伊豆諸島の地震活動および火山活動の特質

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Results of seismic and geodetic observations at Izu Islands, south of Tokyo, during 1966-1971 are briefly summarized. Izu Islands are situated at the northern part of Huzi Volcanic Belt and are composed of several active and dormant volcanoes. Among them, O-sima and Miyake-zima are basaltic volcanoes which showed frequent eruptions in the past. Hatizyo-zima, a basaltic volcano, is located southern extremity of Izu Islands of which last eruption is recorded as 1606. Beside these basaltic volcanoes arranged as the step-stones in NS direction, rhyolitic volcanoes such as Nii-zima and Kozu-sima are situated at the western part of the chain of basaltic volcanoes. These rhyolitic volcanoes are located on the marked ridge which extends NE-SW direction. In August 1965, an intense earthquake swarm occurred near Kozu-sima. Since then, we set up an acceleration seismograph at Kozu-sima and carried out temporal seismic observations at Kozu-sima, Nii-zima, O-sima, Miyake-zima and at Hatizyo-zima successively. According to our continuing observation at Kozu-sima, seismic energy liberated from off coast of Kozu-sima during 1966-1969 was estimated to be 3.2×10^<20> ergs. Precise levelling survey was carried out in O-sima and we found subsidence of the summit region between 1958 and 1967. Study of deformation of the O-sima caldera was made by AGA Model 6 geodimeter. It was found that the direction of the axis of principal strain being roughly parallel to the direction of regional horizontal compression as inferred from earthquake mechanism. It is markedly characterized that epicentral regions of earthquake swarms are roughly located at W/NW part or off coast of the individual volcanic islands. For interpretation of these observational evidences, an argument for a possible model is presented. 1. Izu Islands are located at the northern extremity of Philippine Sea plate and the direction of compression within the plate is shown in Fig.26. 2. Sea bottom topography shows lineament roughly perpendicular to the direction of the compression. 3. Earthquake mechanism of the concerned region shows the small amount of dip component. It implies that the lithosphere of the said region is largely suffered by horizontal compressive stress. 4. The ridge on which rhyolitic volcanoes are located was caused by buckling of the upper part of the lithosphere as the consequence of horizontal compression. 5. Buckling of the upper lithosphere produces pressure decrease at its lower boundary which may yield the melting of the crustal material. Thus, the rhyolitic volcanism is very likely possible. 6. Supposing the vertical gradient of horizontal flow velocity within the lithosphere, volcanoes are left eastward from the location of magmatic foci as shown in Fig. 27. 7. Stress concentration is expected above the magmatic foci (Fig.28) due to horizontal compression which produces earthquake swarms. The above proposed model is nothing but a speculation and we hope further detailed investigation, especially geodetic observation, may be done in future.
1972-10-01

伊豆諸島の地震活動および火山活動の特質

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