- 公益社団法人 日本地震学会
- 地震 第2輯 (ISSN:00371114)
- vol.40, no.3, pp.377-381, 1987-09-25 (Released:2010-03-11)
In Ishigaki Island, the southern Ryukyu Islands, there are many blocks consist of coral limestone. These are considered to be thrown onto the island by Yaeyama Seismic Tsunami in 1771 and so called “tsunami-ishi (means tsunami-stone)”. Based on a distribution of these tsunami-ishi, a record, in archives, saying that the run-up height of the tsunami was 85 meters has been admitted. Or according to the maximum height of the distribution, an assertion that the run-up height is 55 meters has been made.There are two kinds of blocks for these tsunami-ishi. One is genuine tsunami-ishi which is evidently thrown on the island by the tsunami. The other consists of the Pleistocene Ryukyu Limestone block. It cannot be determined whether the existence of these blocks is a result of the tsunami, or not. These coral blocks can be distinguished from each other by its components, either aragonite or calcite. That is to say, when the coral of a block consists of aragonite, it should be a genuine tsunami-ishi. When calcite, it should be a Ryukyu Limestone block. Since the research, made by this method, found out that the maximum height of the distribution of the genuine tsunami-ishi was 25 meters, it is inferred that the run-up height was a little higher than 25 meters.In Miyara Bay of Ishigaki, there is also a block about 100 tons in weight clung by some small coral whose 14C-age corresponds with that of Yaeyama Seismic Tsunami. The 14C-age of this block itself is 2980BP. This suggests that, between at least 3000 years ago and 1771 of Yaeyama Seismic Tsunami, there was no such huge tsunami which could have thrown this large block at Miyara coast. Since, among the genuine tsunami-ishi, the 14C-age of coral of a block, distributing at the altitude of 25 meters, which is the highest altitude for this particular block, is 1810BP, this block is inferred to have been thrown by the tsunami and the value of a little higher than 25 meters is considered to indicate the maximum height of Yaeyama Seismic Tsunami.