A time-lapse seismic reflection survey before and after the 2003 Tokachi-oki interplate earthquake (MW8.0) detected spatio-temporal evolution in reflectivity near the source area. From the evolution pattern and the aftershock observations, we attribute the change in reflectivity to gradual fluid flow, and estimate the subsequent permeability in the plate boundary shear zone to be 10-15 m2. The magnitude of the reflectivity change implies a 1.6% increase in porosity and a 2.2--5.6 MPa reduction in shear strength. As a hypothesis to test, we propose that expanded porous patches guided the seismic rupture propagation in the Tokachi-oki earthquake.