The August 1977 eruption of Usu Volcano produced four major tephras and several minor ones. In this paper, the four major tephras were simulated by a model of two-dimensional diffusion in the atmosphere. The simulation leads to the following conclusions : (1) In the atmosphere with stable wind, the mean value of wind vectors between the height and the ground can be used as an effective parameter to determine the dispersion of tephra. (2) A combination of the diffusion model and mean wind vectors leads to good simulation of the dispersal of tephra. The simulation is good even if in the complicated wind vector field. (3) When the median size of the tephra deposit changes systematically with the direction of locality measured at the vent, the locality of fine size deposit shows leeward direction of wind in the lower atmosphere and the locality of coarse size deposit shows that in the upper atmosphere. (4) The size distribution of whole-deposit of tephra can be estimated by the diffusion model on the supposition of logarithmic probability distribution.

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Two new heat flow values were determined at the Kushikino and Nichinan areas in southern Kyushu, Japan. The Kushikino area exists in the Tertiary volcanic region, where underground temperature was measured by using horizontal boreholes cut in the drifts of the mine and those drilled vertically and directionally from the land surface. By considering the effect of underground water movement to values of geothermal gradients, we determined a high value of 129mW/m² (3.07 HFU). Such a high heat flow value is consistent with those observed in other Tertiary to Quaternary volcanic regions of island arcs. At the Nichinan area, we also determined a high value of 94mW/m² (2.24 HFU) by using a vertical deep borehole up to 1, 000m. The Nichinan area exists in the Tertiary sedimentary region, which is between the Nankai trough and the volcanic front. It is peculiar that such a high heat flow is found in the non-volcanic region from tectonic point of view. The high heat flow value observed at the Nichinan area may show underground volcanism which is not identified at the surface.

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The ⁸⁷Sr/⁸⁶Sr ratios and the abundances of rubidium and strontium have been determined for 40 samples of Unzen, Tara and Kumamoto-Kimpo volcanoes, and Neogene basaltic rocks from the surrounding areas in Kyushu, Southwest Japan. The ⁸⁷Sr/⁸⁶Sr ratios in these Quaternary volcanic rocks range from 0.70418 to 0.70509. In each volcano, almost all of the magma seems to be subjected to the assimilation with different ⁸⁷Sr/⁸⁶Sr ratios during crystal fractionation. The ⁸⁷Sr/⁸⁶Sr ratios of the Quaternary volcanic rocks are clearly distinguished between the northern part and the southern part of Kyushu region marked by the Shiobara-Beppu Tectonic Graben in the central Kyushu. The ⁸⁷Sr/⁸⁶Sr ratios in the tholeiitic basalts, alkaline basalts and calc-alkaline basalts from the areas, range from 0.70362 to 0.70397, from 0.70404 to 0.70487 and from 0.70451 to 0.70470 (except for a calc-alkaline basalt from Kumamoto-Kimpo area, 0.70363), respectively. In the southwest Japan, the ⁸⁷Sr/⁸⁶Sr ratios of Cenozoic basalts from the Kyushu region are lower than those of basalts from the San-in region. This means that there exists a regional heterogeneity of Sr isotopic ratios under the Southwest Japan arcs.

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