Origin of spring waters in the Matsushiro area: Implications for the 1965-1967 earthquake swarm and subsequent "water eruption"

The Matsushiro earthquake swarm, which unusually persisted from 1965 to 1967, generated a gush of groundwater at the end of the period so massive that it was referred to as a "water eruption". In this study, we analyze the geochemistry of Matsushiro spring waters collected in 2009 and 2010, with a focus on Li and Sr isotopic ratios. The Li and Sr isotopic ratios of Matsushiro spring waters, excluding the samples most contaminated by surface-water influence, were +4.0%o to +6.0%o for 67Li and 0.7044-0.7045 for 87Sr/86Sr. These isotopic ratios are similar to those of Matsushiro igneous rocks (67Li -1.2%o to 5.4%o, 87Sr/86Sr 0.7043-0.7050), supporting the hypothesis that the Matsushiro deep-derived fluid has a magmatic origin. Based on our results, the 67Li value of Matsushiro deep-derived fluid without contamination by surface water is estimated to be +4.2%o. Using a Li isotope geothermometer, we estimate that the fluid-rock reaction temperature between the Matsushiro deep-derived fluid and magma exceeded 700 degrees C. Furthermore, the insights obtained from this study on Matsushiro spring waters, emphasizing the characteristics of magma-derived fluid in central Japan, provide valuable information for distinguishing the origin of previously ambiguous fluid associated with the earthquake swarm at the southeastern foot of Ontake volcano, central Japan.