抄録
Excavated areas such as the ore veins, tunnels and shafts of the Old Ikuno Mine, an abandoned mine in western Japan, can be regarded as comprising a huge reservoir of groundwater extending over several square kilometers to a depth of about 1km. In 1988, tidal fluctuations with an amplitude on the order of 1mm were observed in the water level of the Chuo shaft in this mine. The vertical length of the Chuo shaft is 370m, about two-third of which is flooded. Observations were curtailed until 1997, when this well-tide was again observed. The persistence of this phenomenon over a nine-year period implies that the well-tide in the Chuo shaft is an intrinsic feature of this huge underground reservoir. To understand the well-tide in the Chuo shaft, we compared the tidal fluctuations of its water level with crustal movement measured in the same area. We found that (1) the tidal response of the water level to crustal strain is frequency dependent, and (2) the tidal fluctuation exhibits a phase advance. These are quantitatively explained by a finite-cavity model with a drained condition. The estimated volume of the flooded cavity is on the order of 106m3, which is consistent with the excavated volume of the Old Ikuno Mine.
