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
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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
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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)