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

Abstract

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 ( 137 Cs and excess 210 Pb) 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.

Supercycle in great earthquake recurrence along the Japan Trench over the last 4000 years

“…Subsequently, we compared the estimated turbidite depositional ages with those of onland tsunami deposits reported by previous studies (Sawai et al 2012;Hirakawa 2012;Takada et al 2016;Ishimura 2017). The ages of the tsunami deposits that disagree with the turbidites, such as those from the 12-13 century, and A.D. 1611, may have been related to tsunamis begun in other far-field areas or submarine landslides instead of earthquakes along the Japan Trench (Fig.…”

“…Thus, the obvious turbidites T3 in the two cores may suggest that the fault models with a fault length of 300 km or longer are more probable for the 869 earthquake. The 869 Jogan tsunami deposits found in the Sanriku coast in recent years (Takada et al 2016;Ishimura 2017) seem to support this inference. Similarly, the fault model of the 1454 earthquake is also based on a estimation with a minimum magnitude (Sawai et al 2015).…”

“…Onshore tsunami deposits provide evidence that significant earthquakes and tsunamis have impacted the Pacific coast of Tohoku in the past (Abe et al 1990;Minoura and Nakaya 1991;Minoura et al 2001;Sawai et al 2012Sawai et al , 2015Ishimura and Miyauchi 2015;Takada et al 2016;Ishimura 2017). Even before the 2011 Tohoku earthquake, two earthquakes along the Japan Trench (the A.D. 869 Jogan earthquake and the 1454 Kyotoku earthquake) that generated giant tsunamis similar to the one in 2011 are indicated by historical records and tsunami deposits (Satake et al 2008;Namegaya and Yata 2014;Sawai et al 2015).…”

“…The orange point to the left of the coring sites is the hypothetical source of the turbidity currents for calculating PGA in this study. Line C transects the inset in C. Green inverted triangles indicate the sites of onland tsunami deposits (Sawai et al 2012;Hirakawa 2012;Takada et al 2016;Ishimura 2017). Yellow ellipses summarize the suggested rupture areas of historical earthquakes (M ~ 7.4 and larger) over the most recent 200 years (Koper et al 2011;Toda 2016); 2: 1994 Sanriku-oki earthquake, 8: 1896 Sanriku earthquake.…”

Introduction to thematic collection “Historical and geological studies of earthquakes”

“…Along the Sanriku coast, which is located north of Sendai plain with saw-tooth topography, more recent tsunamis caused damages in 1933 and 1896, while few tsunami deposit studies were made before 2011. Ishimura (2017) re-estimated the ages of tsunami deposits which he previously sampled on the Sanriku coast. Ages of 11 deposits in the last 4000 years were estimated using dating methods of 14 C, 137 Cs,and 210 Pb.…”