Evolution of permeability and microstructure of experimentally-created shear zones in Neogene siliceous mudstones from Horonobe, Japan

We report experimental measurements of bulk permeability changes due to a shear zone that is induced in siliceous mudstones collected from the Koetoi and Wakkanai Formations, northern Hokkaido, which are known to show different relationships between fault/fracture distribution and groundwater flow. We evaluate distributions of volumetric deformation in the induced shear zones by using micro-focus X-ray computed tomography. Measured permeability evolution while achieving the peak axial stress for specimens differed for the samples of the two formations. Permeability did not change obviously during shear for the Koetoi Fm. specimens, but in the Wakkanai Fm. specimens, the bulk permeability increased by a factor of 2.5 after reaching the peak stress. The difference in permeability change in these experiments can explain the differences in relationships between in situ groundwater flow and fracture distribution for the two formations. Analyses of the X-ray images reveal that this difference should reflect the differences of the volumetric deformation in the induced shear zones. Pore collapse occurred in the shear zone in the Koetoi Fm. specimen, which leads to porosity reduction, whereas fracture damages developed in the Wakkanai Fm. specimen, increasing porosity. These differences in the microstructure may reflect differences in yielding criteria for these host rocks.