Presentation Information
[T8-O-10]Islands off the Cape Omaezaki possibly submerged by co-seismic faulting due to the 1854 Ansei Tokai earthquake
*Akira ISHIWATARI1 (1. None)
Keywords:
Oki-Gozen Island,Gozen-iwa lighthouse,reefs dangerous for sailing,uplifted coast and subsided island,old maps and pictures
Japan has some historical legends that insist submergence of islands due to big earthquakes. On-land fault movement, with or without a significant earthquake, has actually been observed for some recent large inter-plate earthquakes such as 1923 Kanto Earthquake (M7.9), 1944 Tonankai Earthquake (M7.9) and 2011 Tohoku-oki Earthquake (M9.0). This paper proposes a possibility of a co-seismic fault movement by the 1854 Ansei Tokai Earthquake (M8.4) that resulted in submergence of an island 2 to 3 km off the Cape Omaezaki, Shizuoka Prefecture. The Cape is located at 30 km distance from the Nankai-Suruga trough. The existence of the island has been evidenced by at least six maps and pictures drawn during 1728-1842 period, but the island disappeared from the maps made after 1875. The only major tectonic event in this area during the 1842-1875 period was the 1854 Ansei Tokai Earthquake or its biggest aftershock in 1855. The submerged island currently forms dangerous reefs below sea level. However, Ishibashi (1984)'s study of historical record concluded that the Cape Omaezaki area uplifted 0.9 to 1.2 m by the 1854 Ansei Tokai Earthquake. The land uplifted, while the island submerged. Thus, the presence of a fault with up to 2 m vertical displacement is assumed along the NNE-SSW coast line, which is parallel to the known active faults that cross-cut the Cape. Future borehole drilling studies and detailed sea-floor topographical studies are needed to verify possibility of this earthquake fault.
