車籠埔斷層北段斷層帶之材料特性與構造分布之初步研究

Abstract

台灣1999集集地震(Mw=7.6)的震央位於東經120.81°北緯23.86°，並在台灣中部車籠埔斷層的沿線造成了明顯的地表破裂，其斷層的位移量在南北兩段有很大的差異，在北段最大的垂直位移達到8.5公尺。而車籠埔斷層主要滑動面是位於錦水頁岩內的層間滑動，為台灣西部麓山帶的主要的逆衝斷層之一。 本研究區域以台中大坑大里溪一帶露頭為主，其地層主要屬於錦水頁岩。根據台灣車籠埔斷層深鑽計畫的研究結果，集集地震滑動面於A井岩心深度1111公尺的斷層泥內，由於岩心標本代表性較為局部且較為珍貴，因此，希望藉由台中大坑大里溪野外露頭的調查、採樣，能夠取得較大區域性的資料及充分斷層泥標本加以分析，同時利用磁感率異向性及岩石薄片的觀察分析以了解在穿過斷層帶時，其物理性質上的改變，如應力方向的改變，礦物顆粒的粒徑變化關係等。 根據野外量測所得到之資料顯示，在大坑大里溪區域一帶的走向滑移斷層主要以左移斷層較為明顯，且在距離集集地震地表斷層破裂面60公尺、130公尺及200公尺附近之上盤的南北兩岸均有被左移斷層所切過，並由於其地層位態的突然改變及最大古主應力軸的方向往南順時鐘旋轉約80-100°，初步推測可能為斷層活動時塊體產生順時鐘旋轉的現象；並由破裂密度的量測可以得知在距離集集地震地表破裂上盤約50公尺的範圍內為一強烈剪動的地區。此外，由三個相互垂直的斷層泥薄片的分析可以發現，在靠近滑動面時，礦物顆粒有明顯變細的現象，並配合礦物顆粒的優勢排列可以得知在滑動面上，礦物顆粒排列的方向在葉理狀斷層泥處(053°)相對於下盤圍岩(037°)也有順時鐘旋轉的現象。由磁感率異向性及礦物顆粒橢球體的Flinn圖分析，斷層泥同樣呈現板狀(oblate)的變形，且在通過斷層帶時，葉理強度上升、線理強度下降，異向性隨葉理強度上升而上升。經由磁感率異向性的測量，發現在圍岩所受到的最大主應力方向多為大地應力的方向(130°)，而在葉理狀斷層泥中最大主應力方向(150°)，相對於下盤圍岩亦有順時鐘旋轉的現象。初步推測塊體旋轉可能為葉理狀斷層泥形成的原因之一。由磁感率橢球體及礦物顆粒橢球體的綜合比較可以推論磁感率橢球體所記錄到的多為基質的變形，而礦物顆粒橢球體多為剛體旋轉的現象。

On Sep 21 of 1999, a destructive earthquake (Mw=7.6) took place around the Chi-Chi Town of central western Taiwan and its hypocenter is located at 120.81°E, 23.86N°. The surface rupture during the Chi-Chi earthquake is parallel and closed to the N-trend Chelungpu fault of about 100km in length. The displacement between north and south segments of Chi-Chi rupture is dramatically different. The northern rupture reaches the largest displacement of ~8.5 m. The Chelungpu fault is one of the major thrusts at the fold-and-thrust belt of western Taiwan and slips within the Chinshui Shale. The study area is along the Dali riverbed closed to the drilling site of the Taiwan Chelungpu-fault Drilling Project (TCDP) in Dakeng area of the Taichung city. Most of stratigraphy in the study area is the Chinshui Shale. According to the results of TCDP, the 1999 Chi-Chi slip zone of TCDP Hole_A is located at the fault gouge zone of 1111m depth. In order to complement the limitation of geological information and fault gouge sample on the retrieved core, the detail mapping of representative region and collection of the fault gouge sample on the field were conducted. Furthermore, the analysis of anisotropy of magnetic susceptibility and examination of microfabric on fault gouge samples was performed for understanding the variation of physical properties across the fault zone, such as the change of paleostress direction and grain size variation. Based on field mapping, most of strike-slip faults is sinistral faults. Especially, crosscutting of sinistral fault on the bedding is distinct at the hanging-wall locations of 30 m, 160 m, and 200 m away from the surface rupture. Also, at these locations, bedding was bended and the direction of the maximum stress cloclwisely rotated about 80°-100° from the regional direction of N130° to local N030°, suggesting the possibility of local block rotation. From the distribution of fracture density, a strong shear zone between the Chi-Chi surface rupture and the hanging wall of 50m away from surface rupture is identified. The result of microstructural examination indicates that the reduction of grain size within the fault gouge. Also, the long-axis orientation on slip plane rotates clockwisely in the foliated gouge with respective to the foot wall. The Flinn diagram of magnetic susceptibility anisotropy and mineral grain ellipsoid illustrates the oblate shape within the fault gouge. Furthermore, compared with the wall rock, the degree of foliation and anisotropy increase but the degree of lineation decrease in the fault gouge. The result of anisotropy of magnetic susceptibility displays that maximum stress orientation in the wall rock is parallel to regional stress direction (130°). However, maximum stress orientation in the foliated gouge is aligned to 150°. It also shows that maximum stress direction rotates clockwisely in the fault gouge with respective to the foot wall. The block rotation might be the reason of developing the foliated gouge. The evaluation of results between magnetic susceptibility anisotropy and mineral grain ellipsoid suggests that deformation of the magnetic susceptibility ellipsoid is the output of the matrix deformation, and the change of grain ellipsoid is mainly the reflection of rigid body deformation.