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  • 學位論文

斜移斷層引致上覆土層變形行為之研究

The deformation of overburden soil induced by oblique slip faulting

指導教授 : 林銘郎
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摘要


台灣地區因板塊構造作用頻繁,依據經濟部地質調查所公告,33條活動斷層其中11條為斜移斷層,斷層錯動時對鄰近斷層帶結構物受到的地震影響,除了強地動外,另一主要破壞因素為斷層基盤錯動所導致近地表岩土層土體變形及地表破裂,而斜移斷層錯動時會因其走向及傾向滑移量比之差異,影響破裂跡與剪裂帶在三維中的發展,對近地表土層變形行為之影響,實有進一步研究之需要。 根據近年來國內外災害性地震的研究,斷層錯動引致地表破裂案例中,土層受斷層引致變形之因素,受控於斷層傾角、滑移比S/H(滑移量/覆土厚)及上覆土層材料性質等參數影響,透過國內外地震案例彙整其地表變形影響範圍可由公尺等級到公里等級,並利用鄰近鑽探資料,統計其土層厚度並正規化地表變形影響範圍,其量化成果可用於後續物理模型試驗設計。 利用室內砂箱試驗模擬,為了解斷層兩側覆土層地貌分布、地表線型、斷層錯移在近地表影響範圍、坡向分析等,簡化參數採用90度斷層傾角,填充無凝聚性砂,針對滑移角、基盤滑移比及覆土厚度等,共設計八組不同條件之試驗。 結果顯示滑移比達0.15時地貌形成丘谷地形,而後期再錯動,對既有地形高程差異不大。地表破裂跡線形的角度,隨著滑移量增加至0.2時,破裂跡與斷層面投影的夾角會到達一個峰值約±40度後下降。而最大影響範圍為純走向滑移斷層,影響範圍約為對稱出現於斷層投影線兩側,合計約1倍覆土厚度。當斷層活動時具有傾向滑移分量,影響範圍位置將會偏態,移至地形陷落側,而走向及傾向滑移分量共存時,斷層影響範圍較純走向滑移模型小,約為0.5倍覆土厚度。 斷層錯動後,原平坦地面受到斷層作用擠壓伸張後坡向顯著變化,而坡向方位主要受傾向滑移影響,優勢坡向面積比達40%以上,亦可指示斷層線形,回歸斷層滑移角與地表優勢坡向面積比之關係,得到2次多項式分布,可利用地貌坡向回推斷層滑移角。剖面觀察斷層尖端發展時,傾向滑移之模型,斷層尖端開始向上時,其斷層面角度較陡;當斷層尖端擴展接近地表時,其角度變緩,顯示地形上高程差會擠壓斷層面至地勢較低區域。 選用2016熊本地震與砂箱試驗進行比對,推論此區斷層主要為走向滑移,及下期事件破裂跡可能影響區域提供減震考量,並計算現地土層厚度6.8至8.8公尺。 透過本研究之簡化模型進行基本行為觀察及定量分析,亦可協助推測斷層破裂跡於上覆土層中發展情形,有助於判斷破裂跡可能出露位置,以及未來發展情形,提供斜移斷層引致上覆土層變形之未來災害防治的重要貢獻。

並列摘要


Tectonic activities derived from the convergence of the tectonic plates frequently occur in Taiwan. The Central Geological Survey, Ministry of Economy Affairs (CGS, MOEA), proclaimed 33 active faults over Taiwan, and 11 of them are oblique-slip faults. according to the investigations of well-known disastrous earthquakes in recent years, ground deformation (ground strain and co-seismic surface rupture) induced by faulting is one of the causes for engineering structure damages in addition to strong ground motion. However, spatial development and propagation of the shear zone would be influenced by different ratios of strike-slip to dip-slip by oblique-slip faulting. Therefore, further study on deformation behavior of soil near the ground surface due to faulting, and on its effects on engineering structures within the influenced zone is necessary. Recently, according to domestic and foreign studies about disastrous earthquake inducing fault movement and surface rupture, the soil deformation induced by faulting are influenced by three factors, fault dip, S/H ( slip measurement/ height of covering soil) and properties of soil materials. After referring to both domestic and foreign examples associated with surface-deformation influence range covering the scales from meter to kilometer, we can analyze the thickness of soil with drilling data to normalize the surface-deformation influence range in these examples. The result of the analysis can be quantified and then applied in designing a physical testing model. Sandbox models are used to study the morphology and lineament of overlying soil deformation along the two sides of the fault plane, and influential region and slope directions by the oblique-slip faulting. To simplify the sandbox models, the dip angle of 90° of the fault plane was set and in cohesive sand was used as soil. Eight tests, with parameters of different rake angles, slip ratios of the bedrock, and thickness of the overlying soil, were designed. The results show that hill-and-dale topography forms at the slip ratio of 0.15. Such topography did not change much if the faulting continued. When the slip ratio is 0.2, the angle between the lineament of the surficial ruptures and projection of the fault plane increased to a peak value of ± 40° and then declined. The pure strike-slip faulting led to the maximum effect region, and the region appeared symmetrically on both sides of the projection of the fault plane, being about same amount as the thickness of the overlying soil. When the faulting had a dip-slip component, the effect region appeared as a skewness pattern, moving to the subsidence side. When both the strike-slip and dip-slip occurred, the effect region was smaller than that derived from the pure strik-slip model, being about a half of the thickness of the overlying soil. Due to the faulting, the various slope directions appeared from the initial flat ground surface being compressed and stretched. The azimuth of the dip direction was mainly affected by the dip-slip, with a ratio > 40% for the area of the dominant slope directions, which can indicate the fault lineament. By regressing, the relationship between the rake angle of the fault and the area ratio of the dominant slope directions was a quadratic polynomial. Based on the function, the rake angle of the fault can be inferred by the slope directions. The propagation of the fault tip in a cross-section direction in the models revealed that the angle of the fault plane was steeper when the fault tip became upward; when the fault tip propagated close to the ground, the angle of the fault plane became gentle, showing that the topographical height difference forced the fault plane to propagate to the lower regions. After matching the result of our sandbox test and the field data of the earthquake event occurred in Kumamoto Japan, 2016, we can deduce that the fault mainly slips along the strike direction, and further suggest possible methods to mitigate seismic hazards around the fault fracture trace in next earthquake event. Through observation and quantitative analyses by using the simplified models in this study, rupture development within the overlying soil derived from the oblique-slip faulting can be inferred. Potential positions and future development of the rupture exposures can also be deduced, and also can contribute retaining and protections from disasters of deformation of the overlying soil resulting from oblique-slip faulting.

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