條線形可視為一條連續的左移斷層的不同分段；最大的累積水平錯移量有空間分布上的差異，在此左移斷層的北段約累積了330公尺的錯移量，往南則遞減至275公尺，整體的平均錯移量為300-310公尺。此結果可能反映GPS速度場方向在斷層南北段的差異，或代表此斷層往南逐漸接到另一個斷層系統。另外，本研究於野外調查時並未發現高位沖積扇扇階有紅土化育，故假設這些下切至都巒山層的錯斷河於末次冰盛期（LGM）的低海水面時期下蝕，並開始累積錯移量，推估其約從1.45至3萬年前開始累積，進而求得長期走滑速率為10.0–21.4 mm/yr（300 m / 30 ka–310 m / 14.5 ka）。考量到其高活動速率及切過全新世沖積層，本研究認為此左移斷層在縱谷北段為相當活躍的斷層系統。

This study aims to investigate the tectonic activity of the plausible active fault system at the northern Longitudinal Valley (NLV) in eastern Taiwan. We utilize 6m LiDAR DEM image provided by the Central Geological Survey (CGS), with 20m DEM and 5m contour map as aids, to map active tectonic features along the western flank of the Coastal Range by observing their geomorphology. Also, we conduct field survey to examine our mapping results and to identify outcrops with recent faulting. To quantify the tectonic activity of this fault system, we use QGIS and slip restoration method to measure its cumulative left-lateral offset and further propose its slip rate. Our mapping shows that the fault system comprises a series of right-stepping en-échelon fault traces, reflecting a newborn left-lateral strike-slip fault system. We also discover three streams in series with small left-lateral displacement and several offset alluvial fans, which are strong evidence of recent fault activities. These findings correspond with the geodetic data over the past century and surface rupture of the 2018 Hualien earthquake in the NLV, demonstrating significant left-lateral strike-slip component of the fault system in this region. For the maximum cumulative left-lateral displacement, we find this system has 300 to 310 meters offset on average. Since we did not find any high alluvial fan terraces covered by lateritic soil in the field, we assume that the ~300m displacement is accumulated after the streams at the western flank of the Costal Range incised deeply into the bedrock. Considering the uplift motion is not dominant in the NLV, low sea level period during the Last Glacial Maximum (LGM, 14.5-30 ka ago) could provide the driving force of incision. We can thus derive the long-term strike-slip rate of this fault system from dividing the cumulative left-lateral offset by the elapsed time since LGM. The slip rate ranges from 10.0 to 21.4 mm/yr, which is comparable with the slip rate of the Longitudinal Valley fault (LVF), the principal active fault in eastern Taiwan, reported in previous studies. As a result, we suggest this strike-slip fault system is active, dominating the left-lateral movement in the NLV.