1988 年 41 巻 4 号 p. 573-581
For the purpose of estimating a tsunami hazard statistically, a mean tsunami height Hn is defined as a geometrical one of log Hn=(1/n) ∑ni=1 log Hi, in which Hi is the i-th value of n observed heights in 20-40km segment of the coast, and calculated in the most part of the coast of Japanese Islands. The mean is obtained for the tsunami, having the tsunami magnitude larger than 1, observed during a period from 1600 to 1986 in each segment. A cumulative tsunami energy is estimated by summing up H2n for all the tsunamis concerned. The result shows that the cumulative energy takes the highest value on the coast of Iwate Prefecture, and the 2nd, 3rd, 4th and 5th highest values on those of Miyagi, Kochi, Mie and Chiba Prefectures, respectively.
Frequencies of Hn exceeding 0.5, 1, 2, 5, 7 and 10m are also obtained. It is noticeable that Hn exceeds the 5m level 4 times at maximum for all the segments in Iwate and Miyagi Prefectures. These frequencies are converted into the probability of tsunamis exceeding a certain prescrived height on the assumption of the Poisson process in the generation of earthquakes. The probability of a tsunami higher than 2m hitting the coast of Iwate Prefecture during a 10-year period amounts to 22%, while that of a tsunami higher than 5m 7.5%. These probabilities certainly representing the long-term tsunami activity averaged over the 387-year period in the past, they differ from the one estimated by taking short-term earthquake prediction into account.