臺南—高雄地區西部麓山帶之中洲構造與其鄰近構造再評估

Abstract

臺灣人口稠密的臺南—高雄地區面臨著潛在的地震風險。在過去臺灣地震模型的孕震構造資料庫中，中洲構造被認為是該區域的主要孕震構造，然而其位置、幾何形態及活動性仍存在爭議。本研究旨在重新評估中洲構造及其周邊可能的活動構造。本研究利用構造地形分析，結合 20 公尺解析度的數值地形模型與 RTK-GNSS 技術進行野外地形調查，以確認活動構造的分布位置。此外，研究中亦利用前人鑽井資料計算長期垂直變位速率，並推算孕震參數，進而評估可能的地震危害。 研究結果顯示，可能與構造相關的撓曲崖主要分布於中洲台地西緣及丘陵地前緣，由七條可能的活動構造所形成。根據撓曲崖的清晰度、連續性及構造長度，本研究將這些活動構造分為三類：(l) 主要的可能孕震構造，包括北中洲構造、南中洲構造及小崗山斷層，具有清晰、連續的撓曲崖形貌及較長的構造長度；(II) 可能活動的次要構造，包括潭頂構造、北關廟構造及赤崁構造，其構造長度較短，撓曲崖形貌較不明顯，且分布不連續，較難單獨破裂引發地震；(III) 存疑的活動構造，例如南關廟構造，其構造活動的地形證據較為不足，可能僅由岩性邊界的差異侵蝕所造成。在這三類中，第 I 類活動構造的可能地震危害最大。 本研究進一步計算第 I 類構造的垂直變位速率：北中洲構造為 4.7 至 5.9 mm/yr，南中洲構造為 4.6 至 13.7 mm/yr，小崗山斷層為 3.7 至 4.3 mm/yr。推算結果顯示，第 I 類構造可能引發的震矩規模為 5.9 至 6.4，而南北中洲構造的聯合破裂可能會導致規模大於 7.0 的地震。此外，第 II 類及第 III 類構造也可能因為地震誘發而產生地表變形，進一步加劇可能的地震風險。 本研究所繪製的北中洲構造與南中洲構造的位置與臺灣地震模型中的中洲構造有明顯差異，建議修正其位置。雖然該區域可能存在斷層潛移或泥貫入體活動，導致推算的再現週期與歷史地震紀錄不符，但辨識出的構造顯示出顯著的地震風險，仍需持續監測並進行深入研究。

The densely populated Tainan-Kaohsiung region in western Taiwan faces potential earthquake risks. In the published seismogenic structure database of the Taiwan Earthquake Model, the Chungchou structure has been identified as the main seismogenic structure in this area. However, its location, geometry, and activity are still under debate. This study aims to reassess the Chungchou structure and other potential active structures in the surrounding region. The study combines tectonic geomorphic analysis with a 20-meter-resolution digital elevation model and detailed field surveys conducted using RTK-GNSS to delineate the distribution of active structures. In addition, long-term vertical separation rates were calculated using previous borehole data, and seismogenic parameters were estimated to assess potential earthquake hazards. The results suggest that flexural scarps potentially related to structural activity are primarily distributed along the western edge of the Chungchou Tableland and the front of the hilly region. These scarps are formed by seven possible active structures. Based on the clarity and continuity of the flexural scarps and structural length, the active structures are categorized into three types: (I) Primary and likely seismogenic faults, including the North Chungchou structure, South Chungchou structure, and Hsiaokangshan fault, characterized by clear, continuous flexural scarps and relatively long structural lengths; (II) Secondary faults with potential activity, including the Tanding structure, North Guanmiao structure, and Chihkan structure, which have shorter structural lengths, less distinct flexural scarps, and discontinuous distributions, making them less likely to rupture independently and generate earthquakes; (III) Suspected faults, such as the South Guanmiao structure, which lacks sufficient geomorphic evidence of tectonic activity and may instead be an erosional scarp at lithological boundaries. Among these, Type I structures present the greatest earthquake hazard. This study further calculates the vertical separation rates for Type I structures: 4.7–5.9 mm/yr for the North Chungchou structure, 4.6–13.7 mm/yr for the South Chungchou structure, and 3.7–4.3 mm/yr for the Hsiaokangshan fault. The results suggest a moment magnitude of 5.9 to 6.4 for individual Type I structures. A joint rupture of the combined Chungchou structures could potentially generate an earthquake exceeding magnitude 7.0. Additionally, Type II and Type III structures may be triggered during an earthquake, causing surface deformations and further increasing potential earthquake risks. The locations of the North and South Chungchou structures mapped in this study differ significantly from the Chungchou structure in the Taiwan Earthquake Model database, suggesting the need for a revision. While aseismic creeping or mud diapir activity in the region appear to cause discrepancies between the estimated recurrence intervals and historical earthquake records, the identified structures pose substantial earthquake hazard and require continued monitoring and further research.