地震於坡地崩塌能量門檻值之長期影響研究

Abstract

台灣因地理位置特殊，颱風和地震的作用下，使得邊坡崩塌頻傳，對人民生命財產造成威脅。在1999年集集地震後，鄰近震央的中部山區發生多起崩塌，邊坡結構也在地震力作用下變得鬆散且脆弱，因而在震後每逢颱風、豪雨侵襲，常會誘發大量的新生或復發崩塌，可視為地震對邊坡造成的長期影響，使誘發崩塌所需之臨界降雨下降，直至震後數年方才逐漸恢復。 本研究以台灣中部之大甲溪、眉溪和陳有蘭溪集水區為研究區，先對集集地震前後7期災害事件之崩塌地圈繪資料進行崩塌特性分析，觀察歷年各期崩塌中地震誘發崩塌特性的變化，以此分析地震長期影響；而地震之影響仰賴於地震波的能量傳遞，故以能量角度切入進行崩塌門檻能量分析，將地震與降雨誘發崩塌量化為誘發能量，分析震後地震、降雨誘發能量之消長，以瞭解誘發邊坡崩塌所需之能量門檻和地震對其之影響，並與崩塌特性變遷的結果相互驗證。 研究結果顯示，震後崩塌率和降雨能量臨界線的變化趨勢相反，顯示誘發邊坡崩塌所需之誘發能量趨於常數，而地震長期影響可視為地震儲存於坡體中的殘餘能量，使震後破壞所需之降雨能量減少；而地震的殘餘能量會隨時間衰減，其影響大小和衰減速度受地形放大效應影響，地勢較平緩之淺山區所受地震長期影響於震後5年已然微弱，而較陡峭的高山區，至震後10年影響也已不顯著。

Due to the unique geographic location of Taiwan, typhoons and earthquakes have caused landslides frequently, threatening people's lives and properties. After the 1999 Chi-Chi earthquake, severe landslides occurred in mountain area of central Taiwan. The slope body became fragile subjected to the significant seismic force. Thereafter, typhoons and heavy rainfall often caused new or reoccurring landslides, which are likely affected by the long-term effects of the earthquake. Such effects lead to lower threshold rainfall for landslides, and gradually diminished several years after the quake. In this study, we analyzed the long-term effect of three watersheds located in central Taiwan. Inventories of events after Chi-Chi earthquake were used to observe the variations of landslide characteristics which implied the long-term effect. Then, we quantify the contribution of earthquake and rainfall to triggering landslides, and analyze their variations before and after earthquake to determine the energy threshold for triggering landslides. Finally, we verify the results with the observed long-term effect. This study shows changes of landslide area ratios after earthquake and critical rainfall energy after the quake are complementary, indicating the energy threshold for triggering landslides is about constant, while the long-term effect can be regarded as residual seismic energy accumulated in the slopes, leading to lower threshold rainfall. The residual energy dissipated with time and appeared to relate to topographic site effect. The residual energy in the mountain areas with lower elevation and gentle slopes had already diminished 5 years after the quake, while it took 10 years for the residual energy in higher elevation and steeper slopes to fade away.