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  • 學位論文

以三維構造幾何形貌和大地測量分析台灣西北部新竹地區之新期構造活動

Analysis of Neotectonics Based on 3-D Structural Geometry and Geodetic Measurements in Hsinchu area, Northwestern Taiwan

指導教授 : 胡植慶
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摘要


斷層活動造成之同震地表變形以及地震規模與斷層幾何形貌有緊密之關聯,故了解各區域之斷層幾何形貌即成為研究新期構造活動與地震災防治之重要課題。台灣西部麓山帶之斷層幾何形貌於近年相關研究之下愈趨明朗,但多數研究僅止於二維剖面之建立,三維斷層形貌之討論較少,因此本研究之重點即為建立新竹地區麓山帶之斷層的三維形貌以及其時空演化關係。本研究區域北起新竹斷層及新城斷層,南至鵝公髻斷層與和平斷層,包括內麓山帶與外麓山帶的區域。新竹地區的構造特性,以內麓山帶的北北東-南南西走向之逆衝斷層與緊密褶皺為主,外麓山帶則以東-西走向的走向滑移斷層與平緩寬廣的褶皺,以地震地體構造而言,內麓山帶的地震分布明顯多於外麓山帶。本研究首先建構通過新竹斷層與新城斷層的剖面,並使用2DMove軟體回復剖面,了解地質構造的演育歷史;接著,本研究亦利用GeoSec 3D軟體整合新竹地區之構造平衡地質剖面,建立深部的三維斷層幾何模型,並整合二維的地質剖面與地震活動分布;最後,使用GOCAD軟體整合三維斷層模型與地震重定位資料,呈現斷層深部形貌與地震活動於時空分布的關係。   就盆地演化而言,以地質剖面重建分析的結果得知,新竹地區於早中新世時存在發達的正斷層活動,造成整體地層往東漸增厚的現象,而打鹿頁岩反而往東尖滅。根據回復地質剖面之結果,竹東斷層、北埔斷層、新竹斷層和新城斷層為逆時序發育之斷層,發育順序依序為軟橋斷層、竹東斷層及北埔斷層,接著先發育出青草湖背斜,再被新竹斷層切穿,最後新城斷層沿層間滑動衝出地表;由剖面地層之錯移量顯示新竹斷層之錯移量為1318公尺,新城斷層則為887公尺,假設此兩斷層最早活動時間為卓蘭層沉積結束時,則新竹斷層與新城斷層長期滑移速率分別為0.43 mm/yr 及0.37 mm/yr,又因為新城斷層之近期活動速率大於0.37 mm/yr,因此新城斷層可能為近期新竹地區較活躍的構造;又跨過新城斷層的平均應變率為-0.05 μ strain/yr,除大地震造成之剪應變降後,本研究認為新城斷層的再現周期約為600-6000年。 由地震活動分布與地質剖面分析,以軟橋斷層為界,內麓山帶的地震明顯多於外麓山帶,軟橋斷層深部轉折處之團狀地震分布指示此為應力累積之處。三維空間地震分布與斷層模型則說明,軟橋斷層深部存在四團時空相近之地震,由南往北依序於1999年、2007年、2003年及2006年發生地震事件。本研究建立之三維斷層模型顯示,新竹地區內麓山帶的斷層至深部變緩,外麓山帶的斷層往深部則變陡峭。而位在內麓山帶與外麓山帶邊界的軟橋斷層淺部為高角度斷層,至深部則呈現階梯狀形貌,因此,本研究建議新竹地區的基底滑脫面由西而東為6至7公里漸變為10公里深。

並列摘要


Earthquake magnitude and coseismic deformation are strongly related to the geometry of the seismogenic fault. Consequently, the construction of a comprehensive 3-D fault geometry model for the linkage and interaction of fault systems becomes one of the essential topics in neotectonic studies. The Western Foothills of Taiwan is undergoing an active deformation of the Taiwan orogeny demonstrated by the frequent seismicity and destructive earthquake events. The study region includes several faults-and-folds systems from Hsinchu and Hsincheng faults in the north to Okungchi and Hoping faults in the south in the internal and external western Foothills in Hsinchu area. The structures in Hsinchu area show wide and gentle folds associated with NNW-SSW trending thrust faults in internal Foothills and tide and steep folds associated with the E-W trending thrust faults in external Foothills. From the seimostectonic viewpoint, the earthquakes frequently occur in internal Foothills than external Foothills. Therefore, the main purpose of this study is to investigate the linkage of subsurface structures between internal and external western Foothills in Hsinchu area. To this purpose, we restore a cross-section across Hsinchu and Hsincheng faults, and clarify the overall structural evolution of major fault systems based on this restoration. By integrating eleven CPC geological cross-sections and the profile in this study, we use GeoSec 3D software to construct 3D fault geometry model beneath Hsinchu area. Finally, we use GOCAD software to integrate 3D relocated seismicity and 3D fault model in order to explore the relationships between active structures and seismicities. In terms of the profile reconstruction, the thickness of strata gradually increasing eastward was induced by abundant normal faults activating in early Miocene. However, Talu shale pinched out instead. According to restoration of balanced cross section, the Juanchiao fault, Chutung fault and Peipu fault are in-sequence fault systems, meanwhile the Hsinchu and Hsincheng faults are out-of-sequence fault systems. The Chingtsaohu anticline formed following the Peipu fault. After that, Hsinchu fault cut through the Chingtsaohu anticline and Hsincheng fault, bedding fault, formed in the end. The displacements along the Hsinchu and Hsincheng fauts are 1318 m and 887 m, respectively. We assumed the faults formed after the deposition of Cholan formation, therefore, the long term slip rates for Hsinchu and Hsincheng faults are 0.43 mm/yr and 0.37 mm/yr respectively. Also, short term slip rate of Hsincheng fault is larger than 0.37 mm/yr. As a result, it implies Hsincheng fault is possible a recently active structure in Hsinchu area. The average strain rate across Hsincheng fault is -0.05 μ strain/yr, and the coseismic strain drop divided by strain rate gets 600-6000 years. Therefore, we suggest the recurrence interval of Hsincheng fault is 600-6000 years. Base on the analysis of seismicity and geological profiles, the numbers of earthquakes are larger in internal than external foothills. Clusters at the bend of Juanchiao fault at depth imply that it is the location of stress generating. According to 3D seismicity and fault model, earthquake events occur northward in 1999, 2007, 2003, 2006. The 3D fault model indicates faults at depth become gentler in internal foothills, while faults in external foothills turn into steeper. The boundary fault, Juanchiao fault, shows subsurface high angle thrust and stepped fault geometry at depth. Furthermore, the detachment becomes deeper eastward, from 6-7 km to 10 km.

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