[M-IS16] Tsunami deposit

[MIS16-08] Tsunami deposits of the 1611 Keicho Oshu Earthquake in Atsuma, Eastern Iburi, Hokkaido

*Yuichi Nishimura¹, Yasuhiro Takashimizu², Kohei Iwaki², Takashi Ishizawa³, Daisuke Sugawara³, Yusuke Yokoyama⁴, Yosuke Miyairi⁴ (1.Graduate School of Science, Hokkaido University, 2.Faculty of Education, Niigata University, 3.International Research Institute of Disaster Science, Tohoku University, 4.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:tsunami deposit, Keicho Oshu tsunami, historical tsunami

A tsunami deposit in the 17th Century was found in Atsuma, eastern Iburi, Hokkaido, and its distribution, characteristics, and age were investigated. The survey area was about 1km along the coastline and about 300m inland (600-900m from the present coast). About 50 sites were excavated using a geoslicer and a peat-sampler. Most of the surveyed area was dry grassland, and although some areas were used as sand storage, the pear beneath the lower part of a series of volcanic ash from the 17th century (Ta-b; 1667, Us-b; 1663) is not disturbed. In the peat, other key tephra layers (B-Tm; 10th century, Ta-c3; ca. 2000 years ago, Ta-c2; ca. 2500 years ago) were also well preserved throughout the area. There is only one sand layer in the peat and this sand layer has characteristics of tsunami deposits such as sheet-like distribution, inland-thinning, inland-fining, contamination of marin diatom, erosional contact with the lower peat. Since the sand layer underlied Us-b (1663) with 0-3 cm of peat, it seems to be due to the same tsunami as the event that left deposits in Tomakomai and Mukawa (e.g., Takashimizu et al., 2007). The 2σ calendar age of range of the deposit was 1596-1641. Therefore, the tsunami deposits in this area are likely to be traces of the 1611 Keicho Oshu earthquake. Traces can be identified in Atsuma up to a height of about 4.5m and up to about 500m inland from the late Edo period coastline. 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 the Cabinet Office's estimate (about 9m at the coast). Also, no other possible tsunami traces could be discerned even on X-ray CT images during the past 2500 years. The new information about the 1611 tsunami in Atsuma, together with the research results in Tomakomai and Mukawa, is useful for the evaluation of source models of earthquakes in the northern part of the Japan trench and the damage estimation.