Hot springs reflect the flooding of slab-derived water as a trigger of earthquakes

In subduction zones, water expelled from the subducting slab is believed to be involved in seismic activity. However, little is known about its quantity and flow processes. Here, we show that the Arima hot springs in western Japan contain high concentrations of water derived from the subducting Philippine Sea slab. A long-term record spanning over half a century reveals that the fraction of slab-derived water exhibited a temporary surge in the year preceding and/or subsequent years of the 1995 Kobe (Hyogo-ken Nanbu) earthquake. In total, an estimated 2.6–4.2 × 105 cubic meters of slab-derived water was introduced in conjunction with the earthquake. We infer that the earthquake was triggered by a flood-like release of water from the slab or by the bursting of clogged flow paths to the hypocenter. Our findings highlight the importance of hydrologically slab-connected hot springs for understanding ultradeep water cycles and their causal relationships with seismic phenomena. Water expelled from subducting slabs has a strong control over seismic events, with changes in hot spring water chemistry recording the surge of slab derived water, according to geochemical studies of water samples from the Arima hot springs, Japan.