Examination of “Giant Nankai, Japan, Earthquakes” Suggested by Tsunami Deposits in the Ryujin-ike Lagoon in Oita Prefecture, Kyushu, by Means of Tsunami Simulation

Abstract

Along the Nanakai-Suruga trough, where the Philippine Sea plate is being subducted beneath southwest Japan, nine series of great Tokai and Nankai earthquakes have recurred every 100 200 years during historical times since 684 A.D., and brought about significant tsunamis. Recently, geologists investigated tsunami deposits in the Ryujin-ike lagoon in Oita Prefecture and found 40 sand layers during the past 3,300 years including eight remarkable thick layers. They inferred that the uppermost three thick layers had been deposited by large tsunamis due to the “giant Nankai earthquakes” in 684, 1361, and 1707 A.D. which ruptured the Tokai and Nankai source regions simultaneously, and that “giant Nankai earthquakes” had recurred every about 450 years. In order to examine these inferences, we carried out numerical tsunami simulation at the Ryujin-ike lagoon assuming various static fault models based on existing models of the 1707 Ho’ei, 1854 Ansei-Nankai, and 1946 Showa-Nankai earthquakes. As the results, it has been revealed that the tsunami waveform and its maximum height near the Ryujin-ike lagoon depend strongly on the southwesternmost fault slip of the “giant Nankai earthquake” and insensitive to faulting in the Tokai region. Therefore, we conclude that the large tsunamis near the Ryujin-ike lagoon suggested by thick tsunami deposits cannot show the occurrence of “giant Nankai earthquakes” which ruptured the Tokai and Nankai sources simultaneously. Moreover, for a large tsunami near the Ryujin-ike lagoon, the location and the slip amount of the southwesternmost fault plane of the Nankai earthquake have a trade-o# relationship between each other, which makes it impossible to infer correctly the causal fault of the large tsunami by means of the Ryujin-ike data alone. In addition, there is disagreement between the “giant Nankai earthquakes” inferred by the thick sand layers in the Ryujin-ike lagoon and those inferred by the study of historiographical seismology. In conclusion, the eight remarkable sand layers in the Ryujin-ike lagoon cannot be regarded as the traces of the “giant Nankai earthquakes.”