Additional data on the volcanic activity of Izu-Oshima is presented based on the detailed stratigraphic study of the Older Oshima Group in the southern part of the island. 1. The horizon of the lenticular lava flow which crops out on the southern coastal cliff at Yatsuiso is O₂ of the Older Oshima Group. 2. The scoria cone Sado-yama, is inferred to have been formed in the O₁₀ time. 3. The horizon of the uppermost one of three lava flows which crops out on the coastal cliff from Kootoshi to Otaine is O₁₄. 4. The explosions which occurred to the southeast of Oyanokubo in the O₁₅ time was followed by the ground-surge. 5. In the O₈ time, steam explosions occurred at the Kakiharaiso coast and a large quantity of accretionary lapilli with scoria cores was formed, the explosions were followed by another ground-surge.

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Several earthquake swarms have been occurred at the western region of the Aso Caldera. Some of them have consisted of over a hundred events between magnitude 1 to 4. The seismic observation has been carried out for the earthquakes originating at the region during long period since 1964, in order to determine the long term behavior of seismicity. The obtained data have provided a detailed feature of the seismic activity along the western caldera rim. The upper crust of the western caldera region is divided into the northern and southern areas at the Tateno-valley, which is the collapsed part of the caldera wall. At the northern area, the seismicity is very active and the most of foci have concentrated in the shallow part above 7km depth. The b-value for the events at the northern area is comparatively larger than the value at the southern area. On the contrary, at the southern area the seismicity shows calm and the foci have not been present in the shallow part above 5km depth and have been extensively scattered below 10km depth. These results seem to be closely related to that the tectonic line runs across the Tateno-valley from east to west. Most of events at the northern area have been showing as swarms with the high degree of clustering in time and space. At the periods of seismic swarm activities, the bvalues have trended to be decrease. These earthquake swarms appear to be accompanied by the volcanic activities of the Aso Volcano. The focal mechanisms have been also analyzed for the events at the northern area. The composite fault plane solution for events at the area has not been able to be determined because of the complicated distribution of first motions. It seems to correspond to that some small normal faults are existing in a parallel arrangement from north to south at the northern area.

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Kuju Volcano (1, 786m high) consists of dacitic lava domes and the associated non-welded pyroclastic flow deposits, Handa pyroclastic flow deposits, covering 60 km² area (Fig.2). The original vents of the pyroclastic flow deposits, whose age is estimated to be 0.04 Ma, have not been located. The volume is about 2km³. They contain pumice and accessory lithic fragments and very seldom show welding. At the upper and the basal parts of one flow unit, imbrications of pumice and lithic fragments are well developed with 10°-20°dip against the flow unit boundary (Figs. 4, 5). This dip (Table 1) is apparently not as steep as that of the other pyroclastic flow deposits. Imbrications are clearly observed at 6-12km from the center of the lava domes, while the distribution of the pyroclastic flow deposits covers 4-13km from the center (Fig. 7). The pyroclastic flows ran down 1000m in a vertical distance. The method of determining the flow direction by imbrication is very simple as shown in Fig. 6. Imbrication is most visible from the direction (a) perpendicular to the flow direction on the surface of each outcrop. The flow direction (f) is determined by the bisector (f') of the error angle 2θ formed by the two directions (b, c), between which imbrication is not observed. Data are classified into 3 ranks by the error angle θ as shown in Table 1. The flow directions at 52 outcrops (Table 2) are shown by classified arrows in Fig. 7. The estimated flow-direction patterns are largely divided into the north-flank flows and the south-flank flows (Fig. 7). The north-flank pyroclastic flow deposits flowed along the 2 km-wide major valley on ca 2°slope shown in K-L profile in Figs. 8, 10. Then it diverged to the west at the outlet of the valley, and finally collected in a small basin in the west. Such flow directions suggest that the flow was not derived from the adjacent domes D, E, F in Fig. 7. On the south flank, a fan-shaped pattern of the flow direction is generally observed. But the flows toward Aso volcano are sharply separated into two flows at the boundary between Kuju and Aso ("col" in Figs 7, 9). This is because the flow had not enough power to rush up the very gentle slope of Aso volcano. The evidence shows that the southward flow gradually bent 120°, and it rapidly went down eastward on the south flank. At Takenohata (n. in Fig. 7) this eastward flow crossed the southward flow. At this cross point, lower outcrops show eastward flow (A-B profile in Fig. 8), and higher ones show southward flow (C-D profile) as shown in Fig. 11. This means that after the eastward flow filled the old valley extending in east-west direction, the later pyroclastics flowed southward over the older deposits forming a fan-shaped deposit. Data clearly suggest that the vent for the pyroclastics is located within the circle around A, B, C lava domes, and not in the other domes. The flow directions indicated by imbrications agree with the distribution of the pyroclastic flow deposits. Pyroclastic flows follow the previous topographic relief such as valley, fan, and col. Kuju Volcano may not have emitted the flooded sheet-flows in all directions, but have emitted the tongue-shaped flows intermittently to different directions.

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In the 1977 and 1979 geophysical expeditions to the active volcanoes Nyiragongo and Nyamuragira in the Republic of Zaire, detailed geomagnetic surveys were carried out along several routes with about 1 km spacing in and around the volcanoes. Total intensity of the geomagnetic field was measured at about 310 sites by a proton-precession magnetometer. After subtracting the international geomagnetic reference field (IGRF), geomagnetic anomalies were separated into the local anomaly and the regional anomaly which was approximated by two dimensional second degree polinominal. Finally smoothed local geomagnetic anomalies were obtained by the method of two dimensional running mean. These anomalies could be interpreted by geological features such as dykes, lava or rocks at higher or lower temperature than the Curie point, or terrain effects. Local geomagnetic anomalies suggested that the shallow thermally demagnetized zones are near in the sites of recent eruption at Mihaga (erupted in 1954), Murara (1976) and Harakandi (1977) and that the shallow more strongly magnetized dykes exist at the northern part of Goma and the eastern side of Nyiragongo. The thermally demagnetized zones run parallel with the rift axis in this region and also coinside with the regions of volcanic tremors with a longer period and large amplitudes.

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