台灣位於菲律賓海板塊與歐亞大陸板塊交接處，花東縱谷更是屬於兩個板塊的交界上，根據GPS觀測結果，兩板塊的相對水平位移，每年約為8.2公分，其中花東縱谷兩側每年即吸收了3到4公分的水平位移量。花東縱谷上的斷層由北往南可分為花蓮、玉里、池上與台東等四個斷層區段，其中斷層活動最為明顯的就屬池上斷層，為世界知名的潛移斷層，其地表潛移的行為，也已被詳盡的報導。然而，近期於大坡國小的新事證，包括(一)池上斷層跡位置非在(地形崖)坡腳，約往東移55米；(二)主斷層的傾角為高角度，約78度; (三) 池上斷層前緣的變形於淺部5~15米間的變形行為滑動速率與乾濕季無關。以上恐將影響部分前人研究之解釋。本計劃預計利用兩年的時間，佈置地變監測矩陣、設置斜置貫穿斷層的測傾管、測深150米的地電阻影像剖面、進行反演震間斷層面潛移的位置(或深度)與滑動量，及結合已有的地質調查和地變監測成果。探討或回答底下的幾個問題: (一) 地表淺部(0~5米深)、中段(5~15米深)及深部(15~30米深)的潛變變形行為關連性為何? (二) 各深度段的潛變變形速率，是否與乾濕季的變化有關?若無，與什麼機制有關? (三) 池上斷層的主斷層的潛滑速率為何? 地表的形貌是否反映斷層滑動產生的變形? (四) 池上斷層下盤沖積層下方究竟仍是利吉層或是其他岩性? (五) 斷層的長期抬升速率? ;Taiwan island is situated on the boundary between Philippine sea plate and Eurasian continental plate. Hua-Tung longitudinal valley is considered as the suture zone of these two plates. Based on the GPS observation, the valley absorbs a large portion of horizontal displacement, 3 to 4 cm out of 8.2 cm per year; that is the relative horizontal displacement per year between these two plates. The suture zone, i.e. the Hua-Tung longitudinal fault, is commonly considered as 4 fault segments: Hualian fault, Yuli fault, Chihshang fault and Taitung fault. Among these fault segments, the activity of Chihshang fault is most significant and magnificent. Chihshang fault has been well-known as a creep fault all over the world. Its fault behavior has been well-documented and reported. However, there are some new findings: 1. Its fault trace is approximately 55 meters east of the westernmost escarpment/hill toe of the Coastal Range; 2.The fault dip angle is as high as 78 degrees; 3.The deformation pattern at the leading edge of Chihshang fault at depth of 5 to 15 meters is not associated to the seasonal precipitation. The latest findings above mentioned may affect partly some interpretations of previous works. This study is scheduled for two years in order to (sufficiently in terms of time) complete a few tasks including an array deployment for monitoring the ground deformation, an installation of oblique fault-penetrated inclinometer, a 150-deep ERT survey line and an inversion of creep on the fault plane during aseismic period. Taken together with known geologic and geodetic data, this study intends to explore/answer the following questions: 1. What is the relation/interaction of creep behaviors of the shallow ground (0 to 5 meters deep), moderate ground (5 to 15 meters deep) and deep ground (15 to 30 meters deep? 2. Whether is the creep behavior of the depths above mentioned associated with the seasonal precipitation? If not, what mechanism/factor controls their creep behaviors? 3. What is the creeping rate of the Chihshang fault at the Tapo site? Does the local topography reflect such a high slip rate? 4. What lithologic unit or rock may be beneath the alluvial gravel in the footwall? 5. What is the long-term uplift rate?
