Radon concentration in seawater as a geochemical indicator of submarine fault activity in the Yatsushiro Sea, Japan.

This study examined the relationship between radon (222Rn) concentrations in seawater and crustal activity in the Yatsushiro Sea by investigating the submarine fault zone situated at the southern end of the Futagawa-Hinagu fault zone, activated by the 2016 Kumamoto earthquake (M7.3). We conducted an analysis of 222Rn concentration in samples of bottom water just above the seafloor and pore water in sediments, utilizing multiple and piston cores from the Hakuho Maru Expedition KH18-3. The findings revealed significantly elevated 222Rn concentrations in the central sites of the Yatsushiro Sea, coinciding with a high-stress field exhibiting dense active faults. Seismicity analysis revealed heightened moment release and a low b-value post the 2016 Kumamoto earthquake, indicative of increased seismic activity and the potential for substantial earthquakes in the Yatsushiro Sea vicinity. Our results indicate that heightened concentrations of 222Rn in seawater can serve as an effective tracer for identifying and estimating submarine fault activities. Moreover, our research highlights the utility of 222Rn concentrations in detecting active submarine faults and assessing their activity. It contributes to a comprehensive understanding of the potential for significant earthquakes in the Yatsushiro Sea in the future.

Deep-Seated Fluids in Thermal Waters Before and After the 2016 Kumamoto Earthquakes.

Noble gases, oxygen-hydrogen isotope ratios, and ion compositions were measured at three sampling points (KUM, OTN, and ASO) from December 2013 to July 2021. The 3 He/4 He values at the three sampling points remained stable in the range of 3-4 Ra throughout the observation period, suggesting that the supply of deep-seated gases to the aquifer was stable. The 4 He/20 Ne values of KUM and OTN indicate that the supply of surface-source fluids to the aquifer decreased relative to that of deep-seated fluids at KUM and OTN. In contrast, in the ASO site, both the surface- and deep-seated fluids supplied to the aquifer were stable. The δD-δ18 O relationship indicated the supply of deep-seated water to the KUM and OTN aquifers but not to the ASO aquifer. Nevertheless, the δD-δ18 O relationship remained stable throughout the observation period, suggesting that the supply of deep-seated water to the three stations was stable. The Li/Cl and 1/Cl relationships for the three sampling points were plotted within a narrow range throughout the observation period, suggesting that the groundwater recharge was stable. Neither spikes nor step changes owing to the 2016 Kumamoto earthquake were observed in any of the data. These results indicate that the KUM and OTN aquifers are constantly supplied with deep fluids from the fluid-rich zone beneath the Kumamoto region, and that only deep-seated gas was supplied to the ASO aquifer. We also confirmed that these supply conditions were unaffected by the 2016 Kumamoto earthquake or the subsequent aftershock activity.