Paleo-tsunami deposits since ca. 5 ka in Koyadori and Onuma on the Sanriku Coast, northeast Japan
Abstract:Large tsunamis since the last about one hundred years along the Sanriku Coast on the Pacific Coast of northeast Japan were well documented and observed. Additionally, many historical records described about tsunamis during recent several hundreds years. However, we cannot expect the latest large tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake because previous studies of paleo-tsunami deposits were not efficient and geological evidences of paleo-tsunamis were not obtained on the coast. Thus, we conducted excavating surveys on coastal marshes to identify paleo-tsunami deposits and correlate to historical tsunami events. In laboratory, we revealed physico-chemical sediment property and performed radiocarbon dating and tephra analysis for establishing their geochronology. As a result, we referred to eleven tsunami deposits including 2011 tsunami deposits since ca. 3000-4000 Cal BP and correlated them with historical tsunamis (1896 Meiji Sanriku tsunami, 1611 Keicho Sanriku tsunami, 869 Jogan tsunami) in Koyadori. In Onuma, we identified six tsunami deposits since ca. 5000 Cal BP and younger two deposits of them are possibly corresponding to historical tsunamis. This is the first report of identifying onshore paleo-tsunami deposits correlating to historical tsunamis on the Sanriku Coast based on sediment property, many radiocarbon dating, and tephra correlation.
As these results were derived from only two sites, we need many solid data in other sites to confirm our correlations. In the future, these data are linked to researches for establishing the earthquake cycle model along the Japan Trench and evaluating the magnitude of paleo-tsunamis and paleo-earthquakes.