Detection of a healing process of the Nojima fault by a repeated water injection experiment and understanding of the mechanism of induced earthquakes

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A water injection experiment was carried out at the 1800m-deep borehole at the Nojima fault where the 1995 Kobe earthquake took place, in January to March 2000, three years after the first experiment conducted two years after the Kobe earthquake. The migration of the injected water was monitored by various geophysical measurements at the borehole and two other boreholes nearby, the seismic observation, and self-potential and resistivity measurements of the earth. The result of these observations and measurements suggests that the permeability in the ground surrounding the 1800m-deep borehole and also in a shallow (at depths of 0-4km) fracture zone of the Nojima fault has decreased, which is most likely to indicate a healing process after the Kobe earthquake. Although our new temperature measurement revealed that the injected water leaked at a depth of 540m, most of the injected water appears to migrate towards the fracture zone of the Nojima fault. Changes in water discharge at the bottom of a 800m-deep borehole, horizontally 50m away from the 1800m borehole, indicate that the permeability of surrounding rocks has decreased to about one-half that measured three years ago. The decrease of the permeability of surrounding rocks was also confirmed by self-potential observation. The activity of ultra-microearthquake in a distance of 2-4km from the borehole, increased four to seven days after the start of water injection, which is delayed two days in comparison with the previous experiment. If the activity was triggered by the injection of the water, the delay indicates a decrease in the permeability within the fracture zone of the fault. The migration of seismic sources and b-value decrease suggest that the seismic activity was induced by the water injection.