利用地質剖面探討初鄉活動斷層構造特性;Determination of Structural Characteristics of the Chusiang Active Fault Using Geological Cross-Sections

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Title:	利用地質剖面探討初鄉活動斷層構造特性;Determination of Structural Characteristics of the Chusiang Active Fault Using Geological Cross-Sections
Authors:	張中威;Chang, Chung-Wei
Contributors:	應用地質研究所
Keywords:	初鄉斷層;活動斷層;地震地質;平衡剖面;Chusiang fault;geological model;cross-section
Date:	2023-08-16
Issue Date:	2023-10-04 16:21:49 (UTC+8)
Publisher:	國立中央大學
Abstract:	台灣中部麓山帶的主要斷層系統是車籠埔－大尖山斷層系統，一系列的逆衝斷層被認為發育於一滑脫面，初鄉斷層位於車籠埔斷層與雙冬斷層之間，呈東北走向的活動斷層，西南側併入車籠埔斷層，東北側向雙冬斷層延伸。1999年台灣中部的集集地震，車籠埔斷層沿線形成約100公里的地表破裂，根據同震地表GPS資料顯示，跨初鄉斷層的地表同震位移方向有顯著差異，然而未有報導指出初鄉斷層跡沿線有同震的地表變形。根據台灣機率式地震危害度分析，指出初鄉斷層在未來50年內有高達37%的機率發生規模6的地震，為了解初鄉斷層再活動風險，須建立初鄉斷層的三維構造幾何模型，然而，過往研究對於初鄉斷層的幾何形貌有所爭議，且在幾何上難與車籠埔斷層模型連接，無法完整解釋地表觀察到的現象，也無法解釋地層厚度在空間上的變化，因此，了解初鄉斷層與周圍構造之間的幾何關係及地層厚度變化對於地質模型至關重要。本研究使用前人地質圖資，搭配地質鑽探與震測資料，參考前人的構造模型、集集地震同震GPS位移向量，建立五條地質剖面，並分析合理的區域構造推演。依據桂竹林層厚度的變化，認為集集地區地下存在一正斷層較為合理，且於桂竹林層沉積時有多次的活動，使桂竹林層沉積時於斷層上下盤有厚度上的差異；根據本研究地質剖面，初鄉斷層淺部呈高角度向東傾，深部漸緩，與車籠埔斷層相連接於同一滑脫面。區域構造推演結果，滑脫面發育時受正斷層所影響，深度於濁水溪兩岸有所不同，進而影響逆衝斷層的發育，使濁水溪南、北岸的構造行為有所不同；初鄉斷層可能為大尖山斷層及雙冬斷層的連接型斜向斷層，作為兩者間的空間調適，可能與大尖山斷層有相似的斷層性質，為具有左移性質的逆斷層。 ;The main fault system in the foothills of central Taiwan is the Chelungpu-Tachienshan Fault system, a series of thrust faults believed to develop along a detachment. The Chusiang Fault is located between the Chelungpu Fault and the Shuangtung Fault, with northeast-oriented. It merges southwestward into the Chelungpu Fault and extends northeastward toward the Shuangtung Fault. The 1999 Chi-Chi earthquake in central Taiwan caused surface ruptures along the Chelungpu Fault, spanning approximately 100 kilometers. According to coseismic GPS data, there were significant differences in the coseismic displacement directions across the Chusiang Fault. However, there have been no reports indicating coseismic surface deformation along the Chusiang Fault trace. Based on Taiwan′s probabilistic seismic hazard analysis, the Chusiang Fault has up to a 37% probability of experiencing a magnitude 6 earthquake within the next 50 years. To understand the reactivation risk of the Chusiang Fault, it′s essential to establish a three-dimensional geometric model of its structure. However, previous research has disputed the geometric morphology of the Chusiang Fault and struggled to connect it geometrically with the Chelungpu Fault model, thus failing to fully explain observed surface phenomena or variations in spatial layer thickness. Therefore, understanding the geometric relationship between the Chusiang Fault and surrounding structures, as well as variations in layer thickness, is crucial for the geological model. This study employs previous geological map data, alongside geological drilling and seismic data, referencing previous structural models, coseismic GPS displacement vectors from the Chi-Chi earthquake, to establish five geological cross-sections and create a reasonable regional structural inference. Considering the thickness variation of the Kueichoulin Formation, it is inferred that a blind normal fault exists underground in the Chi-Chi area, which was active during the deposition of the Kueichoulin Formation, leading to differences in thickness between the hanging wall and footwall during the deposition of the Kueichoulin Formation. Based on the geological cross-sections in this study, the shallow segment of the Chusiang Fault dips steeply to the east while gradually flattening at depth, connecting to the Chelungpu Fault along the same detachment. According to the regional structural inference, the development of the detachment is influenced by the blind normal fault, with varying depths along both sides of the Zhuoshui River. This subsequently affects the development of thrust faults, resulting in differing structural behaviors on the north and south banks of the Zhuoshui River. The Chusiang Fault may serve as a connecting splay between the Tachienshan Fault and the Shuangtung Fault, acting as spatial adjustment between the two faults and possibly sharing similar faulting characteristics with the Tachienshan Fault, exhibiting a left-lateral thrust fault.
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