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IN the northern central part of Honshu Island, Japan, there is an Atostsugawa Active Fault system that isone of the most active faults in Japan. The Mozumi-Sukenobu fault is one of the Atotsugawa Active Faultsystems and is an ENE-WSW running right lateral strike slip fault. In February 1997, a fault survey tunnel was excavated across the Mozumi-Sukenobu fault from the Nagato tunnel excavated in the 1960s.Groundwater samples were collected along the Nagato and fault survey tunnels in and around the MozumiSukenobu fault and their chemical compositions, oxygen and hydrogen isotope ratios and tritium concentrations were measured. It is clearly observed that major chemical compositions of groundwater samples collected along theNagato tunnel are Ca and HCO_3 ions. These sampling points are more than 500 m distant from the Mozumi-Sukenobu fault. This Ca-HCO_3 type chemical composition is common in shallow groundwater. On thecontrary, groundwater samples in and adjacent to the Mozumi-Sukenobu fault have Ne-HCO_3 type chemi-
IN the northern central part of Honshu Island, Japan, there is an Atostugawa Active Fault system that isone of the most active faults in Japan. The Mozumi-Sukenobu fault is one of the Atotsugawa Active Faultsystems and is an ENE-WSW running right lateral strike slip fault. In February 1997, a fault survey tunnel was excavated across the Mozumi-Sukenobu fault from the Nagato tunnel excavated in the 1960s.Groundwater samples were collected along the Nagato and fault survey tunnels in and around the Mozumi Sunbu fault and their chemical compositions, This sampling point is more than 500 m distant from the Mozumi-Sukenobu fault. This Ca -HCO_3 type chemical composition is common in shallow groundwater. On thecontrary, groundwater samples in and adjacent to the Mozumi-Sukenobu fault have Ne-HCO_3 type chemi-