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Two effects of clay enrichment in the granite-origin fault zone due to the deformation were studied: (1) it reduces frictional strength and (2) it impedes across-fault fluid flow. We sampled two natural fault rocks located in and out of the aftershock region of 2000 Tottori-ken Seibu Earthquake (Tottori, Japan) , and measured the porosity, permeability and friction coefficient for each gouge zone. Independent of the sampling location and the difference of aftershocks' activities, the deformation concentrated gouge zone showed the lowest porosity in the specimen. The pore in the gouge with the lowest porosity was dominated by smaller sized pore throat (around 10 nm) caused by the enrichment of fine-sized clay minerals, such as smectite. Sliding deformation with permeability monitoring on natural gouges, using the pore pressure oscillation method, were performed along a 30° precut surface of Berea sandstone under 100 MPa of normal stress, 30 MPa of pore water pressure and room temperature condition. The gouges having the lowest porosity showed the lowest friction coefficient and the lowest permeability in the fault rocks. Thus both the frictional and fluid flow properties of natural gouges would be controlled by the clay mineral content. The relationship between the friction coefficient and the permeability on the natural fault gouge draws a similar trend to the previous study on simulated gouges with various mixes of smectite powder and granular quartz, but the trend for the natural fault gouge indicated c.a. 0.1 lower friction than that for the previous study.

We operated laboratory permeability measurements with Neogene mudstone specimens in order to investigate an applicability of laboratory tests to estimate fracture permeability in depth. We collected rock samples from drill cores of Koetoi Formation diatomaceous mudstone and Wakkanai Formation siliceous mudstone at Horonobe area, northern Hokkaido, Japan. We prepared specimens with a saw-cut discontinuity, simulating a fracture in rock, and measured permeability in the direction parallel to the discontinuity under hydrostatic stress condition up to 80 MPa. The measured permeability of a specimen with a saw-cut discontinuity fracture was larger than that of an intact specimen when confining pressure is less than 1 MPa (comparable to the depth of approximately 150 m) in the case of Koetoi Formation diatomaceous mudstone, but, when confining pressure is 1 MPa or more, the permeability values of a saw-cut specimen and an intact specimen became a similar to each other. On the other hand, in the case of Wakkanai Formation siliceous mudstone, the confining pressure under which the permeability of a saw-cut specimen became similar to the permeability of an intact specimen was close to 80 MPa (comparable to the depth of several km) . These results can explain the difference between Koetoi and Wakkanai Formations on distribution of in situ permeability measurements operated by Japan Atomic Energy Agency. Numerical simulations of a flow through a fracture under normal stress could explain the results of the laboratory test for Koetoi Formation diatomaceous mudstone, by using reasonable mechanical properties. These results support the possibility of estimating stress dependency of fracture permeability from rock mechanical properties and permeability of intact part.