臺灣西部麓山帶的東苗栗構造再評估

Abstract

臺灣坐落於歐亞板塊與菲律賓海板塊的聚合性板塊邊界上，為評估臺灣的地震風險，由國科會資助的臺灣地震模型(TEM)計畫與其團隊，利用多種研究方式提供地震潛勢的科學評估基礎資訊，其中一項工作即是根據構造地形特徵建立了臺灣的孕震構造資料庫。然而，這些構造中有些僅依據地形特徵所辨識，初步的提供災害評估使用，因此還需要更詳細的地質分析。 在苗栗區域，TEM過去辨識出了七條可能的孕震構造，其中除了東苗栗構造以外，其他構造在過去均有詳細的地質與地形調查，提供這些構造的支持證據。相比之下，東苗栗構造仍未有詳細調查，其可能的構造崖成因也存有爭議。為解決此問題，本研究重新評估了東苗栗構造，以構造地形概念為基礎，使用5米解析度的數值地形模型分析階地面高程，並透過RTK-GNSS測量關鍵的地形崖，同時在野外針對關鍵的地形崖沿線進行地質與地層調查，分析紅土化階面以探討區域抬升速率特徵。 本研究以階面特徵、地層對比以及沖積層厚度，共三個面向討論東苗栗構造的存在與否。階面的結果顯示，利用比高與紅土分析，東苗栗構造僅部分上盤地區的抬升速率較快，但此一現象不必然需要由東苗栗構造活動造成。地層對比則指出，過去由於垂直後龍溪的地形崖特徵，被視為重要構造活動證據的福基階崖，其周遭地層並無明顯的位移，表示此處可能未經歷長期的構造錯動。沖積層厚度結果則顯示，東苗栗構造線兩側沖積層厚度並無明顯差異，且下盤的沖積層厚度並沒有明顯增加，甚至偶有底岩出露於河床上，表示東苗栗構造沒有對後龍溪流域造成明顯的沉積差異。綜合以上三個面向的結果，顯示此區域似乎不受東苗栗構造影響，東苗栗構造可能並不存在。 至於福基階崖，雖然曾被臺灣地震模型孕震資料庫認定為構造崖，但是經過本研究的調查顯示，後龍溪過去曾大幅改道，形成超過九十度的轉彎，此情形反而支持福基階崖亦可能由河川侵蝕形成。地方文獻與古地形圖亦記載福基階崖下方曾為後龍溪主流河道，顯示其為河蝕崖的可能。綜合上述觀察，本研究認為沒有足夠證據能夠證明東苗栗構造必須存在，因此建議新版的臺灣地震模型孕震構造資料庫，應重新評估並可考慮移除此一構造。

Taiwan lies on the convergent boundary between the Eurasian and Philippine Sea plates. To assess the island’s seismic hazard, the Taiwan Earthquake Model (TEM) has integrated multiple approaches to provide a scientific basis for seismic hazard potential assessments. One major effort is the establishment of a database of seismogenic structures based on tectonic geomorphic features. However, some of these structures were identified solely on the basis of geomorphic features, to also provide preliminary input for hazard assessment, and thus require further geological verification. In the Miaoli region, TEM identified seven potential seismogenic structures. Except for the East Miaoli structure, the others have been supported by detailed geological and geomorphic investigations. In contrast, the East Miaoli structure has not been thoroughly examined, and the origin of its inferred fault scarp remains debated. This study reassesses the East Miaoli structure by applying tectonic geomorphic concepts, analyzing terrace surfaces with a 5m resolution digital elevation model, surveying key scarps using RTK-GNSS, conducting stratigraphic and structural field investigation, and evaluating lateritic soils of terraces to infer regional uplift characteristics. This study evaluates the existence of the East Miaoli structure from three perspectives: river terraces, stratigraphy and alluvial deposits. Results from river terrace elevations and laterite development suggest that only parts of the hanging wall display relatively rapid uplift, which does not necessarily imply structural activity. Stratigraphic comparisons further indicate that the bedrocks around the Fuji Scarp—previously regarded as a key indicator of fault activity due to its trend perpendicular to the Houlong River—does not display significant offset, suggesting the absence of tectonic displacement. Footwall alluvial deposits also exhibit no systematic thickening, showing no substantial differences in thickness across the fault trace. With local bedrock exposures in the riverbed, this implies that the East Miaoli structure did not produce differences in sedimentation patterns. As for the Fuji Scarp, the distribution of higher terraces indicates a major river turn occurred in the past, suggesting that the Fuji Scarp may instead be of fluvial erosional origin. Some historical documents and paleo-topographic maps also record that the area below the Fuji Scarp was once main Houlong River channel, implying the possibility of the scarp being just a terrace riser. Based on these observations, this study concludes there is no sufficient evidence for the existence of the East Miaoli structure. Hence, this study suggests the TEM seismogenic structure database to reassess the East Miaoli structure and to consider its removal.