The journey of water through the deep Earth

I’m a hydrologist with an interest in all aspects of the water cycle. Recently, my students and I have been focusing on the deep water-cycle that reaches depths of over several tens of kilometers through the subduction of oceanic plates beneath island arcs. Ocean water is trapped in the pores and cracks of sedimentary rocks or seafloor basalts that make up the oceanic plate and is transported to the Earth’s interior due to plate subduction. Occasionally, during the subduction process, this water is expelled and then returns to Earth’s surface in the form of magma or non-volcanic hot springs.

Tracing the origin of Arima hot spring water

Arima hot spring, located in western Japan, is one of the most famous spas in Japan with a history of more than a millennium. It has unique water quality characteristics, such as a salinity more than twice that of seawater, temperatures reaching nearly boiling point despite being in a non-volcanic area, and isotopic compositions of hydrogen and oxygen that differ from those of meteoric water and seawater. The H and O isotopic compositions in H₂O are useful tracers for studying the water cycle. We have modelled isotopic evolution of water trapped in the oceanic plate during its subduction, considering water-rock interactions (Adachi and Yamanaka, 2024). Our model’s predictions for the isotopic compositions of slab-derived water beneath Arima agree very well with the non-meteoric end-member compositions in hot spring waters, indicating that Arima hot spring contains water derived from Philippine Sea plate subducting at the Nankai Trough.

Discovery of deep underground flooding

Long-term isotopic records at Arima, spanning over half a century, show an exponential decrease in the fraction of slab-derived components since deep wells were drilled in 1940s for spa development. However, this fraction also exhibited a temporary increase around the 1995 Kobe earthquake (M_w6.9), which resulted in 6,434 deaths and economic damages amounting to 200 billion USD. Remarkably, increases in the slab-derived fraction began before the earthquake at three out of seven studied springs, suggesting that such an increase in slab-derived water is more likely a cause rather than a result of the earthquake. Based on the temporal variation pattern of the fraction, we estimate that approximately 2.6-4.2×10⁵ m³ of slab-derived water was discharged through Arima hot springs, and this deep underground flooding triggered the earthquake.

Predictability of large earthquakes

Large earthquakes do not occur randomly, but they are more likely to happen where and when the flooding of slab-derived water occurs. Water inflow to and accumulation at the source region around the hypocenter can reduce the apparent friction coefficient, weakening the fault strength and thus potentially triggering earthquakes. Similar situations may be inferred for the 1965-1967 Matsushiro earthquake swarm (the highest M_w = 5.4) and the 2024 Noto Peninsula earthquake (M_w7.5) that struck Japan on New Year’s Day. Additionally, various types of precursory phenomena of earthquakes may be attributable to this deep underground flooding. Therefore, the monitoring of hydrologically slab-connected hot springs is expected to contribute to predict future earthquakes.