none;Shallow Geological Structure and hydromechanical behaviour of an active reverse fault at convergent plate boundary: the Chihshang Fault, eastern Taiwan

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Title:	none;Shallow Geological Structure and hydromechanical behaviour of an active reverse fault at convergent plate boundary: the Chihshang Fault, eastern Taiwan
Authors:	牟鍾香;Mu,Chung-Hsiang
Contributors:	應用地質研究所
Keywords:	活動斷層，斷層帶滲透率，斷層帶含水層;active fault;surface tilt;fault zone aquifer;fault zone permeability
Date:	2012-06-21
Issue Date:	2012-09-11 18:26:33 (UTC+8)
Publisher:	國立中央大學
Abstract:	池上斷層地處菲律賓海板塊與歐亞大陸板塊的交界處，斷層的滑移作用造成地表每年2-3公分的錯移量，堪稱世界上位移速度最快的斷層之一。從長期地表位移測量分析顯示，池上斷層震間滑移之速度具有季節性的變化，快速的潛變活動大多發生在雨季，而乾季時，斷層活動近乎停滯，降雨與地下水的自然洩降似乎對於池上斷層的滑移速度影響甚遠。進一步而言，如此活躍的斷層滑移行為在穿越沉積層至地表的過程中，斷層帶的裂隙增減與幾何形貌的確會影響其周圍含水層的水力特性。本研究彙整近廿年來池上斷層的大地測量以及地球物理探勘成果，尤其著重池上鄉錦園村活動斷層監測網，來了解淺層斷層出露於錦園村的活動特性；為了更深入了解淺層地層的岩性與組態，本研究團隊在上盤進行三口岩心構造分析，勾勒出池上斷層在淺層地層的幾何形貌。池上斷層在錦園村受到錦園溪沖積扇作用影響，主斷層在地下30-40米處分為三支斷層，靠近西側的兩支分支斷層其傾角傾東，傾角為34-42°和60-65°，略高於主斷層傾角(30°)，最東邊的分支斷層傾角向西16°，由定年資料推斷，池上斷層上盤抬升速率為每年2.3 ± 0.1公分。除了岩心判釋之外，同時設置地下水觀測網，來紀錄斷層帶上下盤地下水位隨季節的變化，並進一步實施水力試驗以獲取斷層上下盤的滲透係數。結果顯示，在時間域的分析上，上盤的滲透系數在2008年四月到九月其間驟然上升，其結果與池上斷層北邊的微震密度上升趨勢一致，推測微震誘發斷層帶附近新生裂隙，導致地下水流通孔隙增加，進而促使滲透系數上升；就空間分布而言，孔隙水壓在斷層上下盤產生了10米的地下水頭差異，推測為滲透系數的差異和異向性所造成的水力梯度，除了紀錄天然水壓變化，本研究另外致造人為孔隙水壓擾動，利用注水試驗增加斷層帶內的孔隙水壓，並使用傾斜儀紀錄水力梯度變化所造成之地表傾斜，其結果顯示，斷層帶為一個阻水邊界，滲透系數在斷層帶上反映出明顯的異向性，促使地下水流沿著斷層走向流動，更加確認斷層在非固結沉積層的阻水效應。The Chihshang Fault is one of the most active creeping faults in the world at a rate of 2 cm/yr, which is situated along a plate suture between the Philippine Sea and the Eurasian plates in eastern Taiwan. Near the surface, the Chihshang Fault developed in the Holocene unconsolidated gravel layers. This fault behaviour apparently is be influenced by the hydraulic characteristics around the fault zone and in the vicinity of aquifers.In this study, we combined a variety of measurements and analyses at the Chihshang Active Fault Observatory (CAFO), including surface-rupture mapping, three shallow borehole core analyses and kinematic analysis of geodetic measurements, in order to decipher the near-surface fault geometry. We found that the Chihshang Fault has a three-branch fault system with a rather diffused fault zone in the Chinyuan alluvial fan at CAFO, which is composed of at least 100 m thick alluvial deposits. Outside of the Chinyuan River channel, the Chihshang Fault exhibits a single fault system. Combining the uplift rate and subsurface profiles from trench excavation, we interpret that the three fault branches locally developed the structure at the uppermost 30-40 m unconsolidated gravel layers during the last few thousand years. Based on the ratio between the levelling vertical displacements and the creep meters and GPS horizontal displacements, we obtained dip angles of 34-42°, 60-65° and 16° for two west-vergent thrusts and an east-vergent backthrust, respectively, for these three branches. By compiling the ages data in the boreholes, trenches and terraces, we estimated a long-term relative uplift rate of 2.3 ± 0.1 cm/yr in the hanging wall of the Chihshang Fault and an average alluvial sedimentation rate of about 1.1 ± 0.1 cm/yr during the past a few thousands years.In order to better understand how the effects of pore-fluid pressure variations in the aquifer within the alluvial gravels influences the near-surface behavior of the Chihshang Fault, nine observation wells of groundwater were drilled at depths ranging from 30 to 100 m through the aquifer from the footwall to the hanging wall. Monitoring of natural pore pressure variations in piezometers, monthly slug experiments (few seconds), and long duration pumping/injection experiments (hours to days) were carried out during 2007-2011. Together with the subsurface electrical resistivity imaging, surface fracture investigations, and core geological analysis, we identified an aquifer zone that is deformed and fractured by the fault zone. The results showed that the permeability of the fault zone is smaller 1 order than that of the footwall. The variance of permeability caused a 10 meter step of groundwater level from the hanging wall to the footwall in the view of spatial domain. On the other hand, repeated hydraulic tests revealed that the permeability varied with time increased 20 times in the hanging wall from 2007 to 2011. A drastic increase of the permeability in the fault zone was observed from April to September 2008. Two possibilities are interpreted this phenomenon: (1) the increased cumulated earthquake events changed the stress field along the Chihshang Fault and caused the new fractures around the fault zone; (2) the vertical displacement revealed that the dilatation may be happened in the fault zone which increased the porosity to induce a drastic increase of permeability in-situ.
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