臺灣中部集集攔河堰附近應力演化史之初探

災害性地震發生時，不僅會造成人員傷亡及財物損失，也會因為斷層的活動讓地形地貌改變。1999年的集集地震造成了近100公里的地表破裂，此次地震的主破裂面為車籠埔斷層。GPS資料顯示，車籠埔斷層上盤在集集地震的事件中產生了抬升以及錯動。此次抬升事件切穿了濁水溪，使侵蝕基準面相對向下，形成向源侵蝕，而2000年集集攔河堰的啟用，降低濁水溪下游沉積物的堆積速率，由於自然與人為的作用下，促使濁水溪自名竹大橋至集集攔河堰之間的岩層出露。由車籠埔斷層向東，此段露頭大多為單斜構造，且有少部分的淺部變形，直至集集攔河堰以西約1公里處，則出現了東北走向的初鄉斷層與其上盤向北傾沒的頂溪州背斜。這樣的淺部地表變形可能是地震所造成的岩層變形。本研究將利用斷層擦痕與古應力的分析，瞭解初鄉斷層上盤的頂溪州背斜附近的應力場特徵、隨地塊掘升的應力變化，以及大地應力的構造意義，最終建構研究區的構造演化史。研究結果顯示本研究區內應力場先後順序為: stage 1 為正斷層應力場；stage 2 為走滑—逆斷層應力場，以西北西—東南東至西北—東南為擠壓方向；stage 3為走滑—逆斷層應力場，以北北西—南南東至北北東—南南西為擠壓方向；stage 4 為走滑—逆斷層應力場，西北—東南擠壓方向及西北—東南及東北—西南拉張方向。其中又可細分出 Substage，stage 2a 及 stage 2b 為同一應力場，但作用於兩種不同構造型態，分別為水平層面及正立褶皺；同理，stage 3a 及 stage 3b 為同一應力場，但作用於兩種不同構造型態，分別為正立褶皺及傾倒褶皺。彙整上述結果，本研究探討各期應力場的地質意義：第1期為正斷層應力場，由地層年代與沉積環境推測，此時研究區域仍位於前緣斷層以西的前陸盆地中。第 2a 時期，地層位態仍為水平狀態，但應力場轉變為東－西擠壓，由於西部麓山帶的斷層是由東向西漸漸發展，推測 2a 的擠壓應力場可能是受蓬萊造山運動所引起。2b 時期，依然為東－西擠壓應力場，但地層已傾斜，推斷此時期初鄉斷層已經形成。3a 時期，應力場轉變為南－北擠壓應力場，由應力場與初鄉斷層走向關係判斷，此時初鄉斷層為具逆移分量的左移斷層，層面狀態為正立褶皺。3b時期，應力場依然為南－北擠壓應力場，層面狀態為傾倒褶皺，頂溪州背斜向北傾沒可能是南邊的鹿寮斷層向北推擠，使初鄉斷層與鹿寮斷層間形成東—西走向的褶皺構造。第 4 期應力場轉為東—西向擠壓，推測初鄉斷層運動方式為逆移斷層，但第4 期不單單只有逆斷層以及走向滑移應力場，還包括了局部正斷層應力場，有此正斷層應力場結果的露頭位置皆靠近頂溪州背斜軸部，推可能是縱彎褶皺外圍拉張所造成的破裂。

關鍵字

斷層滑動資料；應力演化；層面校正；初鄉斷層

並列摘要

When a catastrophic earthquake occurs, it will not only cause casualties and property losses, but also change the terrain due to the activity of the fault. The Chi-Chi earthquake in 1999 caused nearly 100 kilometers of surface rupture. The main rupture surface of this earthquake was the Chelongpu fault. GPS data shows that the hanging wall of the Chelongpu fault was uplifted and dislocated during the Chi-Chi earthquake. This uplifting event cut through the Zhuoshui River, causing the erosion base level to be relatively downward, forming source erosion. The opening of the Chi-Chi Dam in 2000 reduced the accumulation rate of sediments in the lower reaches of the Zhuoshui River. Under the action, the outcrop between Ming-Zhu Bridge and Chi-Chi Dam were exposed. From the Chelongpu fault to the east, the outcrop in this section is a homocline structure, until about 1 km west of the Chi-Chi Dam, a NE-trending ChuHsiang fault and its hanging wall plunging northward Dingxizhou anticline appeared. Such shallow surface deformation may be caused by the deformation of the formation caused by the earthquake. This research will focus on the Dingxizhou anticline on the hanging wall of the ChuHsiang fault, and discuss the evolutionary relationship between the Dingxizhou anticline and the ChuHsiang fault. This thesis uses the analysis of fault slip data and paleostress to understand the stress field characteristics of the Dingxizhou anticline on the hanging wall of the ChuHsiang fault, the stress changes with the excavation of the block, and the tectonic significance of the geostress, and finally construct the study area History of tectonic evolution. The research results show that the sequence of the stress field in the study area is: stage 1 is the normal fault stress field; stage 2 is the strike-slip-reverse fault stress field, with the compression direction from NW-SE east to NW-SE; stage 3 is strike-slip —reverse fault stress field, the compression direction is NW—SE to NNE—SSW; stage 4 is the strike-slip—reverse fault stress field, the NW—SE compression direction and the NW—SE and NE—SW tension direction. Among them, Substage can be subdivided. stage 2a and stage 2b are the same stress field, but act on two different structural types, namely the horizontal plane and the upright fold. Similarly, stage 3a and stage 3b are the same stress field, but Acting on two different structural types, namely upright folds and plunging folds. Combining the above results, this study explores the geological significance of the stress fields in each period: The first period is the normal fault stress field, and from the stratigraphic age and depositional environment, it is speculated that the study area is still located in the foreland basin west of the front fault. In period 2a, the stratum is still in a horizontal state, but the stress field changes to east-west compression. Because the faults in the western piedmont belt gradually develop from east to west, it is inferred that the compression stress field in 2a may be caused by the Penglai orogenic movement. cause. During the 2b period, there was still an east-west compressive stress field, but the stratum was tilted. It is inferred that the ChuHsiang fault has been formed during this period. During the 3a period, the stress field changed to a north-south compression stress field. Judging from the relationship between the stress field and the ChuHsiang fault strike, the ChuHsiang fault is a left-moving fault with a reverse shift component, and the plane state is an upright fold. During the 3b period, the stress field was still a north-south compression stress field, and the surface state was a dumped fold. The Dingxizhou anticline tilted to the north. It may be that the Luliao fault in the south pushed northward, making the ChuHsiang fault and the Luliao fault. An east-west trending fold structure is formed between. The fourth phase of the stress field is converted to east-west compression. It is inferred that the ChuHsiang fault movement mode is a reverse fault. However, the fourth phase includes not only the reverse fault and the strike slip stress field, but also the local normal fault stress field. The outcrop positions of the normal fault stress field are all close to the axis of the Dingxizhou anticline, which may be the result of the rupture caused by the extension of the periphery of the longitudinal bending fold.