地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
ISSN-L : 0037-1114
1771年4月24日の大津波について (II)
三好 寿牧野 清
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ジャーナル フリー

1972 年 25 巻 1 号 p. 33-43

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The lowlands of southwestern islands of Okinawa are dotted with many blocks of coral which were quarried and conveyed from the reef on the floor of the sea, by the giant tsunami of April 24, 1771. We investigated the distribution of blocks in Ishigaki Island. We chose the third biggest block in this island, the approximate weight of which is 750 tons. It lies some 2.5 kilometers away from the nearest coast line, and its location reaches some 30 meters above sea level. We pierced a tunnel through the base of this huge block, which is the hardest to be imagined to have been conveyed by the tsunami, and demonstrated that this block takes no root. In the opinion of some persons, these blocks are the negro heads of the erosion type. But our tunnel crushed down this opposition. One is apt to feel that this capacity of the tsunami is rather fantastic in view of wave dynamics. But, for example, the giant wave observed in Lituya Bay (Alaska) in 1958, the energy of which was ascertained accurately, could have conveyed the supposed coral pillar whose cross section is some 27.3mm2, to the same position of the above-mentioned block, losing only 0.73% of its wave energy.
Then we considered the action of the tsunami of 1771 upon the tied-island called Funakuyâ in Ishigaki Island. We confirmed that it is traditionally said that the northern part of this island has never been isolated from the main part of the island.

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© 社団法人日本地震学会
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