Secular change of permeability in the fracture zone near the Nojima fault estimated using strain changes due to water injection experiments

Water injection experiments were performed in 1997, 2000 and 2003 at the 1800 m borehole near the fracture zone of the 1995 Hyogo-ken Nanbu earthquake. During these experiments, a contraction of about 10− 8–10− 7 was observed with three-component strainmeters at a bottom of the 800 m borehole, 70 m southwest of the 1800 m borehole. We estimated hydraulic properties of the fracture zone near the Nojima fault by using the strain data to investigate a healing of the fault during the postseismic stage. We calculated pore pressure changes due to the water injection using Darcy's equation and obtained strain changes due to the pore pressure changes as elastic deformations of the crust. The calculated strain changes have a nearly agreement with the observed strain changes. Hydraulic conductivity in 1997, 2000 and 2003 was determined to be 0.9 ± 0.2 × 10− 6, 0.8 ± 0.2 × 10− 6 and 0.4 ± 0.1 × 10− 6 m/s, respectively. The reduced hydraulic conductivities in 2000 and 2003 suggest that the fractures had been healing.