As a part of the national program on the prediction of volcanic eruptions, the Hydrographic Department is conducting the investigations by the remote sensing technique on the submarine volcanoes and the volcanic islands. After Miharayama erupted on Nov. 15, 1986, the Hydrographic Department made 14 time-observations of discolored waters around Osima and craters of Osima Miharayama by the airborne thermal infrared radiometer AGA 780 from Nov. 22, 1986 to Mar. 18, 1987. The results are: (1) Temperature of discolored waters around Osima was lower than that of sea water surrounding discolored water of about 1～2℃, and there was no higher-temperature discolored water. (2) If the discolored water was dense, the origin of it coincided with the lower temperature area of the sea water many times and correlation between the area of the discolored water and the isotherm lines was good. (3) There were many lower temperature areas without the discolored water. (4) If the discolored water was thin, correlation between the area of the discolored water and the isotherm lines was bad. (5) It was suitable to make thermal observations before sunrise to detect the distributions of the sea surface temperature in detail. (6) In the daytime observations sea surface temperature was apt to show no thermal distribution pattern. (7) Temperature of the A crater (crater opened at the south wall of the old crater) had kept high temperature. (8) Temperature of the B craters (craters opened at the north rim of the old crater) showed a tendency to decrease. Temperature of the B craters on Mar. 17 was rather high. (9) Temperature of the C craters (craters opened on the flank of Miharayama) showed a strong tendency to decreae. Temperature of the C craters on Mar. 17 was nearly even to the ground temperature. (10) Temperature of the A crater, the B craters and the C craters became higher together before the small eruption on Dec. 18, 1986. As the observation conditions were differed at every observation time, careful consideration should be required to discuss the relation between the volcanic activities and the observed temperature of the craters.

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Two gas monitoring systems were set after the 1986 eruption at the summit area of Mt. Mihara, Izu-Oshima,. One system has been developed to observe the H₂, CO₂ and O₂ concentrations in the fumarolic gas and in the atmosphere. The other system monitors the H₂ concentration in the atmosphere in the crater area in order to evaluate the degassing activity of magma. Mt. Mihara explosively erupted on 16 November, 1987 after an interval of about one year since the last eruption. Both gas monitoring systems detected the increase in the H₂ releasing activity from the crater area about one month preceding to the explosion. Geochemical monitoring would be useful for detecting the volcanic activity and the prediction of eruption.

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Since the late of May, 1986, many pumices have drifted to the Ryukyu Islands. These pumices are characterized by their size, color and components. The pumices vary in size: some are about 5 mm and the maximum size is about 20 cm. Most of these pumices are gray and a few of them are drak- or light-gray. The pumices contain several percent of essential black xenolithes which vary in size from 1 mm to 5 mm and rarely 2 cm. The rock name of pumices is olivine-augite-bearing trachyte and that of xenolithes is forsterite chromian diopside trachybasalt. These characteristics of the pumices correspond with those of pumices erupted from Fukutoku-oka-no-ba in the north of the Mariana Islands in January 18-21, 1986. Moreover, the chemical composition of host pumice (free from xenolith), bulk composition (including xenolith) and mineral chemistry of phenocryst also correspond with those of the pumices effused from Fukutoku-oka-no-ba. Based on these facts, it is evident that the pumices drifted to the Ryukyu Islands had their origins in Fukutoku-oka-no-ba. Judging from the data concerning the current and wind around the area through Fukutoku-oka-no-ba toward the Ryukyu Islands, it is confirmed that the pumices reached the Ryukyu Islands from Fukutoku-oka-no-ba after four months under the effect of current and wind which run both to the direction of west.

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