1974 年 27 巻 4 号 p. 321-337
Source areas of the tsunamis generated in northeast Japan since 1896 are compiled on a bathymetric chart (Newly estimated: 14; Reanalyses: 14). Sources of large tsunamis lie on the steep continental slope off the Sanriku coast. Most of small tsunami sources are located in the sea shallower than the depth of 2000m. Source dimensions are closely related with earthquake magnitude; the Utsu formula for the relation between the earthquake magnitude and aftershock area can be applied well for the tsunami source area. Generally speaking, the magnitudes of tsunami correspond to earthquake magnitude. However, a few tsunamis generated in the vicinities of the west Aomori and Miyagi regions are especially small in comparison with those at other regions.
Judging from the initial motion on tsunami records, most of tsunamis off the Sanriku coast seem to be generated by the reverse fault type earthquake. The sense, up or down, of the initial motion of the 1896 Sanriku tsunami is opposite to that of the 1933 tsunami which is caused by a normal fault earthquake. This suggests that the 1896 tsunami was generated by the reverse fault earthquake. Source dimension of the 1933 tsunami is newly revised and found to be about 300km. This area is smaller than the former estimations, because arrival times at the tide stations of Hokkaido were read too early by misunderstanding a noise as the first motion.