Water injection experiments and discharge changes at the Nojima Fault in Awaji Island, Japan

Abstract

Three boreholes (500, 800, and 1800 m) were drilled near the Nojima Fault, which is one of the Hyogoken‐Nanbu Earthquake faults. Water injection experiments were conducted from the 1800‐m borehole over three periods (1997/2/9–11, 2/12–13, 3/16–25). Groundwater discharge increased at the 800‐m borehole during the water injections. The discharge started to increase at between one‐half to one full day after the beginning of the injections and decreased substantially just after the stop of the injections. One day after the injections were stopped, their effects on the discharge roughly disappeared. It appeared that these changes in discharge were due to changes in pore water pressure changes induced by the water injections. Therefore, by means of a simple numerical simulation, we analyzed the correlation between the pore water pressure changes and the water injections. In the case of a permeable stratum with a hydraulic diffusivity of 1.0–2.0 km²/day, which is equivalent to 0.08–0.47 darcy on the assumption that the porosity of the stratum is 0.01∼0.1, the calculated pore water pressure changes were highly similar to the observed discharge changes.