抄録
Making use of tide gauge records, the tsunami generated off about 100km east of Hachijo Island on Dec. 4, 1972 is investigated and its behavior is compared with the tsunami generated approximately in the same region on Feb. 29, 1972. The average wave height of the present tsunami is about 1.4 times as large as that of the tsunami in February 1972 and tsunami magnitude is determined as m=1 in accordance with the author's method.
The estimated tsunami source lies to the southwestern side of the 1953 Boso-oki tsunami source, including that of the tsunami in February 1972. The source area extends about 80km in the NE-SW direction. Judging from the initial motion of tsunami, the subsidence of the sea-bottom seems to have occurred in the west side of the tsunami source, and the upheaval in the east side. The features of the initial motion of tsunami were mostly similar to that of the tsunami in February 1972, i. e. the recorded initial motion of tsunami was downwards to southwestern Japan and upwards in northeastern Japan. By applying the corrections for the refraction and shoaling from the observed initial wave, the bottom deformation in the tsunami source would be occurred in the range of -5 to 8cm.
Remarkable gap lies between the 1923 Kanto and the 1953 Boso-oki tsunami source areas located along the Sagami trough. It should be noted to the Genroku tsunami of 1703 (m=2 or more) which might be generated in the vicinity of Boso peninsula.
