季刊地理学
Online ISSN : 1884-1252
Print ISSN : 0916-7889
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宮古島南東海岸のマイバーバマ東部に打ち上げられたハマサンゴ岩塊の14C年代とその意義
小元 久仁夫南 雅代
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ジャーナル フリー

69 巻 (2017) 3 号 p. 119-127

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Maibah beach is located between Higashihenna promontory and Miyato-zaki (promontory), southeast of Miyako Island. Along the sandy beach, beachrock develops intertidal zone. A large boulder lying on the eastern part of the beach has been recognized as a fossil Tridacna gigas. We collected two samples from the boulder in order to determine the age of its outer layer, and noticed that the cross section was different from Tridacna gigas. The samples indicated obviously a characteristic of coralline structure. Then identification and radiocarbon dating of the samples were performed.

The results show that coral is Porites sp. and probably in situ, and that it was transported from lagoon and beached on the shore, judged from the field observation. The calibrated ages were 1626 cal AD and 1679 cal AD, respectively. It is considered that Porites had transported and beached on the shore by huge tsunamis or high waves of typhoon. Unfortunately there was no huge typhoon record, and then we tried to correlate calibrated age of Porites with historical tsunami and earthquake records. According to historical record, Miyako Islands were attacked by strong earthquake of AD 1667 which probably caused tsunami. An average calibrated age of Porites (1653±23 cal AD) coincides with AD 1667 earthquake tsunami with a margin of statistical error. However, age of removed soft outer layers of Porites strongly suggests that it was transported from its original habitat to the present location not only by AD 1667 earthquake tsunami but also by AD 1771 Meiwa Tsunami.

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