The seismogenic layer in the zone on the southern side of the Median Tectonic Line that runs in the east-west direction crossing the Kii Peninsula and Shikoku is abnormally shallow. It is “abnormal” because the shallowness of the depth is not concordant with the generally observed feature that the higher the altitude of the area, the shallower the seismogenic layer. The depth of the seismogenic layer is as shallow as that in the mountainous zone along the Volcanic Front in northeastern Japan, nevertheless such low altitude areas as Wakayama and Tokushima plains exist in the zone on the southern side of the Median Tectonic Line. If it is postulated that the depth of the seismogenic layer is regulated by the brittle-ductile transition temperature of the crustal materials, the shallowness of the seismogenic layer means that the temperature in the crust is relatively high. Then, why is the temperature in the zone along the Median Tectonic Line high? We think it is not unreasonable to consider that thermal fluids are rising from the subducted slab there, if it is reminded that hot waters are estimated to exist beneath the Wakayama swarm region that belongs to the zone.

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Deep-rooted fluids play important role in generating earthquakes and/or volcanic eruption, whereas it was difficult to research deep-rooted fluids from groundwater samples using traditional geochemical tools owing to contamination from surface water. The Li isotopic index, a new geofluid research tool, has a potential to provide us information about deep-rooted fluid without contamination from surface water. Based on the Li isotopic data of groundwater samples, it has been revealed that geofluid involving the earthquake swarms beneath the southeast flank of Ontake volcano in central Japan is attributed to non-volcanic deep-rooted fluid. Furthermore, this paper argues the relationship between inland large earthquakes and the discontinuous deep-seated fluid upwelling that is induced by the periodic breaking of the low-permeable barrier (cap) on the deep-seated fluid reservoir. The Li isotopic tool has a potential to provide us new information about the discontinuous deep-seated fluid upwelling.

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Through a geological survey, this study examines the geochemical properties of groundwater and river water distributed in the environs of the Median Tectonic Line (MTL) on Kii Peninsula, Japan, and their upwelling paths in the upper crust. Carbon dioxide-bearing water spouting in the environs of the MTL is assumed to be contaminated by Arima-type fluid derived from the subducted Philippine Sea slab, based on the result of water quality, oxygen–hydrogen isotope ratio, and helium isotope ratio. Arima-type fluid is presumed to ascend along the MTL fracture zones and related faults parallel to the MTL. NaCl-type and Ca(HCO₃)₂-type groundwaters with high concentrations of Cl^- discharge in the northern part of the study area at the related faults, and CaSO₄-type and Ca(HCO₃)₂-type groundwaters including some high concentrations of Cl^- spout out on the MTL in the southern part of the study area. Arima-type fluid is diluted by the local groundwater system and erupts at the intersection of faults and rivers. Cataclasites and microfractures accompanied in the faults are filled with calcite veins, showing that the fluid penetrates into them, where calcite then precipitates. As a result, the discharge path to the ground surface is sealed and eruption points then migrate to fresh fractures.

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