ABSTRACT

On 1 January 2024, an M7.6 earthquake occurred in the northern Noto Peninsula, Japan, following prolonged earthquake swarm activity that has been suggested to involve subsurface fluid migration. To provide geochemical constraints on the nature and origin of such fluids, we analyzed lithium (Li) and strontium (Sr) isotope systematics of waters from the Noto Peninsula, with a primary focus on hot spring waters that are considered to reflect fluids circulating through deeper parts of the crust over long residence times. River water (MSK) and non-thermal shallow groundwater (KWS) were analyzed as reference samples representing surface and shallow groundwater components. The hot spring waters are characterized by high Cl/Li ratios (>1000) and relatively high δ⁷Li values compared with volcanic and forearc deep-derived fluids reported from southwestern Japan. These geochemical features suggest that the sampled hot spring waters do not record a dominant volumetric contribution from high-temperature deep fluids such as magmatic or slab-derived fluids. Instead, the Li–Sr isotope systematics suggest extensive interaction with crustal rocks and mixing with ancient seawater that infiltrated the crust during the Miocene opening of the Japan Sea and was subsequently modified during long-term subsurface residence. Time-series observations of δ⁷Li and Li/Sr ratios at selected hot spring sites reveal short-term temporal variations in fluid composition. However, these variations do not show a consistent temporal correspondence with individual seismic events, nor are they accompanied by systematic changes in ⁸⁷Sr/⁸⁶Sr ratios. This indicates that the observed fluctuations reflect transient changes in mixing proportions among crustal fluids and ancient seawater components rather than episodic input of a distinct deep-fluid endmember. Overall, this study provides geochemical constraints on the origin and evolution of subsurface fluids beneath the Noto Peninsula. Although previous studies have suggested the involvement of deep-seated fluids in the seismic activity of the region, the hot spring waters analyzed here indicate that such high-temperature deep fluids are not directly expressed in the shallow hydrothermal systems sampled.