Coseismic hydrological changes associated with dislocation of the September 21, 1999 Chichi earthquake, Taiwan

Being situated in the immediate vicinity of the 1999 Chichi earthquake epicenter, 178 automatic-recording monitoring wells of the Choshui fan-delta provided a very valuable data set for testing whether the coseismic water-level changes in confined aquifers can be explained as a poroelastic response of well-aquifer systems to coseismic volumetric strain. This study demonstrates that the polarities of the observed coseismic water-level and river discharge changes are in good agreement with those of the static volumetric strain calculated by a dislocation model, using the well-constrained rupture model of the seismogenic Chelungpu fault. The puzzling "bull-eye" patterns formed by the equipotential lines of the coseismic water-level changes in the confined aquifers in the middle parts of the fan-delta represent pressure enhancement due to their being situated at the pinch-out ends of conglomeratic layers.