地形對地震震波反應之影響

Abstract

台灣位於板塊交接帶，造山運動盛行，地形陡峻、起伏劇烈，山地及丘陵即佔約全島總面積的三分之二，並地震為極常見的天災之一。由歷史上許多的地震案例記載中，皆指出不規則地形會對震波產生放大效應，並可能是導致破壞的主因之一，尤其在坡度較陡的邊坡或山頂處，放大作用極明顯，可達五、六倍甚至是十倍以上。 本研究主要以前人進行之大型振動台試驗及小型振動台試驗為參考依據，建立數值模型模擬邊坡在受地震力作用下之動態發展、震波加速度反應及破壞行為，並藉由改變邊坡之坡高和坡角，分析及探討地形效應兩主要影響因子與邊坡內部加速度反應變化之關聯性，以及可能之破壞行為發展。由模擬結果可知，土層厚度之作用整體上皆大於坡角幾何形狀之作用，其中坡角幾何形狀在接近土壤剪力強度時對震波之影響最明顯，其餘時候則作用微弱。 接著由案例分析輔助探討數值模擬之結果，本研究收集南投縣日月潭氣象站及魚池國小地震測站之地質環境、地形變化及地震紀錄等資料，進行現地數值分析模擬，現地狀況雖存在著自然環境條件等因素，使其加速度反應及破壞行為與振動台模型之結果不完全相同，但整體上，現地案例模擬之放大情形顯著，並且亦有土層厚度作用大於坡角幾何形狀作用的現象。

Taiwan is located at the boundary between the Philippine Sea Plate and the Eurasian Plate which results in frequent orogenic activities and its dramatic mountain terrain. Two-thirds of the total area of Taiwan consists of mountains, uplands and hills and earthquake is the most frequent natural disasters in Taiwan. A great deal of historical seismic literature has shown that uneven terrain can produce expanding shock waves during an earthquake which may be the main reason for landslide hazard. Steep slopes or mountain tops are especially affected by the amplification of ground motion amplitude which can easily reach 5, 6 or even 10 times more. This research mainly uses results of the previous studies on large and small shaking table model test as well as numerical simulation to simulate the dynamic responses, deformation and displacement of slope-lands under the influence of seismic activities. Two main factors of the topographic effect including the slope height and angle, this research were analyzed and discussed. The simulation results show that the overall impact of soil layer thickness is more significant than the geometry of slope angles. Also the geometric shape of slopes has a more prominent impact when it approaches the shear strength of material, otherwise, the impact is rather insignificant compared to the effect of layer thickness. For the field case study, the geological environment profiles, geographical changes and seismic records of the Central Bureau Weather strong motion station in Sun-moon Lake, Nautou County and the strong motion station in Yuchi Elementary School were collected and analyzed. Although the geographical differences and various natural conditions lead to outcome different of shaking table results, the overall stimulation still indicates that the amplification effect is remarkable, and ground layer thickness has greater influence than the geometry of slope angles.