神津島最近の地震活動について : (附)伊豆諸島の火山活動と地震活動の空間的分布について

抄録

From August, 1965, earthquake swarms wepe felt at Kozu-sima, one of the seven Izu-Island, 170km SSW of Tokyo. Earthquake swarms continued in the following year and some of them had Intensity of III and IV. Kozu-sima is one of the volcanic island having lave dome of liparite called "Tenzyo-san" which is believed to have been formed by violent eruption in 838. Since then, no eruption has been recorded. We carried out seismic observations at Kozu-sima in May and October, 1966 and determined epicenters to be located at off-shore of east of the island. No small earthquakes have been observed originated from the volcano. We concluded that the recent seismic activity at Kozu-sima is not directly related to the future eruption, and any feature of coming eruption could not be expected. We further studied in Chapter 3 and 4 the spatial relation between volcanism and seismicity of Huzi Volcanic Belt extending from Asama-yama in the north to Tori-sima in south. Most of the concerned part includes Izu Island arc. The main results are as follows. 1) Seismic activity is quite different beneath the region of Volcano Asama, in Honshu, from beneath the volcanoes in Izu Islands. From the depth of several tens Kilometers to 150km, we have evidences of lack of seismic foci and also the marked absorption of seismic wave energy which imply high temperature or even the existence of molten pockets beneath the volcanic region in Honshu. In contrast, beneath the Izu Islands, seismic activity is highly concentrated at the depth from the surface to about 100 km. This is essential difference in relation between volcanism and seismicity in Honshu and in Izu Island. 2) Between Miyake-zima and Hatizyo-zima, submarine topography shows discontinuous feature of the ridge, at this very point, seismicity also shows discontinuous change. South part shows very low seismic activity having a trend of deeper seismic foci as goes to the south. The relation between the volcanism and marked difference in seismicity beneath the Izu Island arc is worthy to be studied in future. 3) Seismic energy liberated per year has been calculated for the divided region including Izu Island arc as shown in Figs. 10 and 11 using the material of forty years. Most of the seismic energy is liberated at depth of 40 km beneath volcanic islands of Kozu-sima, Miyake-zima and neighbouring islands. This is worthy to note concerning the depth of magmatic foci beneath these volcanoes.