921集集大地震前後降雨型崩塌地特徵之比較

Abstract

台灣破碎的地質、陡峻的地形和強烈的降雨特性，經常引發山地邊坡的崩塌。921地震在台灣中部地區造成大規模的崩塌，其效應是否持續影響該區邊坡的強度，而改變降雨事件觸動邊坡滑動的閾值，值得進一步探究。因此本研究以921地震作為分界，前後各選兩場颱風事件，以1992年寶莉颱風、1996年賀伯颱風、2001年桃芝颱風和2004年敏督利颱風所觸發的崩塌地為研究對象。利用事件前後的航空照片進行崩塌地判識，經正射化後，加以數化登錄，配合40公尺解析度的DEM資料，並利用GIS軟體計算各崩塌地的面積及其所在邊坡的坡度、地形區位及比集水面積等相關地形參數，並進行統計分析。再分析各場降雨事件的雨量、降雨強度，再比較各事件相對應崩塌地的數量及相關地形特徵，推估其與各降雨特徵間的關係。另外，利用結合無限邊坡理論與坡地水文特徵並加入SINMAP邊坡穩定模式進行模擬，以不同的內摩擦角推測邊坡不穩定區，然後與實際崩塌地進行比對，校正反推各場事件發生時的最佳內摩擦角值。 由地震前後的崩塌特徵分析以及模式模擬結果發現： （1）變質岩區岩體較為破碎，其崩塌地密度及面積百分比均比沉積岩來得高，但是921地震後，沈積岩區的崩塌數量則大幅增加。（2）地震前後，各研究區崩塌地發生個數及面積與各降雨事件的最大降雨強度呈現正相關性。（3）各流域在地震前後，由較大強度的降雨事件所引發的崩塌地，其邊坡坡度有低於小強度降雨事件的崩塌地的趨勢。 （4）地震後各降雨事件所引發的崩塌地，其比集水面積有低於發生於地震前者的傾向。（5）二廍坑、三廍坑兩個變質岩區各地層地震前的內摩擦角有較地震後高的趨勢。綜合各項分析，顯示921地震有削弱岩層強度的效應，不過因為大部分鬆動的邊坡物質已經崩塌，加上崩塌處植生的快速恢復，此種效應漸趨於緩和。

Steep terrain and fragile geology in the mountainous area in Taiwan makes the slope susceptible to landslides during heavy rainfall and earthquakes. In 1999, the Chi-Chi Earthquake induced a great deal of landslides in central Taiwan. It is dubious if the weakening of the slope material by the earthquake still influences slope stability and changes the landslide-triggering threshold in some rainfall characteristics. Typhoon Polly in 1992, Herb in 1996, Toraji in 2001 and Mindulle in 2004 are selected for study. The former two took place before the Chi-Chi Earthquake, while the other two occurred after it. Aerial photos are interpreted to identify landslides triggered by each of the typhoons, and individual landslide is then delineated and digitized. Based on the 40 meter resolution DEM, terrain attributes such as size, slope, slope location and specific catchment area for each landslide are calculated using and the results are statistically analyzed. In addition, SINMAP is applied to back calculate the internal frictional angle of each geologic stratum during corresponding typhoon. It is concluded on the calculation results that (1) the fragile metamorphic strata are more subjected to landslides than the sedimentary strata. However, after the Chi-Chi Earthquake, landslides in the sedimentary strata have increased a lot. (2) Both before and after the Chi-Chi Earthquake, the number and area of the landslides increase with increasing rainfall intensities. (3) In all the three sub-watersheds, the estimated internal frictional angles of the slope material after the earthquake tend to be smaller than those took place before the earthquake. (4) The average specific catchment area for the landslides which took place after the earthquake also tends to be smaller than those before the quake. (5) The internal frictional angles of the metamorphic rock after the earthquake are also smaller than it was before. It is shown that the Chi-Chi Earthquake weakened slope material, but since most part of the loose materials has slipped away and vegetation has colonized the once stripped ground surface, the after effects of the Chi-Chi Earthquake have been alleviated.