Re-examination of the age of historical and paleo-tsunami deposits at Koyadori on the Sanriku Coast, Northeast Japan

Geoscience Letters, May 2017

High-accuracy age estimations of tsunami deposits are very important for tsunami deposit research because the recurrence interval and frequency of tsunami deposits allow us to assess the devastating impacts of tsunamis in a populated country such as Japan. The correlation of tsunami events between remote coastal areas further enables us to constrain tsunami scenarios and fault segmentation along a trench. Before the 2011 Tohoku-oki earthquake, the long-term tsunami history along the Sanriku Coast was not well known. Age data of the long-term tsunami history are essential to understand the mechanisms of tsunami generation and earthquakes and to assess the risks posed by them. This study re-examined the age of historical and paleo-tsunami deposits (E1 to E11 deposits) at Koyadori in the middle of the Sanriku Coast and estimated their ages with high accuracy by using continuous sediments since approximately 4 ka. Radiocarbon dating was conducted in combination with other radiometric dating methods (137Cs and excess 210Pb) to estimate the ages of the sediments. The resulting ages revealed the reliable ages of ten historical and paleo-tsunami deposits. The average recurrence interval of historical and paleo-tsunamis in the last 4 ka is 350–390 years, and each recurrence interval between the E4 and E11 deposits is similar despite the tsunami deposits having different characteristics. Moreover, far-field tsunamis do not seem to inundate this site based on the observed and documented records. The continuous and long-term data of tsunami deposits at Koyadori offer important constraints on the timing and frequency of near-field earthquakes (e.g., megathrust, outer-rise, and tsunami earthquakes) around the Japan Trench.

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Re-examination of the age of historical and paleo-tsunami deposits at Koyadori on the Sanriku Coast, Northeast Japan

Ishimura Geosci. Lett. Re-examination of the age of historical and paleo-tsunami deposits at Koyadori on the Sanriku Coast, Northeast Japan Daisuke Ishimura 0 0 Department of Geography, Tokyo Metropolitan University , 1-1 Minami-Osawa, Hachioji City, Tokyo 192-0397 , Japan High-accuracy age estimations of tsunami deposits are very important for tsunami deposit research because the recurrence interval and frequency of tsunami deposits allow us to assess the devastating impacts of tsunamis in a populated country such as Japan. The correlation of tsunami events between remote coastal areas further enables us to constrain tsunami scenarios and fault segmentation along a trench. Before the 2011 Tohoku-oki earthquake, the long-term tsunami history along the Sanriku Coast was not well known. Age data of the long-term tsunami history are essential to understand the mechanisms of tsunami generation and earthquakes and to assess the risks posed by them. This study re-examined the age of historical and paleo-tsunami deposits (E1 to E11 deposits) at Koyadori in the middle of the Sanriku Coast and estimated their ages with high accuracy by using continuous sediments since approximately 4 ka. Radiocarbon dating was conducted in combination with other radiometric dating methods (137Cs and excess 210Pb) to estimate the ages of the sediments. The resulting ages revealed the reliable ages of ten historical and paleo-tsunami deposits. The average recurrence interval of historical and paleo-tsunamis in the last 4 ka is 350-390 years, and each recurrence interval between the E4 and E11 deposits is similar despite the tsunami deposits having different characteristics. Moreover, far-field tsunamis do not seem to inundate this site based on the observed and documented records. The continuous and long-term data of tsunami deposits at Koyadori offer important constraints on the timing and frequency of near-field earthquakes (e.g., megathrust, outer-rise, and tsunami earthquakes) around the Japan Trench. Paleo-tsunami deposits; Sanriku Coast; The 2011 Tohoku-oki earthquake; Radiocarbon dating; 210Pb; 137Cs - One of the goals of tsunami deposit research is to reveal when, where, and how a tsunami occurred. Tsunami deposits are first identified on the basis of various criteria (e.g., Goff et al. 2012), and then analyzed to estimate the ages of the sediments. However, high-precision and high-accuracy age estimations of tsunami deposits are usually difficult owing to the varied preservation of the sediments (i.e., preservation potential, dating material, and geomorphic setting). The age data of tsunami deposits are also significant because they are influential in several research areas (e.g., geophysics, history, geology, geography, and engineering) and society (e.g., risk assessment, hazard mitigation, and city development). For age estimations of tsunami deposits, radiocarbon dating is generally used, while other dating methods may occasionally be conducted. Radiocarbon dating is very useful; however, this technique may sometimes yield inaccurate values owing to problems such as contamination, disturbance of deposits, and preservation of dating material. Therefore, careful sampling and additional dating methods are required to be conducted. Koyadori, the research area, is located in the middle of the Sanriku Coast, where multiple historical and observed tsunamis have been reported (Watanabe 1998). The tsunami triggered by the Mw 9.0 2011 Tohokuoki earthquake hit Koyadori, where an approximately 30  m run-up height was recorded (The 2011 Tohoku © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Earthquake Tsunami Joint Survey Group 2012). Ishimura and Miyauchi (2015) identified tsunami deposits within long and continuous deposits since approximately 4  ka and correlated some tsunami deposits with historical tsunami events at Koyadori, where the largest number of tsunami deposits on the Sanriku Coast has been found. Therefore, the Koyadori site can be a type location for the tsunami history along the coast and allows for a comparison of tsunami event ages and frequencies in Hokkaido (e.g., Nanayama et  al. 2003) and around Sendai (e.g., Sawai et al. 2012). However, Ishimura and Miyauchi (2015) have not yet reported the high-precision and highaccuracy ages of the paleo-tsunami deposits owing to the reversal in the radiocarbon dates. Thus, the author added radiocarbon dating and integrated additional radiometric dating techniques (137Cs and excess 210Pb) after Ishimura and Miyauchi (2015). This study presents a re-examination of the ages of the historical and paleo-tsunami (...truncated)


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Daisuke Ishimura. Re-examination of the age of historical and paleo-tsunami deposits at Koyadori on the Sanriku Coast, Northeast Japan, Geoscience Letters, 2017, pp. 11, Volume 4, Issue 1, DOI: 10.1186/s40562-017-0077-4