921集集地震造成地表大幅度移位(同震變形)，地表破裂長度超過96公里，地表最大位移量將近10公尺。根據GPS(全球衛星定位系統)在地震前後觀測所得之位移量，車籠埔斷層以東水平位移量向西北水平移位平均約3∼4公尺，最大在東勢達8.5公尺；高程抬升量則隨著車籠埔斷層由南往北遞增。值得注意的是在斷層以西的水平位移量朝向東南方，高程移位則呈下降；而在埔里、國姓一帶，其高程位移量也呈下降。本研究利用此區域長期GPS監測之地表變形資料及此次地震所發生的同震變形，藉由數值模擬方法，配合地質調查所得之地質背景資料，解釋此地區活動構造的運動模式，並藉以模擬台灣中部的活動構造。 本研究採用有限差分連續體分析軟體 FLAC (Fast Lagrangian Analysis of Continua)作分析工具，假設岩體為彈性物體，使用界面元素模擬斷層，分別進行車籠埔斷層地區之剖面及平面模擬，並且進行各項參數之敏感度分析，以瞭解各參數對活斷層錯動之影響。分析模式需考慮網格建立、重力平衡(剖面分析)、初始條件、邊界條件、材料參數、界面元素、同震變形等主要分析條件。 網格建立部分：剖面網格是參考Suppe (1987)之台灣中部構造剖面所建立的簡化剖面，大小為30公里寬，15公里深，並且各往東西兩側延伸30公里當邊界元素。平面網格範圍為一長200公里、寬100公里的矩形，長軸方向與板塊擠壓碰撞方向N55°W相同，且為避免邊界效應影響模擬，於網格矩形上下各延伸50公里當邊界元素。 剖面模擬分析結果顯示：車籠埔斷層以東，地盤大規模的向西逆衝上升，但在距斷層以東20公里處開始有發生位移量向下的現象，斷層以西地盤則有微量向東及向下位移現象，分析結果大致符合GPS所監測之同震變形位移資料。 水平模擬分析結果顯示：在斷層上盤主要沿車籠埔斷層向西北位移，下盤則相對的向東南位移，若是考慮斷層分段錯動，網格位移向量會更相似GPS所監測之同震變形位移向量，斷層南邊水平位移量偏西，越往北邊偏北的分量越多。 Associated with the chi-chi earthquake, surface rupture extended more than 96 km long. The largest displacement was about 10 m according to GPS data measured before and after the earthquake. To the east side of the Chelungpu fault, the average horizontal displacement is about 3~4m and oriented northwest. The largest displacement in Tungshih reached 8.5 m. The vertical displacement shows uplifting and increases from south to north. To the west side of the Chelungpu fault, the horizontal slip direction is oriented southeast and the vertical displacement is subsiding. Besides, in the farther eastern side of the Chelungpu fault, such as Puli and Kaohsing, the vertical displacement also shows descending. This earthquake resulted in surface rupture and ground displacement. It’s a good opportunity to verify the relation between crust deformation and coseismic motion in this area. In this study, we apply numerical modeling to simulate the active structure of the central part of Taiwan, and compare the result with the GPS data obtained before and after the earthquake. This study used finite difference analysis software ‘’FLAC’’ (Fast Lagrangian Analysis of Continua). We assume the rock is elastomer and use interface elements to simulate the fault. Then we proceed to simulate the profile and the horizontal plane projects. Special steps which are used to simulate coseismic displacement are different from previous study. We also proceed parameter sensitivity analysis. We consider the relationship with each parameter and the surface displacement. Finally we attempt to interpret how the fault dislocated. The profile simulation shows that the eastern side of the Chelungpu fault thrusts and uplifts relative to the western side. The western side of the Chelungpu fault and the eastern side from 20 Km away, these areas' vertical displacement were descended. It is the same phenomenon with GPS observations. The plane-view model simulation shows that the fault of the hanging wall thrusted toward northwest, and the foot wall moved toward southeast. If we consider that the fault begin to fracture from south to north segment, the surface displacement vectors are conformable with GPS data.
