Hydraulic diffusivity controls fluid pressure and hence affects effective normal stress during rupture. Models suggest a particularly spectacular example of fluid pressurization during the Mw = 7.6 1999 Chichi earthquake when pressurization may have reduced high-frequency shaking in the regions of large slip if the fault was sufficiently sealed. We investigate in situ hydraulic diffusivity which is the key parameter in such models through a cross-hole experiment. We find a diffusivity of D = (7 ±1) × 10-5 m2/s, which is a low value compatible with pressurization of the Chelungpu fault during the earthquake. In most poroelastic media, the hydraulic storativity 5 lies between 10-7 and 10 -5, so that the transmissivity T along the fault zone is comprised between 10-11 m2/s and 10-9 m2/s. The corresponding permeability (10-18-10-16 m2) is at most one hundred times larger than the value obtained on core samples from the host rock. The fault zone is overpressurized by 0.06 to 6 MPa, which is between 0.2% and 20% of the lithostatic pressure.