1967 年 20 巻 3 号 p. 164-170
Both of the severe tsunamis of 15 June 1896 (tsunami magnitude: 4) and of 3 March 1933 (tsunami magnitude: 3) were generated off the Sanriku coast of north eastern Japan. Although the magnitude of 1896 Earthquake (M=7.6) was smaller than that of 1933 Earthquake (M=8.3), the intensity of 1896 Tsunami was much larger than that of 1933 Tsunami, particularly along the northern part of the Sanriku coast. In order to explain this different behaviors of these two tsunamis, tsunami sources are estimated by means of an inverse refraction diagram and the spectral analyses of records at three stations are made.
It is found that the tsunami source of 1896 lies close to the northern part of the Sanriku coast with the major axis of the source directed in NNW-SSE, in contrast to the source of the 1933 Tsunami which lies some 110km from the coast with the axis in N-S direction. Sea level disturbances at the margin of the sources for the tsunamis of 1896 and 1933, estimated on the basis of the shoaling coefficient and the inundation heights of tsunamis along the coast, seem to be about 2m and 1m respectively.
In addition, the source area of the weak tsunami of 21 March 1960 is estimated. The earthquake magnitude is M=7.5 and this source includes the area of aftershock activity extending about 120km in an elongated shape.