Abstract
Evidence for relationships between seismotectonic activity and dissolved weathering fluxes remains limited. Motivated by the occurrence of new springs emerging after the 2016 Kumamoto earthquake and supported by historical groundwater data, this study focuses on the long-term effect of near-surface structural deformation on the contribution of deep, highly saline fluids to the solute fluxes from the Aso caldera, Kyushu, Japan. Available hydrologic and structural data suggest that concentrated, over-pressured groundwaters migrate to the surface when new hydraulic pathways open during seismic deformation. These new springs have a hydrochemical fingerprint (including δDH2O, δ18OH2O, δ7Li, δ11B, δ18OSO4, and δ34SSO4) indistinguishable from long-established confined groundwater that likely reflects a mixture of infiltrated meteoric water with high-sulfate hydrothermal fluids. A comparison of historical hydrochemistry data and patterns of past seismicity suggests that discharge of deep fluids is associated with similar deformation structures to those observed during the Kumamoto earthquake, and that seismic activity plays an important role over historic timescales in delivering the majority of the solutes to the caldera outlet, sustaining fluxes that are amongst the world’s highest. This upwelling mechanism might be relevant for other systems too, and could contribute to the over-proportional share of active volcanic areas in global weathering fluxes.
Highlights
Active volcanic areas contribute a disproportionately large component to the global flux of dissolved elements delivered from the land to the oceans, both in comparison to other lithologies[1,2] and to inactive volcanic systems[3]
One of the curious hydrological phenomena after the 2016 Kumamoto earthquake was the appearance of new highly saline fluid discharging in this northwestern plain (Figs 1a and 2a,b, and Table 1), where the earthquake caused major surface ruptures (Fig. 1a,b)
This study considers how such structures may sustain the high solute fluxes observed in this region, and contribute to the high apparent weathering fluxes from active volcanic systems
Summary
Compressional crustal strain and pressurized fluids affected by a heat source can drive co- to post-seismic upwelling of deep fluid through crustal ruptures[14,25,26]. The confined groundwater hydraulic potential in the northwestern plain is higher than the ground surface level, as evidenced by the presence of natural cold springs (e.g., Kayahara, Fig. 2c and Table 1), artesian deep groundwater, and hot springs (Uchinomaki, Table 1), all observed for several decades in this region This structural and hydrogeological context leads us to suggest that the observed new fluids discharging after the Kumamoto earthquake migrated towards the surface from the confined groundwater by pressure release[30] through either visible (as opening fissure) or invisible (small structural weak and conduit) pathways (Fig. 3), a mechanism responsible for pre-existing artesian wells. Together with high sulfate content and slightly elevated water temperature of the new spring (23.1 °C; Table 1 and Supplementary Table 1), these geochemical indicators are consistent with meteorically-derived groundwater admixed with deeper fluids of hydrothermal origin
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