Historical and paleo-tsunami deposits during the last 4000 years and their correlations with historical tsunami events in Koyadori on the Sanriku Coast, northeastern Japan

Large tsunamis occurring throughout the past several hundred years along the Sanriku Coast on the Pacific coast of northeastern Japan have been documented and observed. However, the risk of large tsunamis like the tsunami generated by the 2011 off the Pacific coast of Tohoku earthquake could not be evaluated from previous studies, because these studies lacked evidence of historical and paleo-tsunami deposits on the coastline. Thus, we first identified event deposits, which are candidates for tsunami deposits, from excavating surveys conducted on the coastal marsh in Koyadori on the Sanriku Coast, northeastern Japan. Second, we determined the physicochemical sediment properties of the deposits (roundness of grains, color, wet and dry densities, and loss on ignition) and established their geochronology by radiocarbon dating and tephra analysis. Third, we identified event deposits as tsunami deposits, based on their sedimentary features and origin, sedimentary environment, paleo-shoreline, and landowner interviews. In this study, we report 11 tsunami deposits (E1-E11) during the past 4000 years, of which E1, E2, E3, and E4 were correlated with the 2011 Tohoku-oki tsunami, the 1896 Meiji Sanriku tsunami, the 1611 Keicho Sanriku tsunami, and the 869 Jogan tsunami, respectively. From age data and the number of tsunami deposits in the trench, we estimated that tsunamis larger than the 1896 Meiji Sanriku tsunami occur and hit the study area on average every 290-390 years. However, historical tsunami correlations revealed variable tsunami occurrence, indicating diverse tsunami generation and/or the combination of several types of large earthquakes from different sources around the Japan Trench.