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

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 m 2 /s, which is a low value compatible with pressurization of the Chelungpu fault during the earthquake. In most poroelastic media, the hydraulic storativity S lies between 10 −7 and 10 −5 , so that the transmissivity T along the fault zone is comprised between 10 −11 m 2 /s and 10 −9 m 2 /s. The corresponding permeability (10 −18 –10 −16 m 2 ) 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.