Total mass and total grain size distribution of TB tephra, a sub-plinian deposit of the 1986 eruption of Izu-Oshima, Japan, were estimated. Based on field survey and granulometric analysis, isopleth maps of mass loading per unit area were made for each size class of one phi interval. It was found that there exists a relationship between an isopleth (C) and area of the isopleth (A) which can be expressed as; ln C=-k Sqrt A. From this relation, total mass of each size class was obtained and total mass of the deposit was calculated from their sum. Obtained total mass of TB tephra is 3.6×10⁹ kg. TB tephra is very rich in coarse population compared with major plinian products. Scoria larger than 2 mm accounts for about 90% of the total deposit. Estimations of masses of fine fractions seem to be performed precisely owing to their large k and the fact that even for the finest fraction, highest concentration of fallout remains around the vent. Distal deposit which falls on the sea is restricted its maximum to assume k on the sea is higher than that of plinian eruption. This assumption seems to be reasonable because plinian eruption has a much smaller k presumably due to a much higher expansion rate of umbrella cloud. The obtained maximum estimation of the total mass of TB tephra is up to 1.4×10¹⁰kg.

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Associated with the 1914 great eruption at Sakurajima volcano, southwestern Japan, the maximum height of volcanic cloud is discussed based on collected documents, sketches and photographs in those days. A series of photographs up to around 10 : 40 on January 12 (in Japanese Standard Time) represents that the volcanic cloud height attained to 7,000 to 8,000 m above sea level. After then, it proved that several documents reported the height to be 9,500-15,000 m, or even more than 18,000 m a.s.l, although it is difficult to obtain reliable evidences. Considering these reports and other observations from a distance, the height of 15,000 m is tentatively proposed here as a possible maximum value. According to an empirical relation, an eruption rate of small pyroclastic materials is suggested as, roughly speaking, 5,000 tons per second or 20 millions of tons per hour, if the volcanic cloud was 15,000 m in height.

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We have detected a clear reflected S-wave (SxS) in seismograms of an earthquake swarm in the crust beneath Osoresan volcano in northern Tohoku Disthct. The location of S-wave reflector is estimated from travel time differences between SxS and S-wave. It is located within a previously known low-velocity zone of P-wave in the uppermost mantle beneath the volcano. The reflector has a size of at least 6 km×8 km and is distributed in the depth range of 44-48 km. It is inclined to the south-southwest, with a steep dip angle of about 40°. The large reflection coefficient of the reflector and its close proximity to the P-wave low-velocity zone beneath Osoresan volcano strongly suggest that the body of reflector is filled with molten materials. We infer that the low-velocity zone beneath the volcano, i.e., the root of the volcano, consists of magma bodies of this sort, one of which we have detected presently.

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