Coseismic changes in groundwater level during the 2018 Hokkaido Eastern Iburi earthquake

Six wells were continuously monitored in Hokkaido, northern Japan, to relate groundwater level changes to regional seismic activity. Groundwater level changes following the 2018 Hokkaido Eastern Iburi earthquake were detected in three of the six wells, even though they are located hundreds of kilometers from the epicenter. The groundwater level changes are qualitatively consistent with the volumetric strain induced by the earthquake. We analyzed groundwater level responses to the M₂ tidal constituent before and after the earthquake, but related changes in amplitude and phase shifts remained within the usual variation. Observed coseismic change was explained by the response to the M₂ tidal constituent component and the calculated volumetric strain for one of the wells, where groundwater level decreased. The observed change in the other two was found to be much greater than the corresponding estimates of the volumetric strain.