The impact of the Tsukiyoshi fault on the hydrogeological conditions in the Tono Area,Japan: A numerical modeling approach

Faults are commonly believed to act as either barriers to horizontal ground-water flow normal to the fault, conduits to horizontal flow parallel to the fault, or a combination of both. In addition, enhanced vertical permeability has also been observed as a common feature. We use numerical modeling to investigate the effects of vertical anisotropy of a dipping fault zone on the distribution of hydraulic head within and around the fault. The Tsukiyoshi Fault in the Tono region of Japan extends through the center of the assessment area and has an E-W strike. According to the results of borehole investigations, the fault has N80W strike, 70 degree dip, 10-30 m width and approximately 30 m vertical off-set. Model results show that for anisotropy ratios (A = kz/kx) of greater than 100, hydrostatic conditions are achieved within the fault zone, despite the existence of significant vertical flow rates. A hydraulic head difference of about 40 m across the fault is observed and confirmed by our model, suggesting that the fault acts as barrier to flow normal to it. We consider the pressure response to two shafts pumping in the upper parts of the fractured granitic formation near the fault. The response to pumping is monitored at two boreholes (DH-15 and DH-2) on the same side of the fault as the pumping shafts. The responses at the two boreholes are vertically invariant and highlight the effects of enhanced vertical permeability around the fault. This suggests that the fault controls the hydrology in this area. Particle tracking is used to investigate and demonstrate the effects of the fault on pathlines.