CHANGES OF HYDROGEOLOGICAL PROPERTIES IN THE CHOUSHUI RIVER ALLUVIAL FAN DUE TO THE 1999 CHI-CHI EARTHQUAKE, TAIWAN

This work explored hydrogeological parameters using pumping tests and the leakage from aquifards using the groundwater flow simulation methods in the Choushui River alluvial fan before and after the 1999 Chi-Chi earthquake. Three wells, SH2, YL2 and SC2, located in a compression zone with high coseismic groundwater levels, were selected to examine likely changes to aquifer characteristics. High coseismic groundwater levels and vertical peak gravity accelerations are apt to change aquifer properties. Decrease in storage coefficient ranges from 60.1% to 80.8% at the three wells, indicating aquifer compression after the Chi-Chi earthquake. Change to transmissivity (T) or hydraulic conductivity ranges from 61% (increase) to 0.8% (decrease). An anisotropic T tensor at SC2 further confirms that normal stresses induced by the earthquake consolidate soil particles, whereas soil particles are dilated laterally after the earthquake. Furthermore, the leakage greatly varies at YL2 because its enclosed aquitards generate sustained fractures. The post-seismic increasing hydraulic diffusivity facilitates to reduce the amplitude or variation of the leakage and to attain promptly an equilibrium state.