Paleotsunami history along the northern Japan trench based on sequential dating of the continuous geological record potentially inundated only by large tsunamis

Estimating paleotsunami frequency and size is an important requirement for assessing future tsunami risks. However, several issues such as ground soil erosion by subsequent tsunami waves and the preservation potential of tsunami deposits make it difficult to ascertain the paleotsunami history accurately based solely on tsunami deposits. This study demonstrates the importance of high-resolution and sequential dating to construct paleotsunami history. Those methods are sufficient to secure continuity of the geological record using calibrated ¹⁴C ages. Along the Sanriku coast including our study area (Noda) at northern Japan, even though several paleotsunami studies have been conducted, the age correlation of tsunami deposits and paleotsunami history has not been reconstructed well because of the large variation in the estimated ages of tsunami deposits. After taking two core samples near the shore and at inland sites, we used them to confirm the continuity of geological records for 400–2700 years ago based on high-resolution and sequential dating results. Because historical records have documented the tsunami history during the last 400 years, we can construct a continuous paleotsunami history from 2700 years ago to the present without a missing event. Because one of our study sites is located inland, we were able to estimate the histories only of large tsunamis equivalent to the 1611 CE Keicho and 869 CE Jogan tsunamis; small tsunamis cannot be detected because those areas were not inundated. By combining these results with the paleotsunami history of surrounding areas, we reconstructed the paleotsunami history of widely comparable tsunami deposits on the northern to central Sanriku coast. Results showed that the paleotsunami histories can be correlated among sites. It is readily apparent that the tsunami interval is greater during the past 400 years than it was earlier along the Sanriku coast. Whereas earlier studies often estimated the ages of tsunami deposits based on results obtained only immediately above and immediately beneath the tsunami deposit, our results demonstrate the importance of high-resolution dating to avoid overlooking any tsunami deposit. Sequential dating is crucially important to correlate widely distributed tsunami deposits accurately.