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.