Abstract
Many landslides classified into “Tertiary type landslides” have occurred in Niigata prefecture. Maki Village located in the southwestern Niigata prefecture is in a typical landslide area of this type. Na-Cl type groundwaters with high conductivities are present in the Utsunomata landslide, Maki Village.
The bedrock of the Utsunomata area is the Taruda Formation of Late Miocene, which is mostly composed of black mudstone and dacitic pumice tuff. The study area is located at the intersection of the Takaba fault and the Nagakurayama anticline, and the bedrock is deformed and fractured.
The Ishikura units of the Utsunomata landslide are active, and are characterized the groundwater of Na-Cl type waters with high electrical conductivities and high concentrations of major components: the concentrations are30-100times higher than those of the stable units in the Utsunomata landslide area.
The Na-Cl type groundwaters with high concentration can not be derived from reactions between groundwater and rock-forming minerals. Na-Cl type groundwaters come from a deep geopressured hydrothermal system, and they influence on landslide in Utsunomata area.
