Origin of Deep Matsushiro Earthquake Swarm Fluid Inferred from Isotope Ratios

Abstract

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 6 180 vs. 6 D 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 6 '80 and 6 D 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. 6 13C ranges from-7.1%0 to-3.1%o, suggesting that the source of CO2 is alsc magmatic. The ratio 'He/ 'He shows that He in the free gas is from the mantle or magma. These 3 pieces of evidence-(1) 6 'BO and 6 D values, (2) 6 '3C 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 wa: weakened, causing the Matsushiro earthquake.