With the aim of comparing seismicities and finding precursory phenomena for the volcanic eruption, a joint hypocenter determination method is applied to the earthquake swarms which were accompanied with the eruption of Miyakejima in 1962 and 1983. In contrast to rather diverse interpretations of the 1962 swarm given by existing studies, the result of our new analysis shows much tightly clustered distribution of hypocenter, about 10 km width and 15 km long, in the north-west quadrant of the volcano. A small seismic gap across the cluster was recognizable. Spatial distribution of the swarm was consistent with the result of macroseismic observation and extent of damages on the island caused by the strong shocks. Before and after the eruption, we couldn’t find any appreciable change of hypocenter distribution which will be effective for a prediction of the eruption. Most of earthquakes which occurred during and after the 1983 eruption were located in the region off the south coast of Miyakejima. The seismic activity may be classified into foreshock-mainshock-aftershock type and is not so intense as that of 1962, with respect to duration of activity and number of strong shocks, although magnitude of the largest event (6.2 by JMA) was larger in 1983 than that of 1962 (5.9 by JMA). Slight migration of epicenters toward the south-east direction was recognized after the eruption terminated, that might be related to the drain back of magma head along the southern fissure line. Focal mechanisms of large shocks were of strike-slip types with maximum pressure axis in the NW-SE direction in the northern area of the volcano and NNW-SSE direction in the southern area. Focal mechanisms of minor events inferred from initial motion of P waves at Miyakejima and other stations were consistent with those of the big events. Hence, there is no need to apply special focal mechanisms, such as a negative single force model nor a tensile-shear crack model, to explain the observational data.

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As an approach to the prediction of volcanic eruption by chemical methods, several proposals are made : 1) Helicopterborne γ-ray spectrometry in order to determine the observation spot on a volcano, 2) Exploitation of a probe specified to an individual gas species contained in volcanic gases, 3) Laboratory experiment as the basis for understanding surface manifestations such as the increase in HCl/SO₂ ratio in volcanic gases prior to the eruptive activity, 4) Experimental and theoretical studies for the possibility of vesiculation due to magma mixing, and 5) Estimation of the magma depth based on the ²²⁰Rn/²²²Rn ratio in fumarolic gases. In order to accomplish the prediction of eruption, it is urged that the progress in volcanology itself is the most important. In-situ continuous observation, laboratory experiment, and theoretical consideration should be combined with harmony.

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Discharges from a volcano such as volcanic ejecta, ashes, gases and hot spring waters can provide us with a variety of underground information of the volcano, since the change in volcanic activity brings about the changes in chemical compositions of the volcanic exhalations. In the present paper we have reviewed the chemical anomalies in the volcanic gases which are related to volcanic eruptions. It is common that the chemical anomalies observed at a different type of volcanic eruptions are very different, but they also seem highly variable even for the eruptions of a similar type. Although an overwhelming majority of the anomalies were recognized as “precursors” after the eruptions, accumulation of such information is quite important to give basis to the geochemical prediction of volcanic eruptions, since our knowledge is still highly limited. The chemical anomalies observed so far have mostly been obtained by repeated spot analyses with variable time intervals. This often makes it difficult to determine when the anomalies did start and how long they lasted. Therefore, we should perform truly continuous observation and collect as much geochemical and geophysical information as possible on each active volcano in question in order to make the geochemical prediction of the change in volcanic activity possible with confidence.

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Isotopic study of volcanic noble gases is of great significance to make progress in the investigation of eruption prediction by geochemical techniques. Since noble gases are chemically inert and the elemental and isotopic compositions are not affected by chemical reactions between volcanic gases and surrounding rocks of a volcanic vent, a sign of change in magmatic activity is supposed to be reflected directly on the composition of noble gases collected at a fumarole. A long-term variation of ³He/⁴He and ³He/²⁰Ne ratios at Showa-shinzan volcano from 1945 to 1977 is discussed on the basis of noble gas data given by NAGAO et al. (1980). A sudden change in the decreasing rates for both ratios is found around 1965. This change in the noble gas composition correlates with a temporal variation up to 100℃ in fumarolic temperature and with a monthly change in the highest degree of color smoke erupted from this volcano. One of explanations for this event is the temperature decrease of Showa-shinzan magma followed by solidification of the magma surface opening into the vent. In order to check a short-term variation of the noble gas composition, fumarolic gas was collected at the A-fumarole of Zao volcano in short intervals between 10 and 60 minutes, and analysed for the noble gas composition. The ³He/⁴He ratio is constant within experimental errors (2σ). However, the ³He/²⁰Ne ratio shows large variations even among fumarolic gas samples collected within several hours. An abnormally high ³He/²⁰Ne ratio was observed for samples collected in Setember 14, 1980, and it was followed by occurrence of a small scale of mud flow in May, 1981. The ³He/²⁰Ne ratio in September, 1980 is approximately 5 times the usual value of ³He/²⁰Ne ratio at the A-fumarole. This suggests that the noble gas composition such as ³He/²⁰Ne is able to detect even a small change in magmatic activity and is helpful for the eruption prediction by geochemical techniques. The proportion and the elemental and isotopic ratios of the magmatic component of noble gas were estimated according to a mixing model between a primary component released from magma and a secondary component originated from ground water.

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Several factors controlling the chemical composition of gas released from magma are briefly reviewed from a viewpoint that the compositional changes of gas released from magma may happen before volcanic eruption and that the studies of those compositional changes are important to find out the chemical precursors of eruption. The following factors are noted : (1) The compositional dependency of fugacity of an individual gas in multicomponent gaseous systems and the solubility of each gas into magma under high pressure and temperature should be fully investigated. (2) It is essential to understand the dissolution mechanism of gas into magma and also the chemical form of dissolved gas in magma. (3) It is also important to know the chemical form of chloride ion in the gaseous phase coexisting with hydrous magma, i.e. metal choride or hydrogen chloride. An example of chemical composition of gas released from magma is presented based on a quite simplified model of magma-gas system. It is implied that the chemical composition strongly depends on the pressure at which the gas released from the magma.

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