The Upheaval of an Insular Volcano Iwo-jima, Ogasawara Islands and Its Recent State of Activity By Ryohei MORIMOTO , Joyo OSSAKA , * *, Tokutaro HAT

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1 The Upheaval of an Insular Volcano Iwo-jima, Ogasawara Islands and Its Recent State of Activity By Ryohei MORIMOTO *, Joyo OSSAKA *, * *, Tokutaro HATORI *, Sadakatsu IZUTSUYA Kazunori URABE * *, Haruo TAKAHASHI *, Yoshimitsu OKADA Jun'ichi HIRABAYASHI * * * *, Kyozo ISA * * * * * and Hiroshi ISOBE Unusual uplift of the land was found about an insular active volcano Iwo-jima, Ogasawara archipelago, 1,200 km south of Tokyo. Remarkable retreat of shore line was revealed by reviewing topographic maps and aerophotographs of the island. Some upward movements of the island are to be one of the most principal reasons of the retreat. As the clear evidences of the upheaval of the land, following phenomena were observed by the writers who investigated the island five times from 4th July to 23rd August 1968 : a) Sunken vessels and a landing craft have emerged on the western coastal beach of the island : b) Faults and cracks appeared on the abondoned run-way of the former Japanese Navy in the central part of the island ; c) Coastal reef emerged on the sea to connect off shore islet with the island ; d) New coastal terraces were formed on the sand beaches ; e) Fresh living corals were exposed on the sea. Results of the investigations are enumerated as follows : 1) About 8 m upheaval of the land since was revealed by levelling survey at the western coast of the island. 2) Amount of uplift observed on the above-mentioned run-way is smaller in its central part than in both terminals, maximum vertical displacement of the fault being 3 m. 3) Tide gauge installed at the western rocky coastrecorded slight relative down of sea level but further observation should be continued without disturbance by typhoon to obtain more reliable data. 4) No expected microseisms were recorded on the high sensitive electro-magnetic seismographs except 2 or 3 per day. 5) Temperature of the fumarolic gases were Ž, Ž higher than that observed in In spite of the rise in temperature, gases of so-

2 called high temperature type, such as sulphur dioxide and halogens, were not contained. 6) Composition of the sublimates also accords with these results of chemical analyses of the gases. 7) Content of each main component element and its ratio to other ones is quite variable in several hot springs whose ph values range from 2.1 to ) Analcite was identified by X-ray and DTA analyses in the tuff forming Moto-yama, an eastern main flat cone of the island. Genetic condition of the mineral inferred from its chemical composition may indicate submarine eruption of the volcano. 9) Three weak zones were recognized by the present investigations Thus three are arranged in addition to the two ones previously reported by Tsuya. radially through the center of the eastern main cone and the two are arranged concentrically around the foot of the same cone. From these weak zones phreatic explosions are expected in future, if they occur. 10) Nothing more than mere weak phreatic explosion was recorded through the historic activity of the volcano. Artificial filling up of the vent, such as the pavement of the surface often causes the small explosion at fumarole. 11) Submarine eruption recently found at the sea bottom, about 5.4 km NNE of the South Sulphur Island, about 50 km south of the island did not occur recently but must have repeated rather stationarily. According to the present investigations, no remarkable magmatic explosion could be expected except those small phreatic ones, same one of which has often occurred on the island. Chemical composition of the gases does not indicate any approach of hot magma to the surface.

29 3) Pertersen, J. (1890) : Beitrage zur. Petrographie von Sulphur Island, Peel Island, Hachijio and Miyake-sima 4) Washington, H. S. (1917) : Chem. Ana. Igneous Rocks 463 8) Kozu, S., Watanbe, M. (1926) : Proc. 3rd. Pan-Pacific Sci. Congr., ) Krauskopf, K. B. (1948) : Trans. Amer. Geophys. Union., ) Macdonald, G. A. (1948) : Bull. Geol. Soc. Am., ) Corwin, G., Foster, H. L. (1959) : Am. Jour. Sci., ) R. Takahasi (1934) : Bull. Earthq. Res. Inst. Suppl. 1 19) Swenson, F. A. (1948) : Bull. Geol. Soc. Am., ) H. Tsuya (1936) : Bull. Earthq. Res. Inst., ) W. Nakajima, M. Koizumi (1966) : J. Geol. Soc.,

1793 Detailed Distributions of Seismic Intensity and Tsunami Heights of the Kansei off Miyagi Prefecture Earthquake of February 17, 1793 Yuichi NAMEGA

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