[M-IS17] History X Earth and Planetary Science

[MIS17-P02] Reevaluation of run-up height of the Keicho 1605 tsunami in Hachijo Island

*Hiroko Kaida¹, Kazuhisa Goto¹, Takashi Ishizawa², Kaoru Hayashi³ (1.The University of Tokyo, 2.Tohoku University, 3.Hachijo Town Board of Education)

Keywords:tsunami, Hachijo island, historical records , 1605 Keicho earthquake, earthquake

Izu Islands are surrounded by the Nankai Trough, Sagami Trough and Izu-Bonin Trench. Hence, revealing tsunami history of Izu Islands is important for the study of natural disasters in Japan. Hachijo Island is the sole island with a rich historical record in the area. Hachijo Island has known to be attacked by 3 large tsunamis during the Edo period: those are the 1605 Keicho, the 1677 Enpo, and the 1703 Genroku tsunamis. The run-up heights are estimated to be approximately 8~10 m in the 1677 Enpo tsunami and approximately >10 m in the 1703 Genroku tsunami, respectively and there is no major difference among previous studies. On the contrary, as for the 1605 Keicho tsunami, the run-up height is controversial: some studies estimated it as about 8~10 m while the other states the height as 10~20 m. The former interpretation is based on the old text of “Kaisho” in the archive “Hachijo Jikki”. However, we revealed that the description in this text is not reliable enough. Thus, in this study, we looked upon the original “Hachijo Nendaiki” which is cited in “Hachijo Jikki”. Referring to the original of “Hachijo Nendaiki”, we found that it is highly probable that the original description of the tsunami-affected site was mistakenly copied in “Hachijo Jikki”. The similarity of two Kanji characters is very likely to have caused unintentional change from “the whole village was damaged” to “the lower part of the village was damaged” by that error. The part of “Kaisho” in “Hachijo Jikki” was the key for some previous research that stated the run-up height to be less than 10 m. When the description “the whole village was damaged” is adopted, the run-up height needs to be 10 m at the least if we consider the elevation of old villages. This is supported by the topographical characteristics of Hachijo Island. Indeed, the island has a sea-cliff along the coast up to an elevation of about 5 m. In addition, it is known to be frequently damaged by typhoons up to an elevation of about 10 m, to the extent that boulders are washed up. This suggests that lands below 10 m elevation are not suitable for daily settlement, so it is unlikely that there was a village in such a low elevation. From the above, we infer that the run-up height caused by the 1605 Keicho tsunami was over 10 m at west coast of Hachijo Island.