1999年921集集大地震後，相當多的研究工作投入在台灣西部麓山帶的構造活動上。而整個中部構造活動與板塊聚合所造成的造山過程密切相關，為了模擬分析此地區的構造狀態，包括滑脫面與斷層型態，有兩種方法被應用：物理模型實驗與數值模擬方法，物理模型實驗結果較偏向定性的分析，欲求得量化精確的結果則必須依賴數值模擬方法。本研究使用有限元素法的ADELI數值模擬程式進行研究，期望瞭解此研究區域的地下構造型態。 本研究參考地質與地球物理相關資料，採用彈塑性之二維有限元素法模型，設計三種簡單的滑脫面型態，並考慮台灣中部地區之主要邊界斷層，給予模型適當的邊界條件和材料參數，藉以設計出一合理的初始模型進行模擬計算，利用921震前的GPS速度場與1990~2002十年間的地震分佈，分別比對模型地表水平速度場與CPD(Cumulated Plastic Deformation)分佈作為控制，找出一合理的摩擦係數組合，進而提出地殼構造活動之解釋。 模擬結果顯示：(1)摩擦係數為一重要的參數，滑脫面摩擦係數為0.4，斷層面摩擦係數由西自東為遞增(0.1~0.5)或相等(0.3)，且滑脫面傾沒角呈40度有較好的比對結果。(2)滑脫面在通過西部麓山帶後，應延伸至80km，再以40度之傾沒角消失。(3)西部麓山帶斷層面的幾何型態為覆瓦狀構造之模擬結果，與地震分佈的比對上有最高的符合度。(4)滑脫面傾沒轉折所發育的應變帶造成的地表隆起最為顯著，且強烈影響雙冬斷層以東的地表水平速度場及應變的分佈型態。(5)最佳模型結果顯示，彰化斷層與車籠埔斷層間之地表將會是未來台灣中部西部麓山帶中速度場值最高的區域。 Efforts have been devoted to the study of tectonic activities at the foothill area in central Taiwan after Chi-Chi earthquake. Tectonic settings under central Taiwan is considered to be highly related to the orogenic processes, two methods, physical modeling and numerical modeling, are usually applied to study the structural evolution of the area by taking the geologic patterns of the decollement and boundary faults into consideration. Physical modeling tends to do the qualitative analysis while numerical modeling stresses the quantitative results. As a basis of finite element method, Adeli numerical modeling program has been used in this study in order to obtain a better understanding of the subsurface geologic structures in central Taiwan. With reference to the available geological and geophysicals data, boundary conditions and material factors have been carefully selected in designing the initial structural model in the elasto-plastic half space. By stepwisely changing the main boundary fault conditions, three types of decollement design have been tested to establish a basic model for further analysis. Modeling results, such as surface horizontal velocity field and cumulated plastic deformation(CPD), are then used to correlate with the pre-921 earthquake GPS data and the earthquake distribution pattern in 1990-2002, and thus an interpretation on the characteristics of the tectonic movement is presented. Following conclusions can be drawn from numerical modeling results: (1) Coefficient of friction(CF) is highly significant. The model, with a CF=0.4 for the decollement as well as CF’s are in ascending order(0.1-0.5) or all equal to 0.3, shows better correlated results. (2) Decollement should extend up to 80 km after crossing western foothill area, and then dips 40 degrees towards the east. (3) When geologic boundary faults are arranged in imbricate pattern, modeling results are best fitted to earthquake distribution. (4) High surface ground uplift appears at the place where decollement kinks at depth, and besides, the distribution patterns of the ground horizontal velocity and strains are also obviously influenced by the kinking of the decollement. (5) The place between Chanhwa and Chelungpu faults will bear the highest horizontal velocity field in central Taiwan in the future.
