Kyoto University, Japan

We measured frictional properties and permeability of core materials from the megasplay fault zone (site C0004) and the frontal thrust (site C0007) in the shallow part of the Nankai subduction zone. Permeability was measured before and after 7.9 m slip displacement at high (1.05 m/s) and low velocities (0.013 m/s) under normal stresses of 1.5 MPa using the rotary-shear apparatus, from which we estimated the shear-induced permeability change in an experimental fault gouge prepared from core material. Gouge permeability (10 (super -18) m (super 2) ) decreased after sliding for wet gouge and increased for dry gouge. The high-velocity friction test under wet conditions yielded a smaller reduction in permeability than the low-velocity test, whereas the opposite trend was observed in dry conditions. We attribute the differences in permeability to the effects of thermal/mechanical pore pressurization upon shear-induced compaction. Symmetric boudin structures may represent evidence of hydrofracturing induced by pore fluid pressurization. The large friction coefficient of the megasplay fault material in the slow and wet friction tests is explained by homogeneous shear deformation and higher permeability that promotes faster shear-induced compaction. The similarity in post-shear permeability for the gouges from the both faults may account for the similar friction coefficients in high-velocity friction, assuming that the pore fluid pressurization process controls high-velocity frictional behavior. This velocity dependence on friction suggests that a large dynamic stress drop is expected for the megasplay fault, implying that large slip displacement followed by a giant tsunami is plausible when a rupture from depth propagates to the megasplay fault.