著者

茂木 清夫

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.56, no.4, pp.p691-711, 1981

被引用文献数

3

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The recent anomalous high activity in the Izu region was preceded by a remarkable earthquake swarm during the period (1964-1965) followed by a very calm period. During the period (1964-1965), several important events, such as the Oga-hanto-oki earthquake (M 6.9), the Niigata earthquake (M7.5), the Matsushiro earthquake swarm and the Shizuoka earthquake (M 6.1) occurred along the active tectonic zone including the active folded zone of Akita-Niigata on the Japan Sea side, the Fossa Magna active belt and the Izu-Tokai region, which was pointed out as an important tectonic boundary between the eastern and the western Japanese islands by Huzita (1980). Therefore, the simultaneous occurrence of the several remarkable events along the above-mentioned tectonic zone may be not accidental and it suggests that the tectonic zone is still active and plays an important role for the present-day tectonics of the Japanese islands. The spatio-temporal pattern in the recent seismic activity in the Izu Peninsula and its surrounding area is discussed on the basis of seismic data in the Seismological Bulletin of the Japan Meteorological Agency. Regular patterns before and after three large earthquakes of magnitudes 6.7 to 7.0 are pointed out. Particularly the donut-shaped pattern before the large earthquakes is noticed. After the last large earthquake (M 6.7) in 1980, a donut-shaped pattern was found in the northwestern Sagami Bay region just adjacent to the focal region of the 1980 earthquake. Furthermore the migration of recent large shallow earthquakes in the Izu region from south to north has been observed. Therefore the northwestern Sagami Bay region may be considered a potential region for a future large earthquake. In this case, however, it should be considered that there is a possibility that the strain energy accumulation in this region is not enough for the occurrence of a large earthquake, because the 1923 great Kanto earthquake occurred in the adjacent region.