著者

吉田 則夫 奥澤 保 塚原 弘昭

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.55, no.2, pp.207-216, 2002-10-31 (Released:2010-11-17)

参考文献数

36

被引用文献数

3

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In the final stages of the Matsushiro earthquake swarm (August 1965-October 1967), a large volume of groundwater was generated that continues to flow as of this writing. We studied the spring water origin by measuring oxygen/hydrogen isotope ratios and concentrations of Na+ and Cl, the main dissolved ions. We took water samples from June 1999 to October 2000. Data plots for δ18O vs. δD are distributed along a well determined linear regression line having an endpoint, i.e., river water, at the ordinary value of rainwater. The regression line can be extrapolated toward estimated δ18O and δD of “andesitic magmatic water” originating from magma in subduction zones. This implies that the Matsushiro groundwater can be regarded simply as a mixture of 2 fluids, i.e., surface water and andesitic magmatic water. We obtained the carbon isotope ratio of CO2, the main component of free gas in spring water. δ13C ranges from -7.1‰ to -3.1‰, suggesting that the source of CO2 is also magmatic. The ratio 3He/4He shows that He in the free gas is from the mantle or magma. These 3 pieces of evidence - (1) δ18O and δD values, (2) δ13C of CO2, and (3) 3He/4He - suggest that the origin of Matsushiro water is magmatic. Considering the presence of an electric conductive layer and seismic reflective layers 15 km beneath the Matsushiro area, we presume that this andesitic magmatic water accumulates as a thin layer at this depth. An impermeable sheet presumably lying just above the water layer was formed by precipitates from magmatic water.We present the following model of the relation between groundwater and earthquakes : When the impermeable sheet broke and high-pressure water with CO2 rose into the upper crust, the crust was weakened, causing the Matsushiro earthquake.

著者

奥澤 保 塚原 弘昭

出版者

公益社団法人 日本地震学会

雑誌

地震 第2輯 (ISSN:00371114)

巻号頁・発行日

vol.53, no.3, pp.241-253, 2001-03-25 (Released:2010-03-11)

参考文献数

31

被引用文献数

5

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Many small earthquakes occurred in the Matsushiro area in a period from 1965 to 1967. This is called “Matsushiro Earthquake Swarm”. The swarm earthquakes are interpreted as a result of deep ground water intrusion into the focal area. At the hardest time of the crustal movement, water flow from pre-existing springs increased, and many new springs appeared along a hidden fault zone formed by the swarm earthquakes. Some of springs are still alive and have peculiar chemical compositions. The deep ground water must have caused the earthquake swarm. We show the characteristics of the deep ground water through analyzing chemical composition. Our measured data and many published data show that the deep ground water obtains Ca2+ from rocks and loses Na+ during its upwelling migration through the fault zone of Matsushiro Swarm Earthquakes. The data also show that the major components of Cl- and (Na++Ca2+) have a liner relation in concentrations, although every spring water has different concentrations of Cl- and (Na++Ca2+). It is interpreted in terms of mixing deep ground water with shallow ground water at defferent ratios. The concentrations of the major ions of the deep ground water are estimated from both of our measured data and compiled data using the liner relation, e. g., Na+: 310meq/l, Cl-: 330meq/l. Analysis of our measured data also reveals that the small temporal variation in the concentration of Ca2+ in a short period such as several months is attributable to the reaction between the ground water with CO2 and surrounding rocks.