Modeling the coseismic groundwater level increase in the Oi well, central Japan

Abstract

A model for coseismic groundwater level increase is presented for understanding hydrological responses of wells to seismic waves. Three types of coseismic groundwater level changes were observed in six wells 300–500 m in depth, operated by the Hot Spring Research Institute of Kanagawa Prefecture, central Japan. The first change was a sustained increase uniquely appearing at the Oi well. The second, a sustained decrease observed at most wells following the 2011 off the Pacific Coast of Tohoku Earthquake, and the third was an oscillatory response appearing in all six wells. In this study, we focused on the first response at the Oi well. We analyzed digital data at 1-Hz sampling rate of 12 events including shallow and deep earthquakes, and local to remote earthquakes from 2011 to 2016. There were 11 earthquakes which generated a sustained increase in the groundwater level in the Oi well from 5 to 10 cm. The time series of the sustained increase in the Oi well was well approximated by the decaying exponential function, characterized by a time constant ranging from 156 to 363 s. The slug test model for radial flow adequately represents the time curves observed in the Oi well, for which possible values of specific storage and hydraulic conductivity were used. The success of the application of the slug test model indicates a sudden increase in pore-pressure in the aquifer surrounding the well during the passage of seismic waves. We examined several candidates for the cause of the earthquake-triggered pore-pressure increase around the Oi well. We found that the poroelastic static strain change due to earthquake is not suitable for the sustained groundwater level increase at the Oi well. Qualitative examination suggests that following three models possibly explain the observed buildup times at the Oi well, but that all of them are not definitive: (a) permeability change due to barrier removal on the fracture surface, (b) undrained consolidation, and (c) gas bubble nucleation and growth. [Figure not available: see fulltext.]