Abstract
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.
Highlights
Historical and paleo-tsunami research and its application to geophysical study The 2011 off the Pacific coast of Tohoku earthquake (2011 Tohoku-oki earthquake) (Mw 9.0) that occurred on March 11, 2011, triggered a large tsunami (2011 Tohoku-oki tsunami) along the east coast of Japan, causingIshimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16The best sources of precise long-term tsunami data are coastal lowlands, in particular, marshes (Minoura and Nakaya 1991; Witter et al 2003; Sawai et al 2009; Shennan et al 2014)
Geomorphological classification Initial mapping of geomorphic surfaces around Koyadori was based on interpretation of 1:8000- and 1:10,000-scale aerial photographs taken by the Geospatial Information Authority of Japan before and after the 2011 Tohoku-oki tsunami, and anaglyph images prepared from 1 m and 5 m mesh DEM (Digital Elevation Model) provided by the Geospatial Information Authority of Japan and the Iwate Prefecture
Identification of tsunami deposits The roundness similarities between the event and beach deposits (Fig. 10) indicate that event deposits were transported from beach and beach ridges to the inland trench sites
Summary
Historical and paleo-tsunami research and its application to geophysical study The 2011 off the Pacific coast of Tohoku earthquake (2011 Tohoku-oki earthquake) (Mw 9.0) that occurred on March 11, 2011, triggered a large tsunami (2011 Tohoku-oki tsunami) along the east coast of Japan, causing. The best sources of precise long-term tsunami data are coastal lowlands, in particular, marshes (Minoura and Nakaya 1991; Witter et al 2003; Sawai et al 2009; Shennan et al 2014). In this study, we excavated trenches at a coastal marsh in Koyadori, in the middle part of the Sanriku Coast (Fig. 1). Study site Koyadori is located in the central part of the Sanriku Coast, the easternmost part of Honshu Island 200 km east of Koyadori, the Pacific plate subducts underneath the Eurasian Plate, where rupture areas of historical and observed earthquakes have been identified on the plate boundary
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