Researchers have discovered that the 1995 Kobe (Hyogo-ken Nanbu) earthquake, which affected southern Hyogo Prefecture, may have been caused by flooding occurring deep beneath the Arima Hot Springs. Through a detailed examination of the stable hydrogen and oxygen isotope ratios, as well as chloride ions in the hot spring water collected over several decades, they have found a probable link between the earthquake and water coming from the subducting Philippine Sea Plate.
Hot springs typically contain water that originates from rocks within the Earth’s crust, a fact that can be verified through isotopic analysis. Arima Hot Springs, located in Kobe, Hyogo Prefecture, Japan, are known for their unique properties, including a salinity level more than double that of seawater. This suggests that their water is likely sourced from the Philippine Sea Plate. However, direct evidence establishing this connection was previously lacking.
In their research, scientists confirmed that the isotopic ratios of the plate-derived water beneath Arima Hot Springs corresponded with those found in the actual spring water. This alignment matched predictions made by a numerical model. Furthermore, they noted a significant decline in the amount of plate-derived water following the introduction of deep well drilling in the 1940s, with a brief increase noted around 1995. Remarkably, prior to the 1995 Kobe earthquake, similar increases were observed in three of the seven hot springs examined. This mirrors the observed rises in chloride ions and radon levels in groundwater, which have been recognized as indicators leading up to the earthquake. The estimated quantity of plate-derived water during this time surpassed 100,000 cubic meters, which may have compromised the fault structure and triggered the Kobe earthquake.
Additionally, this phenomenon isn’t exclusive to Arima. The Matsushiro earthquake swarm (1965-67) displayed comparable traits. Researchers noted that a considerable portion of the water in Matsushiro hot springs also comes from the Philippine Sea Plate. Thus, regular monitoring of such hot spring waters could provide important information for forecasting earthquakes.
This research received support through a Grant-in-Aid for Scientific Research (grant nos. 15H02957, 19H01370 & 24K00169) from the Japan Society for the Promotion of Science.
