[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P21] Investigation of the relationship between the deep-derived fluids and Matsushiro earthquake swarm using Li and Sr isotopes

*Zahra Zandvakili¹、Yoshiro Nishio¹ (1.Kochi University )

キーワード：Matsushiro earthquake swarm, Lithium isotope, Geofluids

The Matsushiro earthquake swarms that unusually continued from 1965 to 1971 accrued in central Japan. The most striking feature was the water eruption and increasing of the water discharge from water wells and springs. Recently, a number of earthquakes and uprifts have been observed in Japan related to upwelling of the water. To investigate the relationship between earthquake swarms and water eruptions, Matsushiro spring water and well water samples (collected in 2009 and 2010) were analyzed by focusing on Li and Sr isotopic ratio measurements. All analyses have been performed at the Center for Advanced Marine Core Research, Kochi University. Our result of measurement of Li and Sr isotope ratio indicated binary mixing of the magmatic andesitic fluid derived from magma and surface water. In addition, the results have shown that the observed ⁸⁷Sr/⁸⁶Sr ratio of nearly all spring water samples reacted to those of volcanic rocks erupted in the past from Mt. Minakami, in the average fluid-rock reaction temperature of 650°C using the Li isotope geothermometer. This estimated Li reservoir temperatures (650 °C) were representative of the reaction of fluid and rock in the lower crust or deeper. Magma-derived fluids were stored for long periods of time and cooled beneath an impermeable sheet at the boundary of the lower and upper crust. Then the impermeable sheet broke and fluids derived from the old magma of Mt. Minakami rose to the upper crust. Therefore, out results of ⁷Li/⁶Li ratio and the ⁸⁷Sr/⁸⁶Sr ratio analyses in this study indicate that fluids derived from the old magma of Mt. Minakami in the lower crust or deeper is the main cause of the Matsushiro earthquake swarm.