During the 1990-1995 eruption of Unzen Volcano (Fugendake), SW Japan, seismicity near the crater intensified in association with the growth of a dacite lava dome. In this paper, we examined the seismicity recorded there between November 1993 and January 1994, the period when remarkable inflation occurred in the SW fiank of the dome and the upper southern flank of Fugendake. The seismicity in this period was characterized by an increase in the number of high-frequency earthquakes and a gradual increase with time in the maximum magnitude of the earthquakes. On the basis of cross-correlation coefficients, we identified 70 earthquake groups, each having more than 100 earthquakes. This categorization expanded the definition of the well-known earthquake family; the waveforms in each group varied slightly over time. From among the 70, we then analyzed 20 major groups. The cross-correlation of waveforms in each group was higher for the earthquakes that occurred at shorter intervals. In many groups, the maximum magnitudes of earthquakes changed continuously with time, and the frequency distribution of magnitudes became largest near the maximum magnitude at that time. New earthquake groups were activated just after the activity of previous earthquake groups had declined or ceased. Such transitions had often been linked to changes in the rate of ground deformation on the upper southern flank of Fugendake, suggesting that the manner of magma movement and the stress field had changed significantly in and around the shallow conduit just beneath the lava dome.

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Hachijo-Nishiyama (meaning of the West Mountain) is an active composite volcano situated, next to another Pleistocene composite volcano Higashiyama (meaning of the East Mountain), on the Hachijojima Island, 300 km south of Tokyo. These two volcanoes stand on the volcanic front along the Izu-Mariana arc. Since the beginning stage of Nishiyama, ca. 10,000 yBP, the volcano repeatedly erupted in a shallow submarine environment. The explosively emitted pyroclastic materials covered almost whole the island. By 3,000 yBP, Nishiyama had grown as a subaerial volcano and lavas and fallout scoria were erupted without considerable effects of water. In this study, we presented petrographical and petrochemical descriptions of the lavas of Nishiyama in the latest 3,000 years. Based on these data, we discussed the differentiation processes of Nishiyarna volcano and the genetic relationship between the magma of Nishiyama and Higashiyama. Nishiyama volcano is totally composed of basalt to basaltic andesite with ubiquitous plagioclase, with or without mafic phenocryst minerals such as olivine, clinopyroxene, orthopyroxene, and titanomagnetite. In terms of SiO₂-Al₂O₃ relationship and amount of phenocrysts, we divide the differentiation trends of Nishiyama into three groups. P-group, A-group and T-group.

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