2004年新潟県中越地震と地下水異常・活構造
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概要
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We examined considerable changes in temperature, electric conductivity and hydrochemistry of groundwaters around the epicentral area of the Mid Niigata prefecture Earthquake in 2004 during one year from 2004-2005 to 2005-2006 winter months using snow-melting wells, and identified hydrogeological processes and origin of groundwaters. During the 2004-2005 winter months, three new anomaly areas in temperature and electric conductivity of groundwaters appeared along the Yukyu-zan active fault and its inferred northeastern and southwestern extensions, in the eastern part of Kawaguchi Town and Horinouchi region of Uonuma City, and along the northern extension of the Muikamachi active fault from Koide to Obiro regions. The former one area is located on the hanging wall block of the seismogenic source fault of the earthquake. The latter two areas are situated on the footwall of the source fault. The groundwaters from these three anomaly areas are characterized by high NaCl concentration and high water-temperature. The high-salinity and high temperature groundwaters are considered to be mixtures of deep hot fossil seawater and meteoric waters on the basis of hydrochemical characteristics. Therefore, the seismic fracturing may have increased permeability and assisted upward flows of deep hot groundwaters immediately after the earthquake. During the 2005-2006 winter months, temperature and electric conductivity of groundwaters from the anomaly area on the hanging wall have remarkably increased. During the same months, considerable decreases in temperature, electric conductivity, and NaCl concentration of grooundwaters have been recognized in the anomaly areas on the footwall. The difference between the first and second winter months after the earthquake suggests that the permeability in the hanging wall did not decrease under an E-W extensional condition and remains enhancing deep circulation of groundwater along the Yukyu-zan active fault, whereas the permeability in the footwall under an E-W compressional condition decreased and consequently the temperature, electric conductivity and NaCl concentration of groundwaters were lowered.
- 新潟大学災害復興科学センターの論文
- 2007-07-00