In order to research the coseismic and postseismic groundwater changes for a groundwater system consisting of multiple aquifers we studied the mechanism of coseismic groundwater changes at Yudani hot spring, which is an artesian well supplied by at least three aquifers, in Tottori Prefecture, Japan. We observed the changes in discharge and water temperature at this site in response to earthquakes, tides, and atmospheric pressure changes. For the groundwater system of Yudani hot spring a realistic model composed of three aquifers having different frequency dependencies of groundwater responses to crustal strain and atmospheric pressure was adopted. The tidal and barometric responses of the discharges from the three aquifers, which were cyclic responses over a limited frequency band, were calculated, and the physical parameters of the three aquifers were estimated by fitting a theoretical model to the observed responses. Then, using the strain response over the whole frequency determined by the estimated physical parameters, the theoretical responses of the discharges from the three aquifers to step-like volumetric strain changes were calculated. As a result, the three-aquifer model with different frequency dependencies quantitatively explained the observed groundwater changes at Yudani hot spring due to earthquakes. In addition, the values of the coseismic volumetric strain steps were estimated based on groundwater changes at Yudani hot spring. These values did not agree with the theoretical coseismic volumetric strain changes at Yudani hot spring due to the earthquake fault slips, which were calculated under the assumption that the Earth's crust is a homogeneous elastic half-space medium. It is possible that a local strain release, such as aseismic slip at a fault near Yudani hot spring, might have caused the large strain changes estimated from the coseismic groundwater changes. ¿ 2000 American Geophysical Union