Short-term isotopic variability in spa waters containing lithospheric components across central Japan: A preliminary study

• Isotopic variability in spa waters reflect changes in fraction of lithospheric components. • Monthly variability does not always show a monotonic decrease in lithospheric component • Daily variability reflects both isotopic forcing due to earthquakes and precipitation. • Time series data are helpful for determining meteoric-lithospheric mixing line. • Precise mixing line enables better identifying of the origin of lithospheric water. Thermal and/or mineralized waters in natural spas often contain not only meteoric components but also lithospheric components. The latter components are rarely replenishable and experiencing depletion at some spas over decadal timescales. While temporal variations in hydrogen and oxygen stable isotopes can reflect the behavior of the lithospheric component, their short-term variability is not well known, and the methodology for identifying the origin of the component has not yet been established. To address the above issues, here, we show the observed monthly/daily isotopic variations in natural spas in central Japan and evaluate the changes in the lithospheric component. Additionally, we propose a new approach of the end-member mixing analysis based on the short-term isotopic variability. The results do not always indicate monotonic reduction in the lithospheric component but reflect reversible changes in the mixing fraction of the lithospheric and meteoric components. Correlation analyses reveal that potential causes of these changes include earthquakes, precipitation, and artificially controlled pumping rates. Among the eight spas, one showed passive responses to earthquakes while another might respond to fluctuations in slab-derived water flow, which can induce seismicity. Namely, this study is the first to demonstrate the potential for predicting modulation in slab-derived water flow and the consequent seismic activity via isotopic monitoring of spa waters. Long-term, frequent monitoring is expected to improve our understanding of the origins and behavior of deep-seated waters and to potentially contribute to early warning of earthquakes or any tectonic events.