伊豆大島三原山火孔底最近の変動

抄録

Observation of the level change of the bottom-floor of the central pit in the summit crater has been carried on at Mihara-yama, Oshima Island since 1968. Pictures of the bottom-floor were taken by observers went down into the central pit along its wall at the earlier stage of the study and by radiocontrolled model helicopters and an observation balloon later. The depth of the bottom-floor was measured with wire, ropes and transits. Rise of 95 meters of the bottom-floor of the central pit was observed after a minor eruption of Mihara-yama on February 28, 1974. Before the eruption, the bottom-floor had kept roughly constant height of about 360 meters above sea level during this decade. The bottom-floor may have become shallower by a pile of flooded lava during activity of the eruption. After that the surface of the bottom-floor may have been uplifted as a whole by rise of the head of the magmatic column accompanying formation of spatter cones by Strombolian eruptions at the central part and flood of molten lava from the margin of the bottom-floor in the central pit. The active stages of Mihara-yama and Miyakejima may be determined by relation between frequency of volcanic earthquakes in Oshima Island and those in Miyakejima Island as shown in Figure 7. In an active stage of each island, there exist three maxima of frequency of volcanic earthquakes and a major eruption occurrs at the second maximum of volcanic earthquakes. It is estimated that an active stage of Mihara-yama started at 1970 from Figure 7 and that the first maximum will finish at 1975. The minor eruption of Mihara-yama in 1974 occurred just during the first maximum of volcanic earthquakes. The second maximum of the volcanic earthquakes is expected to come before long, accompanying a major eruption. The rise of the bottom-floor of the summit crater of Mihara-yama represents that of magmatic head. Generation of volcanic earthquakes and elevation of the head of the magmatic coulmn can be explained as phenomena caused by compressive stress concentrated under the volcano, though the mechanism which joints those earthquakes and level change has not been known. Therefore, this remarkable uprise of the bottom-floor in the summit crater of Mihara-yama might be a precursor of a coming major eruption.