| 摘要: | 九二一集集地震（MW=7.6）是一個十分特殊的地震，震源位在南投縣集集鎮附近，深度 8公里，引發車籠埔斷層再次逆衝滑動，造成地表長達 90 公里的斷裂，但在南北二段斷層錯動量有很大的差異，在北段離震央50公里處有巨大錯動（高達9公尺），但南段靠近震央錯動卻小許多。由地表破裂來看，車籠埔斷層緊隨著錦水頁岩分佈，若以Suppe提出的〝薄皮逆衝理論〞來解釋此次地震機制，錦水頁岩可以視為一滑脫面，而地震觸發車籠埔斷層即沿著此滑脫面向上逆衝。 早期中國石油公司在台中盆附近地區域做了許多地球物理測勘，這些資料顯示車籠埔斷層的上盤為低角度向東傾斜的單斜構造，但也有一點向南傾斜。為了進一步瞭解集集地震的發生機制，對於滑脫面之錦水頁岩的研究十分的需要。最近，國科會委託中油公司做了數條深部反射震測剖面，結果明白的顯現出向東傾斜之錦水頁岩層（車籠埔斷層面），甚至可以到達震源的位置（Wang et al., 2002, TAO）。 本研究利用淺層反射震測法，在烏溪至濁水溪、以及車籠埔斷層至雙冬大茅埔斷層之間的區域內，佈下四十多條南北向以及東西向的廣域測線，並配合中油公司深部震測剖面做為控制，希望能更清楚瞭解整個研究區域的地下構造，並與地表地質結合，解釋車籠埔斷層錯動之機制。結果如下： (1)由東西向剖面發現，地層皆向東傾且傾斜角度由西向東漸漸和緩，較為特別的是在集集剖面中集集大壩以東的測線，其東傾的角度反而較大，似有一斷坡（ramp）存在哮貓斷層以東。 (2)本研究認為哮貓斷層是存在的，主要是因為中寮東西剖面與集集東西剖面之間傾角變化的不同，顯示著中間必定有一構造存在，此構造即哮貓斷層，而集集剖面中的斷坡也就是哮貓斷層所形成的。 (3)由南北向剖面發現，研究區域內的地層南北方向的起伏變化與地表地形的變化一致，岩層由南往北為一上下起伏波浪狀變化；在地層為向下凹陷位置處，其地表位置正好皆為河流，如坪林溪、樟平溪。在烏溪附近地層往北有一較大的抬昇。 (4)南北剖面中濁水溪南岸測線結果較為不同，地層為向南傾斜的現象，推測應與哮貓斷層有所關連，該斷層下有一斷坡，將地震震源卡在其下方。 (5)車籠埔斷層地下構造形貌影響著地表錯動量變化，也就是說，地震時在斷層面上之位移錯動分佈，主要由斷層面之形貌所掌控。 The 1999 Chi-Chi earthquake (M=7.6) in Taiwan was a relatively unique seismic event which activated the Chelungpu thrust fault with extraordinarily large surface ruptures. The earthquake’s hypercenter is at 8km deep and locates near the ChiChi town in Nantou. The Chelungpu thrust fault is 90km long, lying mostly in a north-south direction and with a reverse thrust dipping shallowly to east. A thin-skinned thrust model is employed to explain the behavior of this earthquake fault. In geology, the Chelungpu fault almost slides along a sedimentary layer, the Chinshui shale. It acted as a kind of detachment surface and the Chi-Chi earthquake triggered the Chelungpu fault to slide along the detachment. From the early CPC’s (Chinese Petroleum Corporation) data about the Taichung basin, we have known that the Chinshui shale layer is not only dipping to east, but also to south with a little tilt angular. In order to further investigate the behavior of this earthquake fault, it is necessary to construct the 3D structure of the Chinshui shale. In this paper, we used the shallow reflection seismic method to detect the structures in the area between the Wuhsi river, the Choshuisi river, the Chelungpu fault and the Shuangtung fault. Over 40 seismic lines are distributed in this area, which are divided into two parts: one along the east-west direction and the other along the north-south direction. After combining the results of these shallow seismic lines and the deep seismic profile provided by CPC (supported by the NSC), we start to obtain a gross 3D structure picture of the area, which can be used to interpret the behavior of the earthquake fault We obtain the following conclusions: 1)From the seismic profiles in east-west direction, we find that all sedimentary layers are dipping to east and the dipping angle decreases toward east. However, in the Choshuihsi area, the dipping angle becomes larger. A ‘ramp’ structure is implied in this area. 2)From the seismic profiles in north-south direction, we find that the variation of stratum almost parallels to the variation of surface topography. Up and down undulation of the stratum exists in the north-south direction. A general dipping to the south is observed. 3)The structures in the southeastern side the Hsiaomiao fault shows quick dipping to the southeast. This is interpreted as a ramp which may trap the earthquake hypercenter. |
