車籠埔斷層北段之地下構造研究; A study of subsurface structures in the northern portion of the Chelungpu fault

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Title:	車籠埔斷層北段之地下構造研究;A study of subsurface structures in the northern portion of the Chelungpu fault
Authors:	李翔鈞;Hsiang-Chun Lee
Contributors:	應用地質研究所
Keywords:	車籠埔斷層;集集地震;Chelungpu fault;chi-chi;921
Date:	2003-04-30
Issue Date:	2009-09-22 09:58:28 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	論文提要內容：九二一集集地震為二十世紀末臺灣島帶來莫大的震撼，而該地震的相關斷層-車籠埔斷層也因怪異的運動模式，引發不少研究。其中，車籠埔斷層北段於石岡、卓蘭一帶東轉，並引起巨大且複雜的地表破裂，特別令學界關心。本研究利用淺層反射震測法於石岡、卓蘭間的車籠埔斷層北段附近，佈設了近百條測線，範圍含括了大甲溪以南的新社台地、石岡至卓蘭間的大甲溪和大安溪沿岸、大安溪以北的壢西坪臺地等，目的在於瞭解車籠埔斷層北段的地下構造，進而合理推論車籠埔斷層東轉及形成地表巨大破裂之可能原因。本研究利用淺層反射震測的高解析度震測剖面，並結合鑽井資料、中油深層震測剖面、地表地質調查等，歸納出以下幾點結論： 1. 東勢背斜為一以錦水頁岩層為滑脫面之滑脫褶皺（detachment fold），且褶皺向西南方向傾伏並延伸跨越至大甲溪南岸，由於受到大甲溪以南新社台地的抬升作用影響，背斜的翼間角（the interlimb angle）有逐漸向西南漸漸變大之趨勢。 2. 大甲溪以北的石圍牆向斜與其以南的頭嵙山向斜，由於兩向斜於淺處（兩公里內）並無為明顯的構造所區隔，兩向斜軸的位置可合理順勢地作一相連，推論應為同一向斜構造。該向斜構造為本區錦水頁岩層之基底構造，其他淺部構造再發育於其上方。 3. 大甲溪以南之新社台地為推起構造（pop-up structure），其下方之車籠埔斷層面向西出露地表，向北亦不斷抬升，至大甲溪南岸已很接近地表（約500公尺），此抬升可與東勢背斜軸相連，形成一道從石岡、經吊神山、至內灣之突起構造（buckle）。 4. 大甲溪南岸–東勢背斜軸之東西向突起構造，構成了地下屏障（barrier），於九二一地震發生時，阻擋了車籠埔斷層向北破裂，迫使其切穿上覆地層出露地表，形成斷層東轉並裂成數條之雁形排列。此大甲溪–東勢背斜屏障主導了車籠埔斷層北段之破裂行為。 5. 由於車籠埔斷層的主要滑脫面-錦水頁岩面於石岡附近十分地接近地表，且由鑽井資料顯示斷層滑動面受到水的作用，合理推論在兩溪附近，水有可能滲入斷層面降低斷層阻力，因此雖然車籠埔地震斷層於北段業已遠離震源，但仍有形成最大地表破裂的條件。 The 1999 Chi-Chi earthquake (Mw=7.6) produced great impact on the Taiwan island. This earthquake induced the 90km-long and NS-trend Chelungpu fault which possesses many interesting and unusual rupture behaviors. Among them, the northern portion of the fault (i.e. the Shihkang and Cholan areas) had attracted much attention of many researches. At this northern end, the fault turned east, created huge (up to 12m) and complicated surface ruptures. In this study, we used shallow reflection seismic method to detect subsurface structures of the area, attempting to find the relationship of the structure and large ruptures. Over one hundred seismic lines (each about 500 m long) have been acquired, which were distributed over a 10 x 20 km area around the Tachiahsi and Taanhsi rivers. The irregularities of the structure are believed to be the factors which control abnormal ruptures in this area. After combining the seismic sections, the drilling data, CPC deep seismic sections, and geologic maps, we obtained the following conclusions: 1. The Tungshih anticline develops above the Chinshui shale, and behaves as a detachment fold. This anticline plunges toward the southwest until reaching the axis of the Shihweichiang syncline. The interlimb angles of the anticline become broader in the southwest. 2. The Shihweichiang syncline (north of the Tachiahsi river) and the Toukoshan syncline (south of the Tachiahsi river) should be the same syncline. This syncline forms the basis of the Chinshui shale structure system. Several shallow folds (e.g. the Tungshih anticline) develop above it. 3. The Hsinshe terrace, south of the Tachiahsi river, is a pop-up structure basing on the Chelungpu fault as the sliding bottom. The Chelungpu fault surface under the Hsinshe terrace not only rises toward the west, but also climbs up near the surface toward the north. This up-lifting may connect with the Tungshih anticline and form a structural boundary (called the Tachiahsi-Tungshih boundary, TT-boundary). 4. The TT-boundary is obviously a structural barrier which may interfere ruptures of the Chelungpu fault during the earthquake. This west-east trend barrier blocks fault slippages, causing them to turn east and splitting into ‘en-echelon’ branches. 5. The Chinshui shale gives the major detachment basis for the Chelungpu fault. This layer comes close to surface (about 500 m depth) under the Tachiahsi river. A drilling data (Tanaka et al., 2002) has shown high water content (45%) within the fault gouge. It is not unreasonable to assume that the water from the two rivers may sink into the fault surface and cause certain degrees of lubrication. This could be one of the factors which result in huge ruptures around the ‘two river’ area.
Appears in Collections:	[Graduate Institute of Applied Geology] Electronic Thesis & Dissertation

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