Thirty hydrological responses induced by the M8.0 Tokachi-oki earthquake in 2003 were observed at hot spring wells and an undersea coal mine in Hokkaido, Japan. Most of the decreases and increases in groundwater levels or discharge rates can be explained as a poroelastic response to earthquake-induced volumetric strain inferred from a fault model determined by dense static GPS observation. In five wells, observed groundwater-level changes and inferred volumetric strain steps induced by the Tokachi-oki earthquake are consistent with groundwater-level changes that are proportional to the inferred volumetric strain steps induced by four large earthquakes in 1993--1994.