[M-IS13] Tsunami deposit

[MIS13-P01] Large-Scale Tsunami Events Records of Deposits and Stalagmites in Caves of Ishigaki Island, Okinawa Prefecture, Japan

*Yoshiro Ishihara¹, Yuuka Okita¹, Miyu Kitahara¹, Hajime Naruse², Shingo Kameshima³, Seita Gushiken³, Chiaki Katagiri³, Kaishi Yamagiwa⁴, Rintaro Ono⁵, Ayano Shimabukuro⁶, Shinji Sugihara⁷, Kazuhisa Yoshimura⁷ (1.Department of Earth System Science Faculty of Science, Fukuoka University, 2.Graduate School of Kyoto University, 3.Okinawa Prefectural Archaeological Center, 4.Ryukyu University, 5.National Museum of Ethnology, 6.Ishigaki City, 7.Kyushu University)

Keywords:Tsunami deposits, Ishigaki Island, Cave, Archaeological site

In the Yaeyama islands, several tsunamis events have been obtained mainly from tsunami boulders distributed along the coast. The 1771 Meiwa Tsunami is well known from ancient documents, tsunami boulders, and sandy to gravelly tsunami deposits, and its epicenter and magnitude have been estimated from various information. Although tsunami boulders and deposits indicate that a larger tsunami event may have struck about 2000 years ago (Okinawa Sakishima Tsunami), the geological information is limited, and the detailed scale is not as well understood as that of the Meiwa Tsunami. Also, in the Yaeyama islands, it has been revealed that the culture changed from the Shimotabaru Period to the Non-Ceramic Period around 2000 years ago, which suggests the influence of a large scale tsunami. Hence, it is archaeologically important to know the tsunami details. On the other hand, it is known that many caves formed in the limestone of the Ryukyu Group are distributed along the coast of the Yaeyama islands. Since caves have more stable environments than the surface, the stalagmites that grow inside the caves provide a well-preserved record of the paleoenvironment. In several caves along the east coast of Ishigaki Island, event deposits, including marine-derived fossils and thick gravelly deposits under the event deposits, which the Meiwa Tsunami probably causes, are found. These coarse-grained deposits are also distributed in caves located at elevations above 30 m and are often dated to about 2000 years ago. This presentation will describe the stratigraphy and age of these deposits and their distribution.
This work was supported by JSPS Grants-in-Aid JP20K04138 and JP21H04368.