In the 2011 off the Pacific coast of Tohoku Earthquake, groundwater pressure changes were observed in
and around the Mizunami Underground Research Laboratory (MIU) in Central Japan, where two vertical shafts
and horizontal research galleries are excavated in the granitic rock mass. Coseismic changes of groundwater
pressure are believed to correspond to crustal dilation/contraction induced by earthquakes. In this study we calculated
volumetric strain changes due to the Tohoku Earthquake based on previously reported fault slip models.
The calculation indicates approximately 2 107 of dilational strain around the MIU. Based on the strain sensitivities
calculated from tidal responses at the monitoring boreholes, the dilation corresponds to drawdowns of
several tens of centimeters, and is almost the same as the drawdown observed in the boreholes at distances
greater than 1 km from the MIU. In contrast, rapid elevation of groundwater pressures associated with the earthquake
was observed in the boreholes within the 500 m vicinity of the MIU. The anomalous elevation is
explained by a temporary recovery of the drawdown due to excavation of the shafts and a unique permeability
increase induced by the coseismic dilation of heterogeneous local geological structures such as impervious
faults controlling the hydrogeological environment.