Change of Rock Permeability Induced by the 2011 Tohoku Earthquake

Abstract:

We have been monitoring the pressure of groundwater at the Kamioka mine in Gifu prefecture, central Japan. Kinoshita et al. (2015) observed a large pressure decrease which is equivalent to over 2 m change in water level after the 2011 Tohoku earthquake(M9.0), although the hypocentral distance is more than 500 km.

Kinoshita et al. (2015) hypothesized that new water paths were made associated with the shaking from the earthquake and the rock mass became more permeable, because the pressure decreases continued for several days. To verify the hypothesis, we calculated the hydraulic diffusivity from the response of the pressure to Earth tides. Earth tides induce expansion and contraction of the crust, and pore pressure responses to this deformation. The pressure change from theoretical strain changes of the Earth tides can be calculated and subtracted from the observed pore pressure data. Then, we can extract the hydrological rocks property from the residual pore pressure.

We used the tidal analysis program BAYTAP-G (Tamura et al., 1991) to calculate the Earth tide response and focused on the M₂ and O₁ constituents which are the largest. The M₂ and O₁ amplitudes decreased after the Tohoku earthquake. We estimated the hydraulic diffusivity from those results and showed there was remarkable increase after the earthquake. To try to understand the cause of the permeability change, we compared the effects of both static and dynamic strain associated with the earthquake, but there is no clear difference. Kinoshita et al. (2015) could not conclude from those results whether static or dynamic strain has a dominant effect.

The Geological Survey of Japan, AIST has developed a network of groundwater monitoring in Japan since 1976 (Itaba et al., 2008). We investigated the groundwater levels and pressure changes before and after the Tohoku earthquake using the AIST boreholes. Among the factors influencing groundwater changes, at first, we focus on the effect of differences in boreholes lengths. Each observation site of AIST used for the present analysis has two or three boreholes with different lengths. We investigated the groundwater responses of each borehole and depth in terms of confined and unconfined aquifers.