In this study, we first measured the hydrogen and oxygen stable isotope ratios that make up the water molecules in Arima hot spring water, and by comparing them with a numerical model, we found that the Arima hot spring water is mixed with water originating from the Philippine Sea Plate. Arima hot spring water not only contains rain and snow that falls from the sky, or water that originates from these (meteoric water), but also water squeezed out from the Philippine Sea Plate, which is submerged more than 60 km underground directly beneath Arima hot spring.

　Next, by examining data on the hydrogen and oxygen stable isotope ratios and chloride ion in Arima Onsen water over the past half century or more, the researchers determined that the proportion of plate-derived water in the hot spring water had temporarily risen sharply around the time of the Great Hanshin-Awaji Earthquake in 1995. In three of the seven Arima Onsen springs investigated, an increase in plate-derived water had occurred prior to the earthquake (as early as 7 days before).

　The amount of plate-derived water that suddenly gushed out from Arima Onsen around 1995 was estimated to be 26 to 42 cubic meters. It is strongly suggested that the water that overflowed like a flood from deep underground at that time may have weakened the fault and triggered the Southern Hyogo Prefecture earthquake.

　In the future, if deep underground flooding can be detected by monitoring hot springs that contain water originating from plates, such as Arima Onsen, it will pave the way for predicting earthquakes in advance. The research group plans to verify the effectiveness of monitoring on shorter time scales and to apply the same method to the entire Japanese archipelago, and even to plate subduction zones around the world.

Paper information:【Communications Earth & Environment】Hot springs reflect the flooding of slab-derived water as a trigger of earthquakes