彰化斷層及其附近構造; Detecting the Chunghua Fault and Its Neighboring Structures

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Title:	彰化斷層及其附近構造;Detecting the Chunghua Fault and Its Neighboring Structures
Authors:	許文立;Wen-Li Hsu
Contributors:	地球物理研究所
Keywords:	彰化斷層;八卦背斜;Changhua fault;Pakuashan anticline
Date:	2001-07-17
Issue Date:	2009-09-22 09:53:25 (UTC+8)
Publisher:	國立中央大學圖書館
Abstract:	彰化斷層位於八卦台地西緣，斷層的走向和台地走向一致，在八卦台地北端呈北偏西20º走向，而在台地的南端則是南北走向。彰化斷層由於在地表地形上有明顯的特徵，一向被視為斷層。但現今多為植被或沖積層覆蓋，並無明顯的斷層露頭出露，缺乏有力的野外地質證據加以證實，中油之深部震測的剖面在地表淺處的解析度不甚良好，彰化斷層在地表的位置一直無法明確的指出，故經濟部中央地質調查所將彰化斷層定位在第三類存疑性斷層。本研究主要採用淺層反射震測方法，在八卦台地西緣彰化斷層可能存在位置處施測，企圖尋找彰化斷層在地表淺處的正確位置，並推論斷層錯動的形式及機制，或檢查該斷層是否確實為活動斷層。本研究設計了十四條測線橫跨可能斷層位置，並將可能斷層位置包含於測線內加以偵測。另外設計了一組十一條短測線之廣域佈線，用以觀察八卦背斜淺部構造及其地層分布趨勢。經資料分析結果發現，在淺部地層構造中並未發現斷層的存在，並推測斷層可能存在於地下深處，以斷層彎曲折皺的方式造成八卦背斜。本研究獲得以下之結論： (1) 從震測剖面看來，八卦台地西緣之地下地層並無錯動現象，在淺處僅存在地層彎曲，故本研究認為彰化斷層的破裂祇發生在較深處，並可能形成盲斷層。斷層彎曲褶皺的模式，可以說明彰化斷層和八卦背斜之關係。 (2) 由於近地表地層未發生斷裂錯動，祇為低角度（小於40度）之褶皺彎曲，故彰化斷層應不是“斷層”，且彎曲部份仍上覆水平的台地堆積層，故其褶皺構造之形成至少已有十萬年以上，因此更不屬於“活動斷層”。 (3) 地層彎曲發生之位置（即原彰化斷層之位置）緊靠著台地之階崖，與地形關係密切。此位置並非在平原地區，而與石再添（1986）所提之員林斷層—田中斷層位置相同。 (4) 八卦背斜呈現西陡東緩不對稱之背斜構造形態（西翼25度，東翼15度），代表構造作用力主要來自東方。八卦背斜確定北延，跨過大肚溪後將北連至大度背斜。 (5) 震測剖面指出八卦台地西緣南北變形差異相當大，北端花壇高區形成較早，地層抬升角度較緩，約15度至25度。南端田中高區形成較晚，地層抬升角度較陡，約25度至40度。 (6) 從地層抬升變形的情形看來，八卦台地南端田中地區仍持續受到應力之作用，若地下存在有盲斷層，其可能引發大地震之危險性仍然值得注意。 The Changhua fault is located at the western border of the Pakuashan terrace, western Taiwan. This fault represents the most western thrust fault along the deformation front of Taiwan western foothill area. Photogeologic studies indicated that it is an active fault with apparent lineation and fault scarps. However, the seismic profiles provided by the petroleum company reveal that a standard ‘fault-bend fold’ may exist under the Pakuashan anticline. There are not apparent fault offsets found on the seismic sections. This study attempted to use the shallow reflection seismic method to map the shallow structures around the fault and to examine its activity. Over 20 seismic lines are distributed along the boundary between the plane and the Pakuashan terrace. It is surprised to find that most seismic sections are composed of flat layers even inside the area of low-level terraces. The places where the sedimentary layers start to bend are very close to the boundary of the Toukoshan formation exposed at high-level terraces. These smoothly bending layers support the model of fault-bend folds. The Changhua fault might be caused by a blind thrust fault buried under the Pakuashan anticline. The magnitude of bending is not large enough to stimulate a visible thrust at sites of the Changhua fault. We have the following conclusions: (1) From the seismic profiles, we found that the sedimentary layers are bent into curved shapes at shallow depths. The Changhua fault may have offset the layers at deep places, but not near the surface. It behaved like a blind fault. The model of fault-bend folds can nicely explain the relationship between the Changhua fault and the Pakuashan anticline. (2) The displacement of the fault dose not occur at shallow depths. Hence, we think that the Changhua fault dose not belong to the “fault”. Due to thick sedimentary layers covering curved structures, we estimate that the Pakuashan anticline may be formed at least one hundred thousand years ago. (3) Seismic profiles indicate that the degree of deformation is different between the northern part and the southern part of the Pakuashan anticline. The northern part (Hwatan High) was formed earlier than the southern part (Tenchung High) and was suffered with larger deformations. The dip angles of layers in the northern part are larger than those in the southern part. (4) It shows that the southern part of the Pakuashan anticline is still under active tectonic pressures. We still need to worry about the danger of large earthquakes which might occur along the underground blind thrusts in this area.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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