https://scholars.lib.ntu.edu.tw/handle/123456789/37493
標題: | 「地震及活斷層研究」-地震地質─車籠埔斷層沿線同震單斜褶皺構造幾何形態與運動機制之初步研究 | 其他標題: | Preliminary geometric and kinematic analysis of monoclinal structures formed during the 1999 Chichi earthquake in central Taiwan | 作者: | 楊昭男 | 公開日期: | 2003 | 出版社: | 臺北市:國立臺灣大學地質科學系暨研究所 | 摘要: | 依據地面的觀察及已發表的照片顯示,921 集集大地震時在地面產生的崖 狀地形,絕大多數為褶皺崖,而非斷層崖。此種褶皺崖的形狀可以當作一個只 有一個斜面的單斜褶皺(monocline)。這種單斜褶皺是因為盲斷層在擴展過程 中,在斷層面尖端線前方的岩層受到剪動作用所造成的結果。 從通過褶皺崖的槽溝所顯現的褶皺型態特徵以及褶皺與斷層的關係大多可 以trishear fault-propagation folding 運動模式加以解釋。這些褶皺的特徵為,由 地面往地下深處:褶皺的緊密度由地面的開放單斜褶皺逐漸轉變為緊密的褶皺 或甚至是倒轉的褶皺,前翼的傾角變陡、岩層的厚度變薄,前翼向斜軸部的岩 層變厚,前翼背斜軸部與前翼向斜軸部的形狀均為弧形。若有斷層的話,斷層 通常是切過前翼岩層變薄程度最為嚴重的反曲點位置,而且從斷層尖端沿著斷 層往下,岩層的斷距會越來越大。 從褶皺崖的形狀及崖高沿著車籠埔斷層跡的變化以及槽溝剖面所顯現的構 造高度的變化,顯示斷層跡沿線在地震時所形成的地質構造,是由一些構造高 度(structural relief)由北向南逐漸降低的雙傾伏的單斜褶皺(doubly-plunging monoclines)串連而成的褶皺帶。依據斷層擴展褶皺理論,推測這可能是斷層移 動向量在垂直斷層面走向的分量由北向南逐漸減低所造成的現象。 From observations in the field and photo published, it is revealed that only a few of the escarps, formed during the 921 Chichi earthquake in 1999, is truly fault escarp which only occurs at the river bed without or with very thin sediments, and most of the escarps are monoclinal fold, because the dipping surface of the escarps is still the original ground surface other than newly-produced fault surface. This fact suggests that most part of the fault that caused the earthquake is still a blind fault and the monoclines are fault-propagation folds. From the variation of the escarp height along the Chelongpu shear zone, it is deduced that the zone can be divided into several doubly-plunging folds that produce culmination and salient structure along the shear zone. In trench excavations cross the monoclines, it is shown that the layering of recent sediments such as clay, sand and conglomerate under the ground surface have been severely deformed into folds and/or faults. Although the detailed structures are complex, but in general, the monoclinal folds have some characteristic feature in common. The overall monoclinal fold is tightening downward, and broadens upward toward ground. The forelimb becomes steeping and thickens near the synclinal hinge, thinning between the anticlinal and synclinal hinge. In the case where the forelimb of monoclinal fold was truncated by thrust fault, the displacements (fault separation) along the fault are decreasing toward the ground surface. All such characteristics strongly suggest that the monoclinal fold is resulted from trishear fault-propagation folding (Erslev, 1991) during fault event(s). It is also noted that in a monocline, the higher the escarp height, the steepness of the forelimb. This also indicates that the monocline is a plunging fold, plunging from the portion where the height of the escarp is high toward the portion where the height of the scarp is low. It seems that the formation of individual doubly plunging monocline may due to the lateral propagation of the tip-line of the fault. The saddle structure may result from the linkage of two separate propagating faults in the soft sediments underneath ground surface. The factors that controlling the formation of the regional culmination and salient structure along the Chelongpu shear zone may include the variation along strike of the fault in magnitude of slip, dip of the fault plane, epical angle of the triangular shear, the ratio between the rate of tip line of propagating fault and the amount of slip on the fault (i.e. P/S ratio) as suggested by Cristallini & Allmendinger (2001). In the present case, it seems that the change of slip and the dip of the fault plane along strike maybe more important than others. This imply that the higher uplift height in the Choulan and Shihkang area the dip angle of the fault is steeper than that in the area from Shihkang to Chushan where uplift height is lower. |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/11827 | 其他識別: | 912119M002025 | Rights: | 國立臺灣大學地質科學系暨研究所 |
顯示於: | 地質科學系 |
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912119M002025.pdf | 253.89 kB | Adobe PDF | 檢視/開啟 |
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