Effects of seismic ground motion and geological setting on the coseismic groundwater level changes caused by the 1999 Chi-Chi earthquake, Taiwan

The groundwater level changes induced by the 1999 Chi-Chi earthquake were well recorded at the monitoring wells in and around the Choshui River alluvial fan, Taiwan, which is adjacent to the focal region. We analyzed the coseismic groundwater level changes related to the geological setting and seismic ground motion. In a typical fan area, the groundwater levels coseismically rose and those amplitudes increased as the ground acceleration and hydraulic conductivity became larger. In the slope area near the earthquake fault, the groundwater levels coseismically dropped and those amplitudes increased as the ground acceleration became larger. The liquefaction and permeability enhancement, whose degrees depend on the geological setting and seismic ground motion, might explain the characteristics of the coseismic groundwater level changes in the Choshui River alluvial fan.