2009 Volume 118 Issue 3 Pages 543-563
High salinity Na-Cl-type geothermal waters have often been found in the anticlinal hilly terrains and landslide-prone areas of Niigata Prefecture by boring to depths of more than 1,000 meters. These fluid pressure gradients are much higher than the hydrostatic pressure gradient and approach the lithostatic pressure gradient with increasing depth. The geothermal waters in the Matsunoyama area, Tokamachi City, have the highest orifice temperature in Niigata Prefecture. Eight geothermal water wells in this area were drilled along the anticlinal axis of a nearby hilltop or higher breast of the Matsunoyama dome and ranged in depth from 170 to 1,170 meters and in temperature from 35 to 95°C. For example, the Takanoyu-1 geothermal water well is only 170 meters in depth but has an orifice temperature of 90°C. These waters show geyser action associated with methane gas, and typically have very high salinity with considerable amounts of chloride. Hydrogen and oxygen isotope values and chloride concentration of approximately 9,000mg/L suggest that the origin of the waters is altered fossil seawater trapped in organic-matter-bearing sedimentary rocks. The temperature and depth of the primary reservoir are estimated to be 139°C and 3,000 to 4,000 meters, respectively, using a Li-Mg geothermometer and the mean geothermal gradient of 30-40°C/km in the Niigata sedimentary basin. Na-Cl-type groundwaters emerged from several landslides located in the Higashi-kubiki and Naka-kubiki areas including the Matsunoyama area. Well loggings were carried out to profile the electric conductivity and hydrochemistry of groundwater in the Utsunomata landslide in the neighborhood of the Matsunoyama area. Profiles in the active landslide mass showed sharp increases of the electric conductivity and the NaCl components at around the depth of the sliding surface. In contrast, no significant variations of the profiles were recognized in the inactive landslide mass. Na-Cl-type groundwaters are formed by mixing deep Na-Cl-type geothermal waters with meteoric groundwaters. These phenomena suggest that the geothermal water injection into shallow aquifers in landslide mass generates a partially high pore water pressure around the sliding surface and causes landslides.
