Abstract
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.
Highlights
When a large earthquake occurs on a fault, stress acting on the fault is released and rocks surrounding the fault are fractured
To investigate the recovery process of a fault zone just after a large earthquake, repeated water injection experiments have been conducted near the Nojima fault since 1997
Methods: water injection experiments and observations Three boreholes were drilled to depths of 500, 800, and 1800 m near or into the Nojima fault and a branch fault during 1995–1996 (Ando 2001) (Fig. 1), immediately after the 1995 Hyogoken-Nanbu earthquake
Summary
When a large earthquake occurs on a fault, stress acting on the fault is released and rocks surrounding the fault are fractured. Methods: water injection experiments and observations Three boreholes were drilled to depths of 500, 800, and 1800 m near or into the Nojima fault and a branch fault during 1995–1996 (Ando 2001) (Fig. 1), immediately after the 1995 Hyogoken-Nanbu earthquake. The water injection experiments were conducted in the 1800-m borehole, which was drilled from a site east of the surface trace of the branch fault This borehole penetrated the branch fault and reached the fractured zone of the Nojima fault. Seno 1969), we estimated a conversion factor from the barometric responses of the groundwater discharge and pressure data, as described by Kitagawa et al (2007), since period range of the barometric responses were similar to the duration time of the water injection experiments (Table 1). We can confirm that the macroscopic permeability of the damage zone stopped decreasing in 2003
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