Application of B and Li isotope systematics for detecting chemical disturbance in groundwater associated with large shallow inland earthquakes in Kumamoto, Japan

Abstract

In this study, we investigated the detailed processes of hydro-chemical changes in regional groundwater flow systems associated with large earthquakes using stable isotope ratios of dissolved trace elements. A Cl-B-Li ternary diagram, in combination with isotope systematics of δ¹¹B and δ⁷Li was applied to the groundwater system of Kumamoto, Japan to understand the groundwater disturbances associated with the Kumamoto earthquakes in 2016. Our approach was based on the conservative chemical behavior of these three elements in the groundwater system, and their contrasting concentrations and isotopic signatures of B and Li among seawater, stagnant groundwater, and hydrothermal fluid. A small contribution from the influx of deep hydrothermal fluid into the aquifer was detected in one observation well near the epicenter of the 2016 Kumamoto earthquake, along the Futagawa fault. B-Li isotope systematics are useful isotopic proxies to precisely evaluate hydro-chemical changes in groundwater flow systems, and detect contribution of different sources associated with various tectonic disturbances on a finer scale.