[S04-1] Historical and macroseismic studies of earthquakes I

[S04-1-02] A possible tsunami caused by a submarine landslide in 1512 at the Nankai trough, Japan

^○Toshitaka Baba¹, Taiki Okada², Juichiro Ashi³, Toshiya Kanamatsu⁴ (1.Graduate School of Science and Technology, Tokushima University, 2.Faculty of Engineering, Tokushima University, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Japan Agency for Marine-Earth Science and Technology)

“SHINCHO-KI" is an ancient document that records tsunami damages caused by the 1512 Eisho earthquake, the 1605 Keicho earthquake, the 1707 Hoei earthquake and the 1854 Ansei-Nankai earthquake at Shishikui region in Tokushima, where is located along the coast of the southeastern part of Shikoku, facing to the Nankai trough. According to SHINCHO-KI, 3700 people were dead at Shishikui by the tsunami during the 1512 Eisho earthquake. However, no evidence was found for the occurrence of 1512 Eisho earthquake except for SHINCHO-KI, while the other earthquakes were recorded in many ancient documents in the southwestern Japan.
To investigate the source mechanism of the 1512 Eisho earthquake, we carefully read a bathymetric chart and found a scarp with height of about 400 m and width of about 6000 m at a position about 24 km offshore in the southeastern direction from Shishikui. We also carried out a survey by using a deep-towed sub-bottom profiler (SBP) on ROV NSS during the R/V Hakuho-maru KH-16-5 cruise. The result shows detailed structures possibly caused by a recent landslide. The vertical displacement of the strata was measured to be about 50 m. By considering these results, we simulated the 1512 Eisho tsunami generated by a submarine landslide. The topographic data in Shishikui which is needed in the calculation was made from the present data. But we removed the artificial structures such as wave breakers and altered costal lines by referring to old map images. The numerical simulations indicated that the tsunami flow depths during the 1512 Eisho tsunami described in SHINCHO-KI could be simulated by the assumed submarine landslide. The maximum tsunami height of 9 m was calculated at Shishikui, while it was 3 m along the Kii Peninsula located at the opposite side of the Kii Channel.