| Summary: | 碩士 === 國立成功大學 === 資源工程學系碩博士班 === 91 === 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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