Résumé
The 2011 Mw 9.0 Tohoku-oki earthquake triggered a massive tsunami and devastated approximately 1,000-km-long coastal regions from Aomori to Chiba prefectures, northeast Honshu island. Several pioneering papers already investigated the Sendai to Fukushima region, southern 2011 source area. They found a widespread tsunami inundation by the 869 A.D. Jogan tsunami event (e.g., Minoura and Imamura, 2001; Sawai et al., 2008), which lead the estimated size of the Jogan earthquake up to M8.4 (Satake et al., 2008). No geologic evidence and no historical account in further north may have prevented to conclude straightforwardly that the Jogan earthquake was the penultimate event of the Tohoku-oki earthquake. Here we focus on the Sariku coast, northern part of the 2011 source, and present our pre-Tohoku-oki studies performed at six locations. To recover paleotsunami deposits, I employed conventional coring technique and newly invented Geoslicer to extract several-meter-deep soil and sediment samples. Evidence for paleotsunami event is generally discernible as a several-to-several-tens-of-centimeter medium-to-coarse sand often including mud clast and shells. Such a tsunami sand layer is also distinct intercalated by peats and humic soils representing interseimic dormant periods. Radiocarbon dates yielded from such organic-rich units bracket the depositional timing of the tsunami sand. Although the time constraints of the tsunami sand units are different from site to site, there are shared time ranges of tsunami inundations. Seven well-preserved paleotsunami horizons were commonly found at five onshore sites during the period between 1,000 and 6,000 y.B.P, which yields 500-700 years of recurrence intervals of extremely large tsunamis. One of the flaws in the onshore surveys however is a deficit of the younger sediments due to surface human perturbations. To overcome the issue, I then chose a site inside a bay where no massive erosion and human modification were expected. A 35-m drilling sample recovered from Ozuchi Bay, central Sanriku coast, instead preserved more frequent 22 units of tsunami deposits and associated perturbations during the past 8,000 years, including the 869 A.D. event. It suggests that shoal inside bays can well preserve the sediments of tsunamis, even smaller events. A prominent tendency found in the sequence of tsunami units is a sudden change of recurrence intervals from 500-800 years to 70-150 years at 2,000 y.B.P., which might be associated with a sudden bathymetry change. It may also implies that only huge tsunamis occurred every 500-800 years left sediments inside the bay, which is consistent with the results from onshore surveys.