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Abstract

Sandy tsunami deposits from the 1611 Keicho Oshu earthquake are distributed in peat at a depth of 50cm-1m in Atsuma, eastern Iburi, Hokkaido. The elevation at the time of the event was 3-4.5m, and it slopes gently inland. The thickness of the sediments is more than 50 cm at the thickest point, and it became thinner inland and invisible at about 500 m from the coastline ca. 200 years ago. Also, the mean grain size of the tsunami deposits becomes as fine as 3.5φ near the distribution limit. The size of the tsunami suggested by the properties of these deposits is about 5m in height, which is smaller than that in the tsunami scenario by the Cabinet Office (about 9m at the coast). Neighboring Tomakomai and Mukawa also show that the inland extent of the tsunami indicated by geological traces is significantly shorter than that drawn in the hazardmap. In these areas, no other possible tsunami traces could be discerned even on X-ray CT images during the past 2500 years, though known tephras (Ta-b; 1667 CE, Us-b; 1663 CE, B-Tm; 10th Century CE, Ta-c3; ca. 2000yr BP, Ta-c2; ca. 2500yr BP) were well-preserved. The new information about the 1611 tsunami in Atsuma, together with the research results in Tomakomai and Mukawa, provides important constraints for the size and recurrence interval of tsunamis in the northern part of the Japan Trench and is useful for the evaluation of source models of earthquakes and the damage estimation.