Geochemical constraints on origin of fluids involved in the Matsushiro earthquake swarm

Abstract

The distribution and behavior of deep-derived fluids, which play a significant role in crustal deformation such as seismic activity and volcanic eruptions, have attracted much attention in recent years. Recently, a number of earthquakes and uprifts have been observed in the Noto Peninsula of Japan, indicating the involvement of deep-derived fluids. The Matsushiro earthquake swarms that unusually continued from 1965 to 1971 produced such a massive gush of groundwater at the end of the period that it was also called a ”water eruption”. It is, however, difficult to research deep-derived fluids from spring water samples using traditional hydrogen and oxygen isotopic compositions owing to contamination from surfacewater. Lithium (Li) is relatively unsusceptible to contamination from surfacewater because the Li content of deep-derived fluid is significantly higher than that of surfacewater. The amount of Li leached from sediment to fluid increases dramatically with temperature, and the leached Li is retained in the fluid as it cools. Lithium isotope ratios may provide further insight into the origin of deep-derived fluids. To provide new constraint on the origin of fluids involved in Matsushiro earthquake swarms, we analyzed Li isotope ratios of spring water and basement rock samples recovered from drilling core in the Matsushiro area.The spring water samples analyzed in this study were collected in 2009 and 2010. In addition to the Li isotope ratio, the Sr isotope ratio and chemical composition of the spring water samples were analyzed. The results have shown that the observed Sr isotope ratio of nearly all spring water samples were similar to those of volcanic rocks erupted in the past from Mt. Minakami, where is the epicenter of the Matsushiro earthquake swarm. The temperature at which the fluid reacted with the rock can be estimated from differences in the Li isotopes of the rock that reacted with the fluid (Li isotope geothermometer). Based on the result of this study, the fluid-rock reaction temperature is around 600 °C. The results of the Li and Sr isotope analyses obtained in this study support the theory that the fluid that caused the Matsushiro earthquake swarm was water derived from the magma of the Mt. Minakami.