1971 年 24 巻 2 号 p. 95-106
Based on records of tide gauges, two small tsunamis accompanying the earthquakes on April 21, 1969 (M=6.5) and July 26, 1970 (M=6.7) are investigated. Both earthquakes occurred in the Hiuganada region, southwestern Japan. A part of the estimated tsunami sources seems to overlap on the continental slope. The dimensions of these tsunami domains are inferred to be 3O-40km, taking account of the aftershock areas. Judging from the features of the initial part of records, the subsidence of the bottom seems to have occurred in the eastern part of the tsunami sources.
Beside these tsunamis, eight others were generated in Hiuganada during the period from 1926 to 1970. The percentage of tsunamigenic earthquakes is 100% in earthquake magnitude M>7.0 and about 70% in 6.5<M<6.9. These percentage are large in comparison with those obtained for the whole region of Japan. The magnitudes of various tsunamis are compared on the basis of the mean values of the maximum wave-heights observed at the same tide stations on the coast of Kyushu and Shikoku. Tsunami energy is closely related to earthquake magnitude, and the lower limit of tsunami energy observed in this region seems to be of the order of 1018 ergs. The geographic distributions of the seismic intensity I=4 (JMA scale) for tsunamigenic earthquakes are shown to indicate the relation of this area to the location and the dimension of this tsunami source.