HR: 0830h
AN: S51A-04    [Abstracts]
TI: Anomalies Of Groundwater Temperature and Chemistry In Relation To The 2004 Niigata Chuetsu Earthquake
AU: * Watanabe, N
AF: Research Institute for Hazards in Snowy Areas, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Sato, S
AF: Department of Geology, Faculty of Science, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Ujihara, H
AF: Department of Geology, Faculty of Science, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Kobayashi, K
AF: Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AU: Toyoshima, T
AF: Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Niigata, 950-2181 Japan
AB: The 2004 Niigata Chuetsu earthquake of Mw6.6 occurred at the 220km north of Tokyo, Japan, on October 23, 2004. It was the special interest that this earthquake and several large aftershocks occurred at the northern segment of Niigata-Kobe Tectonic Zone (NKTZ) proposed by Sagiya (2001). Some of investigators regard the NKTZ as the plate boundary between the Eurasian plate and the North American plate. In the northern segment of the NKTZ, the thick accumulation of the Neogene sediments has been folded to make basins and ranges almost in parallel with the NNE-SSW coastal line of the Japan Sea. There are several active faults and a local tectonic line called "Shibata-Koide Tectonic line (SKTL)" around the epicentral area. Our field investigations indicated no evidence of the reactivation of these faults and SKTL. In addition, we found a spring water discharged at the eastern margin of the epicentral area close to SKTL. The water showed higher temperature and extremely higher electric conductivity than the surrounding shallow groundwaters. Then we focused on the behavior of groundwaters around the epicentral area, and measured the temperature, electric conductivity and chemistry using "the snow-melting wells". The Niigata area has been suffered from the heavy snow-fall more than three meters thick. The groundwaters from >10,000 wells at the depth of 40-100m have been utilized for snow-melting since 1960s. Considering the average aquifer depth of 70m for the wells, the local geothermal gradient of 30°C/km, and the annual mean temperature of 11.5 to 12°C in research area, we decided the temperature of >15°C was anomalous. We found the groundwaters with 15 to 17°C in the western margin of the epicentral area where the Yukyu-zan active fault with a NNE-SSW trend is located at the eastern border of the Nagaoka plain and the Higashi-yama Hill. We also found the waters that the temperature and the electric conductivity were anomalous in the southeastern margin of the epicentral area where SKTL with the NNE-SSW trend is located between the Higashi-yama Hill and the Echigo Mountains. A few groundwaters with 18 to 25°C were found in the downtown area of Ojiya City located in the southwest extension of the Yukyu-zan active fault, although no active fault has been known there. We consider that their anomalies are influenced by deep geothermal systems associated with the natural gas and oil reserviors beneath the folded ranges. Furthermore, the high relief of basin and range structure bordered by major faults creates deep circulation of groundwater, and consequently thermal and mineral springs sometimes discharge along the faults. It is likely that deep geothermal waters have seeped upward through fault fractures, injected into shallow aquifers and mixed with groundwaters. The excess fluid pressure and the friction heat induced by a series of earthquakes might be released through the Yukyu-zan active fault and SKTL to the near surface. Such process might be one of the major causes of groundwaters' anomalies in temperature, electric conductivity and chemistry.
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2005 Joint Assembly