Evaluation of faults stability due to passing seismic waves: Study case of groundwater level changes induced by the 2011 Tohoku earthquake in Central Japan

In the study, we analyze changes in groundwater pressure observed in several boreholes drilled in and around the Mizunami Underground Research Laboratory (MIU) induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). The aim of this project is a development of methodology to evaluate systematic fault activity by numerical analysis. To reach this goal we investigate the behavior of the fault zones present in the area during the passing of seismic waves. We built a simplified hydrogeological model of the MIU site and performed a series of fluid flow simulations with TOUGH2 flow numerical code. We investigate how changes in permeability along three faults present in the study area: the Tsukiyoshi Fault, the Hiyoshi Fault and the Main-Shaft Fault may have influence the groundwater level monitored in boreholes intervals. We also test the influence of the cone of depression at the MIU site and the hydraulic connectivity between the sedimentary cover and the granite aquifers. Our results suggest that two main mechanisms are responsible for the observed changes in groundwater pressure: (1) crustal dilation induced by the Tohoku earthquake causing a groundwater recharge from the sedimentary aquifers to the Toki granite aquifer where the sedimentary cover is thick; and (2) permeability increase along faults critically oriented for shear reactivation and oriented in the direction of the passing seismic wave. In this case, the seismic wave increases the shear stress acting on the fault promoting slip and a change in permeability through a mechanism of slip-induced dilation. Faults not critically stressed and faults critically oriented for shear reactivation but oriented perpendicular to the passing seismic wave are not reactivated.