Seismicity-induced groundwater level changes in boreholes around Mizunami Underground Research Laboratory (MIU), Japan: Effect of the 2011 off the Pacific coast of Tohoku Earthquake

For improving the scientific basis for geological disposal of high-level radioactive waste, multidisciplinary researches are approached in the MIU, in which two vertical shafts are excavated in the crystalline rock mass. Groundwater levels are continuously logged in multiple boreholes, for understanding the regional groundwater flow around the MIU site. Soon after the 2011 off the Pacific coast of Tohoku Earthquake, groundwater level changes were observed in the almost boreholes. All boreholes arranged away from the MIU (approximately 1 to 5 km) showed drawdown ranging from 1 to 5 m. Several studies (e.g. Wang, 1997, JGR; Ge and Stover, 2000, JGR; Hamiel et al., 2005, EPSL) suggest that coseismic changes of groundwater level correspond to static volumetric strain changes induced by earthquakes, i.e., drawdown/elevation of groundwater level is reflected by crustal dilatation/constriction. We calculated volumetric strain changes due to the Tohoku earthquake based on the previously-reported fault models (slip models estimated by teleseismic source inversion; Yagi and Nishimura, Univ. of Tsukuba; Poiata et al., ERI, Univ. of Tokyo). We determined crustal deformation and stress change using the program Coulomb 3.0 (Lin and Stein, 2004, JGR; Toda et al., 2005, JGR). The calculation outputs approximately 2.3E-7 strain of dilatation around the MIU. Thus the drawdown observed in the boreholes arranged away from the MIU is consistent with the volumetric strain changes associated with the Tohoku earthquake. In contrast, groundwater levels were elevated up to 15 m in the boreholes localized in the vicinity of the MIU (within 600 m). These boreholes had shown successive drawdown since the shaft excavations started in the MIU, while voluminous sump water had been released successively from the shafts. Soon after the Tohoku earthquake, volume of the sump water increased approximately ten percent. Irregular elevation of water level soon after an earthquake like the case of the MIU has been reported in other excavation sites where successive drawdown is observed (e.g. Kitano and Tamai, Geoline2005, Lyon). Dilatation caused by an earthquake could induce fracture aperture opening and subsequent permeability increase, which could cause the anomalous increase of the sump water and the elevation of the groundwater level within the MIU site, as a temporal recovery of artificial drawdowned groundwater level.