In situ measurement of the hydraulic diffusivity of the active Chelunepu Fault, Taiwan

[1] Hydraulic diffusivity controls fluid pressure and hence affects effective normal stress during rupture. Models suggest a particularly spectacular example of fluid pressurization during the M_w = 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.