1999年921集集大地震發生時濁水溪沖積扇出現了土壤液化、地表位移量改變以及流川流量與地下水位改變等現象，這些水文現象的改變顯示濁水溪沖積扇地下水文性質受到集集地震的影響。對水資源管理而言，了解地震對地下水文性質的可能改變是相當重要的。
本研究依據集集地震發生時濁水溪沖積扇地下水位觀測網之時水位記錄，以及自由場強地動觀測站所記錄之地震震度、尖峰加速度與地表位移量等資料，以定性及定量方式分析地震與地下水文變化可能相關性。首先繪製地下水文與地震資料空間分佈等值圖進行相似性比對定性分析，再利用碎形理論方法計算不同變量之碎形維度作定量分析。接著探討集集地震可能引致含水層特性之改變，使用兩種物理模式分別由水位變化量與垂直位移量去估算含水層孔隙率及水力傳導係數改變量，作為水資源規劃管理的參考。在現場地質材料之非均勻空間分佈推估上，使用孔隙彈性理論由同震水文反應估算地質材料參數，探討其與水文地質參數之相關性，作為以後地震機制與模擬之基本資料。
研究結果顯示垂直位移量較大區域及水位變化量較大區域與含水層高水力傳導係數區域大致符合，推測集集地震所造成的水文變化可能為含水層中之高傳導係數區域。定量分析顯示地震震度及地表垂直位移量與水位變化之碎形維度較為相似。本研究使用的兩種模式所估算出孔隙率及水力傳導係數改變量相較實際值而言非常小，顯示濁水溪沖積扇地下水文性質受地震影響並不大。本研究從地質材料性質觀點使用基本之土壤力學及孔隙彈性理論求出水井體積應變效率為2.3~36.9cm/ppm。由孔隙率變化估算體積應變結果，以水位變化模式估測值最低，垂直位移變化模式估測值最高，而文獻中均質材料之錯位模式估側值居中。由體積應變配合實際水位資料計算體積應變效率，水位變化模式結果介於74~161cm/ppm，與斷層錯位模式結果落在同一級數內，垂直位移變化模式結果介於0.03~2.26cm/ppm之間較為偏小。含水層特性與地質材料性質關聯性分析顯示，體積應變效率較大區域與水位變化幅度較大區域有一致的關係。

Liquefactions of soil, displacements of ground surface, changes of stream flow and water level have been observed in the Choshuishi alluvial fan during and after the 1999 Chi-Chi earthquake. The hydrological response of the Choshuishi alluvial fan to the Chi-Chi earthquake shows that the earthquake impacted the aquifer. Understanding the possible earthquake-induced changes of hydrogeologic properties is important for the water resources management.
In this study both hydrological and earthquake data were used to qualitatively and quantitatively analyze the possible correlations between the hydrologic response and seismic factors. These data are the hourly digital records of the groundwater level from monitoring well network and records of magnitude, peak ground acceleration (PGA) and ground surface displacements from free-field strong-motion stations in the Choshuishi alluvial fan. First we qualitatively examined the similarity of the hydrologic response and seismic factors by comparing contour maps. Then the fractal analysis was performed to quantitatively determine the similarity of the spatial distributions of different factors. Changes in porosities and hydraulic conductivity were evaluated in the main aquifers of the Choshuishi alluvial fan based on the data of groundwater level and the vertical displacement of ground surface. Poroelasticity theory was utilized to evaluate the properties of geologic material and its spatial distribution using coseismic hydrologic response. Then we investigated the correlations between the geologic properties and hydrogeologic parameters.
Our result shows that the area with larger vertical displacements of ground surface and larger changes of water level in the Chi-Chi earthquake was found to coincide with the area having a larger hydraulic conductivity. This indicates that the change of the Choshuishi alluvial fan due to the Chi-Chi earthquake may mainly occurred in the highly permeable zones. The changes of groundwater level, earthquake magnitude, and vertical displacement of ground surface were found to have similar fractal dimensions. Changes of porosities and hydraulic conductivity due to the 1999 Chi-Chi earthquake were very small based on our proposed two approaches. By using soil mechanics and poroelasticity theory, the volumetric strain efficiency is found in the range from 2.3 to 36.9 cm/ppm. The estimated volumetric strains are different from three approaches. The first model based on the change of water level has the smallest volume strain while the second model based on the displacement of ground surface has the largest one. By using the volumetric strain to compute volumetric strain efficiency, the result estimated from the first model is 74~161 cm/ppm which is in agreement with those calculated by the third model using dislocation theory. The result estimated from the second model is 0.03~2.26 cm/ppm and is smallest among the three mdels. The area with larger volumetric strain efficiency was found to coincide with the area having larger changes of water level.

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