集集大地震對陳有蘭溪流域邊坡穩定之影響

The impact of the Chi-Chi Earthquake on Slope Stability --- A Case Study in the Chenyulan Watershed

吳翊鼎(Yi-Ding Wu)

指導教授 ： 徐美玲

國立臺灣大學/理學院/地理環境資源學研究所/碩士(2007年)

https://doi.org/10.6342/NTU.2007.00510

摘要

1999年9月21日的集集大地震，除了因斷層移動所造成的直接災害外，更誘發了近26,000處的崩塌地，總面積達16,000公頃，而其震波對邊坡土體結構與舊有破裂面的衝擊與殘存效應即為本文的探討重點。
本研究以深受地震影響而發生大規模崩塌的陳有蘭溪流域為研究區，選定地震前後四場颱風事件，以由航空照片及衛星影像判識各場事件引發的崩塌地為分析對象，依地質分區，分析比較各區崩塌地的數量和密度，以及面積、比集水面積、坡度、高程以及距稜線距離等崩塌地屬性；另利用無限邊坡模式，以實際崩塌狀態率定各地層在地震前後各場事件發生時的「虛擬內摩擦角」。再根據降雨特性、崩塌地特徵與內摩擦角的變動狀況，綜合評估地震前後邊坡穩定性的演變。
分析各颱風事件所引發崩塌地的空間分佈，發現其密度隨總降雨量升高而增加。至海棠颱風為止，921地震所誘發的崩塌的再崩塌機率，比賀伯颱風所誘發的崩塌來得高，顯示921地震比豪雨對本研究區邊坡穩定具有比較長時間的影響。在內摩擦角的推估值上，岩性較為脆弱的地區的變動最為激烈，可能表示該等地質區對於降雨狀態較為敏感。
綜合分析各地層在各事件時內摩擦角的推估值，以及新崩與舊崩塌再崩者的數量及面積的變化，推測研究區的邊坡強度，因集集地震而陡降，到2005年時尚未出現明顯的回復現象，其後續發展值得持續關注。

關鍵字

集集地震 ； 陳有蘭溪 ； 無限邊坡模式 ； 虛擬內摩擦角

並列摘要

The Chi-Chi Earthquake which took place on September 21, 1999, not only caused direct destructions from its fault movements, but also induced 26,000 landslides with a total area of 16,000 ha. This study will place emphasis on the residual effects of the seismic waves on slope stability.
The Chenyoulan Watershed where numerous landslides were induced by the Chi-Chi Earthquake was selected as the study site. Four typhoon events which occurred before and after the earthquake were selected as the study cases. The landslides which took place in each of the typhoons were interpreted from aerial photos and satellite images. The number and density of landslides in each geological formation were calculated, and the size, specific catchment area, slope, elevation and the distance to ridge lines of each landslide were analyzed. In addition, pseudo internal frictional angles for each geological formation at the time of each event were back calibrated by applying the infinite slope model and the related landslide information. The evolution of the slope stability after the earthquake was evaluated based on the rainfall and landslide characteristics of each event in addition to the changing behavior of the pseudo internal frictional angles.
The spatial distribution of the landslides was found related to the spatial pattern of rainfall at each typhoon events, i.e. the density of landslides increased with rainfall amount. Comparing the landslides triggered by Typhoon Herb and by the Chi-Chi Earthquake, the later are more susceptible to reoccurrence during Typhoon Toraji. It shows that the Chi-Chi Earthquake has more profound and lasting effect on slope stability in the Chenyoulan Watershed than heavy rains. The pseudo internal frictional angles of the weaker geological formation showed more fluctuation in pseudo internal frictional angles variation, which was postulated to be caused by their sensitivity to the rainfall variation.
Based on the pseudo internal frictional angles, the number and area of the new and reactivated old landslides in each event, it is concluded that the slope stability in the Chenyoulan Watershed was sharply reduced by the Chi-Chi Earthquake, and has not recovered its stability even in 2005. Further study on the residue effects of the earthquake on slope stability is recommended.

並列關鍵字

the Chi-Chi Earthquake ； the Chenyoulan Watershed ； the infinite slope model ； pseudo internal frictional angle

參考文獻

林冠瑋 (2005) 陳有蘭溪流域的山崩作用在颱風及地震事件中與河流輸砂量之相對關係，國立臺灣大學地質科學研究所碩士論文，130。

鍾育櫻 (2005) 921集集大地震前後降雨型崩塌地特徵之比較，國立臺灣大學地

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