抄録
Hypocenters of foreshocks, main shock, and aftershocks of the 1930 Kita-Izu earthquake and the Ito earthquake swarm of 1930 are redetermined. It is clarified that foreshocks of the Kita-Izu earthquake occurred concentrated near the epicenter of the main shock, and almost all aftershocks occurred in the region to the west of the Tanna fault. Some of the aftershocks are located in the northeast coast of Suruga Bay. A spatial complementarity is seen between the aftershock area of the Kita-Izu earthquake and the recent active region since 1974, that is, recent seismicity is active to the east of the Tanna fault. Foreshock activity of the Kita-Izu earthquake began about 3 weeks before the main shock. It reached to the culmination in the evening of 25th November when the largest foreshock (M5.1) occurred at 16h 5m. Then the foreshock activity gradually decreased and it was rather quiet a few hours before the main shock occurring at 4h 2m on the 26th. The aftershock activity did not decrease according to the Omori's formula, but showed intermittent increase. The Ito earthquake swarm in the spring of 1930 occurred in almost the same region as that of recent swarm activity off the east coast of the Izu Peninsula. There were two peaks in the swarm activity of 1930, i.e., in March and in May. It seems that swarm activity in March occurred somewhat nearer to the east coast of the Izu Peninsula than that in May, although the difference is not so clear. If the number of earthquakes with M4 or larger in the 1930 swarm activity, 70, is compared to the total number of earthquakes with M4 or larger in the recent activities from 1978 through 1989, 90, it can be said that the swarm in the spring of 1930 was remarkably active.
