The thermal structures of the Nakanoyu geothermal area, Nagano and its vicinity are considered quantitatively. The Nakanoyu area is located in the south-eastern foot of an active volcano, Mt. Yakedake. In order to improve traffic in the Abo mountain pass which is closed for six months by heavy snow in winter and sometimes is closed by rockfalls and landslides, the Ministry of Construction has planned to construct a long tunnel there. As the route is near the summit dome of the Yakedake volcano, the temperatures within the tunnel may become extremely high. We, therefore, have to research the detailed 3-D subsurface temperature distribution of the Nakanoyu area and its vicinity. Numerical simulations were carried out to calculate the 3-D subsurface temperature distribution from the 1 m-depth ground temperatures and the measured temperatures within deep boreholes as boundary conditions. The computed results indicate that the subsurface temperatures are quite high in the region near the Nakanoyu hot spa, which becomes about 160℃ in the maximum at 1330 m altitude. With the increase in altitude, the computed subsurface temperature becomes low in general. These calculation results are quite consistent with the measured values within the new gallery which was drived after our theoretical research.

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Chemical analysis for major and minor elements were performed on the pumice and scoria samples from the 17 fall units of the 1707 fallout tephra of Fuji volcano. Analytical results showed that there exsisted three magmas different in chemical composition, implying that a layered magma reservoir with downward increasing basicity may have been formed prior to the eruption or a magma mixing might occur during the eruptive activity. The trude density of the pyroclastics also varied along with the variation of the chemical composition. The crystalline phases in ejecta were observed to be more abundant in the later ejecta than in the earlier. The variation in porosity of the ejecta observed along the stratigraphic column togather with the content of lithic fragments in the respective fall unit may discriminate the pyroclastics ejected from the two earlier formed craters from the ejecta from the later formed crater.

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