A symposium on the prediction of volcanic eruptions was held on December 7, 1977 at the Science Council of Japan under the joint sponsorship of the Subcommittee on Prediction of Volcanic Eruption, National Committee on Geophysics and the Volcanological Society of Japan. Ten invited papers on various aspects of prediction of volcanic eruptions were presented in this symposium and are printed here in this Bulletin through the collaboration of the Editorial Board of the Volcanological Society. It is expected that these papers will provide valuable data for the study on the prediction of volcanic eruptions and for the prevention of volcanic disasters.

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The research program on prediction of volcanic eruptions started in 1974, as a national project which haa been proposed by the Geodetic Council, Ministry of Education. In this short paper, the following outlines are described. 1) newly established committees in order to promote the above research, and 2) contents of the proposals for a five-year program and recent achievements of research on prediction of volcanic eruptions in Japan.

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History of instrumental observation of volcanic earthquakes was initiated in the middle of 19th Century by L.Palmieri at Vesuvio Observatory. F. 0mori’s vigorous observation was made by Omori tromometer deviced by himself firstly at Usu Volcano in 1910 and at Asama and Sakura-zima successively. T.A. Jaggar's observation at Kilauea started also in 1912 and was found the continuous vibration of volcanic origin. Following to these vanguards, with the advancement of instrumental technique, relationship between volcanic earthquakes/volcanic tremors and eruptive activity has been investigated precisely by T.Minakami and S.Sassa. National project of the prediction of volcanic eruptions started since 1949 and modernized seismic equipments with telemetering system and data processing units were introduced at university’s volcano observatories. Plan of installation of seismographs at several active volcanoes are in progress by Japan Meteorological Agency as well as the replacement of the old seismographs at the existed volcano observatories.

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Predictions of volcanic eruptions, generally speaking, can not be achieved by observations of a single kind. They depend on synthetic consideration of the observational results of various kinds. Geodetic, geomagnetic, geoelectric and geothermic observations, that is, miscellaneous methods besides seismometric and geochemical ones, should be indispensable to achievement of the predictions. Geodetic observations consist of precise levellings, distance measurements and continuous observations of tilt. The intermittent surveys are useful for long-term prediction while the continuous observations are necessary for short-term prediction. Gravity changes at volcanic areas may be caused by density changes of subsurface material and vertical displacements of the earth-surface. In many volcanic areas, gravity measurements have been repeated several times with accuracies of ±20μgal, but not favoured by any sufficiently large changes. Geomagnetic observations would be carried out by proton precession magnetometers with accuracies of ±1γ. Their results have raised new difficulties in eliminating the disturbances of external origin and the local characteristics of secular changes. Geoelectric methods aim at detection of hot material by measuring subsurface conductivity. They are worthy of practice near the craters which are accessible with an apparatus. Geothermic methods are recently conccntrated on infrared imageries, especially air-borne ones. These three years, the air-borne infrared surveys have been carried out over about 20 volcanoes in Japan in order to prepare fundamental data for prediction of volcanic eruptions.

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Records of the historic eruptions in Japan were compiled by Omori (1918), Musha (1941, 1943a・b, 1951), Kuno (1962), and Japan Meteorological Agency (1975). Detailed information of these eruptions, however, is not always available in older writings. Interpretation and evaluation of the historic records by means of tephrochronology in the field and petrology of the volcanic products are essential to recognize the type, sequence and scale of the historic emptions, the result of which provides us important information for the estimation of future eruptions and mitigation of volcanic hazards. For this purpose, a list of the historic eruptions in northern Japan and that of the chemistry of their essential products are presented here, with some discussions on the estimation of future emptions.

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In this report some phenomena observed in seismic events and ground deformation before the summit emptions which have been lasted since 1955 at Sakura-zima Volcano are introduced and discussed. The observational facts are the following: 1) The focus of seismic events migrate from deep to shallow part before eruptions, 2) the focus of summit explosion-quakes distribute beneath the active crater in depth 1 to 3km, 3) the focal mechanism occurred beneath the Volcano changes before and after eruptions, 4) the spectral structure of volcanic micro-tremores suggests that dimension and/or physical boundary conditions of the origin can change easily, 5) the ground deformation around the Volcano is closely related with the summit emptions and 6) the expansion rate of Aira caldera is evaluated as 10¹² cm³/year and the volume of ejected materials is estimated as 10¹³ cm³ in the active period, 1965-72, which corresponds to the volume of materials accumulated under the caldera in 10² years.

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The activities of Usu Volcano has been continuing since August 1977 though the early eruptive period ended a week after the outbreak of the eruption. The present activities are felt and unfelt earthquakes, crustal deformations in the summit crater and at the foot of the volcano, and occasional steam explosions. Long-period prediction of the outbreak of the 1977 Eruption depended on the case studies of the past eruptions of the volcano. The eruption occurred almost in accordance with the prediction. Short-Period prediction of the outbreak consisted of seismometric, geodetic and geothermic ones. Unfortunately, nevertheless, the eruption broke out only 30 hours after the first swarm of earthquakes before these data would be synthetically analyzed. Predictions of developments of the volcanic activities after the outbreak of the eruption, depend on the geophysical observations of various kinds together with petrological examinations of the ejecta from the new craters. By seismometric observations, the hypocenters and the source mechanisms are determined, and the energy released by volcanic earthquakes is estimated. By geodetic observations, the rates of upheavals in the summit crater and of contractions along the volcano slopes are watched. Both the air-borne and land infrared imageries of the summit crater would give us the important informations concerning the types of the future eruptions, fissure or dome types, and developments of the activities. It is highly desirable that all results of various observations should be synthetically taken into consideration for predictions of the volcanic activities.

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In Japan there are about 200 Quaternary volcanoes, including about 77 active volcanoes. They provide us with various kinds of natural resources and beautiful sceneries. 0n the other hand, we have been suffering from volcanic disasters since the dawn of history. Scientific study of our volcanoes commenced soon after the Meiji Restoration of 1868, and the first volcano observatory in our country was established on the flank of Asama-yama, Central Honsyu, in 1911 by the Imperial Earthquake Investigation Committee in cooperation with Nagano Meteorological 0bservatory. Thereafter many volcano observatories were established by the Japan Meteorological Agency (JMA) and various univeersities. In recent years, due to rapid increase in number of climbers and tourists coming to those active volcanoes and of inhabitants near them, the danger to human lives and properties in case of eruptions of those volcanoes has been enlarged year after year. The volcanological task of the JMA has been expanding rapidly after the 2nd World War. Especially, since 1962, the JMA has been making every effort to establish systematic observation and research of all active volcanoes in Japan. 10 of our active volcanoes are situated on Kunashir and Iturup Islands, now occupied by the USSR, so the JMA is responsible for the surveillance of about 67 active volcanoes for the time being, ana divides them into three classes (A,B and C), depending upon the natural and social conditions of the respective volcano.

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The Hydrographic Department of Maritime Safety Agency has been conducting surveillance and investigation about the activity of submarine volcanoes in the adjacent seas of Japan. There are three active submarine volcanoes belonging to Middle Mariana ridge; Fukutoku oka-no-ba about 4 miles mrtheastward, Minami-Hiyosi Kaizan about 50 miles south-south eastward and Fukuzin Kaizan about 180 miles southeastward from Minami-Io Sima Is. respectively. Discolored water around them is found at irregular intervals, which should be caused by submarine volcanic eruption. The Hydrographic Department conducts to research their activity using remote sensing techniques such as aerial photogrammetry, multiband photographs and thermal camera by an airplane. A survey ship also is used to research bathymetry, geological structure, geomagnetism and gravity in the submarine volcanic areas at the rest time of the volcanic activity. In this paper, wedescribe about the system of surveillance and investigation to them together with the record of recent submarine volcanic eruptions in the adjacent seas of Japan.

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0ur Working Group (IAYCEI) on “Mitigation of Volcanic Disasters” has promoted to establish the useful method for predicting volcanic eruptions, based on instrumental observations in order to prevent or mitigate volcanic disasters. In this brief report, a list of dangerous volcanoes in the world is shown in Table 1, which is compiled by informations from the members and correspondents of the Working Group. 0n another hand, the 1969-71 event of Pozzuoli area near Monte Nuovo, Italy, is reported in a condensed form. The land deformation and seismic activity of Pozzuoli are compared with those of the 1930 Ito and the 1943-45 Usu events.

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The observation networks for monitoring volcanic activities of 66 active volcanoes in Japan are reviewed. For the purpose of improving the present observation networks to strengthen efficiency of the prediciton of possible volcanic eruptions of these volcanoes, several practieal technical proposals are made.

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