從苗栗沿海地區之階地地形探討苗栗前緣構造之活動特性

Abstract

台灣島位於歐亞板塊與菲律賓海板塊的聚合板塊邊界上，因呂宋島弧與歐亞大陸邊緣的碰撞造山而形成。苗栗沿海地區位於台灣的西北造山前緣，此地的丘陵地形緊鄰著海岸線，顯示此地區位於造山活動的影響範圍之內，然而在地質調查所公布的活動斷層圖當中，此區域並沒有任何的活動斷層，前人文獻當中亦鮮少討論此區域內構造的活動狀況。因此本研究希望能藉由觀察苗栗沿海地區的地形，了解此區域內的構造活動狀況。 本研究在數值高程模型的輔助之下，觀察後龍溪到大安溪間沿海地區的地形，搭配野外調查以及槽溝挖掘當中對階面沉積物以及底岩的觀察結果，分析該區域的階地與河流分布之關係，以及階地形成與構造活動之間的關係，藉此確認構造的活動程度以及位置。從區域內廣泛分布的階地可推測，整個區域確實受到構造抬升所影響，其中以苗栗前緣構造的影響最為廣泛，因此本研究針對此構造進行進一步分析，由野外所見的地層位態以及地形分析的結果，重建此構造之地下形貌，並藉由階面沉積物的定年確認其活動速率。此外分析階面所受到的變形，除苗栗前緣構造外，亦可觀察到斧頭坑斷層以及倒梯崎構造的活動跡象。 比較以上結果與前人的GPS以及震測剖面，發現斧頭坑斷層與苗栗前緣構造之間的關係較為複雜，考慮到整個竹苗地區的構造活動模式，以及台灣造山向南傳遞的現象，本研究認為斧頭坑斷層未來可能會截穿苗栗前緣構造，成為此區域最主要的活動構造之一。 依照階地沉積物的定年結果進行推算，以上提到的構造皆符合地質調查所制定的第二類活動斷層之標準，值得當地政府以及居民多加關注。

The island of Taiwan is located at the convergent boundary between the Eurasian and the Philippine Sea plates. The ongoing collision between the Luzon arc and the Eurasian continental margin produced the Taiwan mountain belt. In the Miaoli area in northwestern Taiwan, the hilly area extends all the way to the coast, suggesting that this area is influenced by tectonic activities. However, no active fault has been officially proposed in this area. Previous studies have not paid attention to structural activities in this area either. Therefore, this study analyzed the terraces in the Miaoli coastal area to understand the active structures and their activities. This study focuses on the coastal area between the Houlong River and the Daan River. We analyzed the terrace distributions to study the relationship between the formation of terraces and structural activities, and observed the terrace deposits and bedrocks in outcrops and trenches. The wide spread terraces in this area suggest that the entire area is uplifted by the activity of a proposed Miaoli frontal structure. We constructed the subsurface geometry of this structure with bedrock and geomorphology observations, and calculated the slip rate of this structure with terrace sediment dating results. Based on the observation, the terraces are also deformed by the activity of two other structures, the Futokeng fault and a secondary Daotichi structure. Based on the observations of GPS analysis and seismic profiles, we suggest that the structural development of the Miaoli frontal structure is related with the general structural evolution of the Hsinchu and Miaoli area, and the southward propagation of collision in Taiwan. The Futokeng fault may cut through the Miaoli frontal structure in the future and become one of the main active structures in this area. Based on the results of the terrace sediment dating, many structures in this area can be considered as active structures. Therefore, it is important to consider future earthquake hazards of these structures.