We investigate the effect of movement of nearby faults caused by an intrusion of dyke on volcanic activity. We examine two factors as indicators of the effect of faulting. One is a change in stress concentration at the dyke tip and the other is a change in opening of dyke. In this study, a finite element method is applied under a two dimensional plane stress state to incorporate geometrical relationship between the dyke and the fault. Dyke and fault are modeled as an open crack with internal pressure and a shear crack with Coulomb friction, respectively. Geometrical parameters considered are an angle, a distance between the dyke and the fault, and the length of fault. One of the most important results is as follows; when a fault with high angle against the dyke strike extends beyond the dyke tip along the propagation direction, and when the fault with low angle extends to it, the dyke propagate and open more easily than the case without fault. We apply our result to recent volcanic activities in Japan associated with the dyke intrusion, the 1986 Izu-Oshima and 1989 off Ito eruptions, and we suggest that changes in volcanic activities may have been attributed to the occurrence of moderate-sized earthquakes near the dyke tip.

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The lava effusion process from a cinder cone and its mechanism are discussed based on the field observation of Ojika-Jima Monogenetic Volcano Group (OMVG). The cinder cones of OMVG are classified into two types, C-type and D-type cones, based on the mode of lava effusion from the cone. In the C-type cone, lava overflowed from the central crater, whereas in the D-type cone, lava flowed out from the flank. These types are related to the morphology and internal structure of the cone. The ratio of cone height (H_co) to width (W_co) of the C-type is smaller than that of the D-type, and the part of the dense welding is widespread around the cone. On the other hand, the welding area of the D-type is within the limits to the central part of the cone. The D-type is further divided into two types; Dc-type is accompanied by a mountain body collapse with lava effusion and Dp-type does not have this collapse. The majority of Dc-type cones are larger than Dp-type cones, although the ratios of H_co/W_co are similar. In the OMVG, a thin dike (less than 1 m thick) is generally observed inside the cone. However, if a dike intrusion does not have enough stress to collapse a mature cone, a branched dike system could cause a much larger load to the slope of cone and push a sector of the cone outward. Therefore, a branched dike system seems to control in cone breaching. The dike system is always observed inside Dc-type cones, while it is rare inside Dp-type cones. Considering the concept of crack propagation in an elastic body, the dike branches off under the condition that the breaking strength of the deposit around the tip of a feeder dike is low. Accordingly, the collapse of a cinder cone caused by a branched dike system is incident in the larger-scaled cinder cone, especially when the welded area is restricted to the central part of the cone and altitude difference between the lava lake in the crater and the top of the dike is large. It has been assumed in previous works that the density difference between the lava and cinder cone is the main controlling factor for the mode of lava effusion from the cinder cone. In this paper, the author concluded that the degree of welding around the feeder dike and total volume of the cinder cone are the major controlling factors in the dike propagation process.

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The mercury concentrations in fumarolic gas condensates collected from volcanic areas in Hokkaido, Miyazaki, and Kagoshima Prefectures, Japan, were investigated. Mercury concentrations ranged from 0.12 to 135 μgl^-1 in 44 collected samples. It was found that there was a positive correlation between mercury and chloride concentrations in fumarolic gas condensates collected from Satsuma-Iwojima Volcano, Kagoshima Prefecture. It was considered that the mercury in fumarolic gas was discharged in the form of mercury (I or II) chloride. However, the results from mercury chemical equilibrium calculations for the vapor in fumarolic gases indicated that mercury was discharged mainly in the form of metallic mercury vapor.

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Miyakejima volcano, south off central Japan contracted remarkably during the eruptive period in June-September 2000 according to the GEONET data obtained by the Geographical Survey Institute of Japan. However it proved that inflations appeared repeatedly before several major explosive eruptions, although the signal was almost masked by a contraction process. Based on a possible precursory inflation, a forthcoming major eruption was experimentally predicted just one day before the largest explosion on 18 August 2000. Such an experience will be useful for the better understanding of a volcanic process and the progress of a prediction of volcanic eruption.

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Clay minerals such as smectite, kaolin mineral and pyrophyllite and secondary minerals such as pyrite and gypsum are frequently found in fine fraction of volcanic ash from many volcanoes in Japan. Natural occurrences of clays and secondary minerals disclose geological and geochemical settings and alteration environment. Chemical compositions and mineral assemblages of alteration products in volcanic ash can provide authentic information on chemical conditions inside volcanoes and their temporal variations indicate change in chemical condition beneath volcano before and after its activation. Researching assemblage of clay minerals in volcanic ejecta and their chemical compositions is a safety method for analysis of volcanic activity and will contribute to prediction of change in volcanic activity.

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In June 2000, several eruptions occurred on Miyake-jima Island, accompanied by volcanic and/or hydrothermal activity offshore from the island. In January 2001, water samples were collected from five stations around the island in order to reveal the existence of volcanic activity on the seafloor in the area. Concentrations and stable carbon isotopic compositions of dissolved methane in the water samples, however, showed no sign of submarine volcanic activity over the area. These results indicate that the submarine volcanic activity in the area was not in steady state at least during the sampling program.

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The Moritayama Lava lies in the adjacent area of Sambe Volcano, and has been thought as a part of Miocene basement rocks. Radiometric age determinations are carried out on the Moritayama Lava, and 0.9±0.2 Ma (fission-track ED-2 method) and 1.01±0.03 Ma (whole rock K-Ar method) are obtained. These data indicate that the Moritayama Lava is the first stage volcanic product of the Sambe Volcano at early Pleistocene time.

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The sixth national program for prediction of volcanic eruption has been conducting since 1999 in order to advance toward the higher stage of the prediction of volcanic eruption. The progress of prediction depends not only on the advance of basic knowledge of volcanology but also on the accumulation of successful and non-successful practical knowledge through the occurrence of volcanic eruptions. Among nine eruptions and three volcanic unrests that occurred during the last five years in 1997-2001, the predictions of two large eruptions at Usu and Miyakejima volcanoes in 2000 and of volcanic unrest at Iwate volcano in 1998-1999 were reviewed and evaluated mainly on the basis of geophysical point of view. Based on detection of some precursory phenomena, the prediction of the beginning time of the eruptions at Usu and Miyakejima volcanoes was successfully performed by the practical experience. The semi-real-time prediction of the beginning time at Usu volcano was also successfully operated, based on the application of rock fracture criterion to the seismic data that were automatically processed in the real-time operation. However, it became clear that the predictions of eruptive mode and sequence at Miyakejima volcano were still far beyond the grasp of the basics because of abnormally complicated behavior. It is confirmed that a tomographic imaging of the shallow volcanic conduit system together with a precise 3-D hypocenter distribution contributes significantly for deep understanding of what was going on the volcanic unrest at Iwate volcano.

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In this report, we review prediction researches on the recent eruptions at Usu and Miyakejima volcanoes, based on three points of view: 1) basic understanding of magma-plumbing system, 2) detection of magma accumulation processes, 3) detection of magma movements by extensive observations. Each volcano showed contrasting long-term precursors. Usu volcano showed no remarkable inflation before the beginning of seismic swarm activity while Miyakejima showed quasi-continuous inflation. Before both eruptions, we could clearly detect remarkable precursors and contributed to dispatch actual warnings leading to the quick evacuation of inhabitants. After the beginning of the eruption, however, it was difficult to predict the time developments of the activities, especially in case of Miyakejima volcano where a caldera collapse was accompanied by repeated explosive hydromagmatic eruptions, and followed by a huge amount of continuous degassing. In order to make a successful long-term prediction or to predict a scale, style and time developments of volcanic eruption, we further need to elucidate magma-plumbing system and to understand particular processes operating in magma.

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Recent progress on petrological and geochemical studies on eruption products is reviewed especially related to three major eruptions in the last decade, 1990-95 Unzen, 2000 Usu and 2000 Miyakejima. Detections of essential magmatic fragments in the early eruptive products were critical for the evaluation of the progress of ongoing eruptions. Mineralogical studies were useful not only to identify essential materials, but also to estimate the depth and temperature of magma chamber and magma ascending processes. Systematic studies on volatiles both in melt inclusions trapped in phenocrysts and in volcanic gas have enabled to construct the quantitative models of the dynamic processes of magma. Petrological and geochemical estimations of the depth of magmatic processes are now becoming to be combined with the geophysical observations, and will be used together to construct more dynamic magma models in the near future. Experimental petrology to determine precise phycico-chemical conditions of magma and kinetics of crystallization and vesiculation will be the key studies in the future to assemble more quantitative information on magmatic activities and appropriately predict the change of ongoing volcanic activities.

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