抄録
Eruptive activities of Miyakejima Volcano are studied with reference to tectonics, earthquake swarm activities around the volcano and their focal mechanisms. The earthquakes in this region show a tendency to occur in swarms. Near the volcano, there exist three tectonic ridges with strike of NE-SW ; (1) Niijima-Zenisu ridge, (2) Miyakejima-Onoharajima ridge and (3) Mikurajima-Inanbajima ridge. In the period before the eruption of the volcano, the earthquake swarm tended to concentrate in a narrow part of the ridge (1) or (3), and in the next period, in other part of them. In the course of earthquake swarm activities, the regions of ridges (1) and (3) were extensively occupied by the seismic focal region. The eruption of the volcano started after seismic gap had grown up around the volcano, and after the eruption, a part of the major seismic gap was filled by focal regions. This cycle was commonly observed before and after the eruptions in 1940, 1962 and 1983. Focal mechanism solutions for major shallow earthquakes in ridges (1) through (3) are redetermined in order to study the tectonic stress in this region. Most solutions indicate approximately a pure strike-slip, and a predominant direction of a dislocation closely resembles with the strike of the tectonic ridges which constructed by the volcanoes. Volcanic eruption may have also been caused by the same stress force in the crust. Another phenomenon commonly observed is the complementary relation between the seismic activity just after an eruption and the eruptive activity ; itself seismic activity was at high level when an eruptive activity continued for a short time (approximately two days or less), while it was quite low when an eruptive activity continued for a longer time.
