Changes in permeability of the Nojima fault damage zone inferred from repeated water injection experiments

In 1995, the Hyogoken-Nanbu earthquake (M 7.3) ruptured the Nojima fault, Awaji Island, central Japan. To investigate the recovery process of a fault zone after a large earthquake, repeated water injection experiments have been conducted every few years in an 1800-m-long borehole near the Nojima fault since 1997. In addition, the groundwater discharge rate and pressure have been observed in an 800-m borehole. From the resulting data, the macroscopic permeability of the fault fracture zone was estimated to range roughly from 1 × 10−6 to 2 × 10−6 m/s. The macroscopic permeability of the fault fracture zone decreased until 2003, and then, it stabilized or increased slightly through 2006. These changes in permeability indicate that the fault fracture zone stabilized within 8 years after the occurrence of the earthquake.