Abstract

The A.D. 2011 Tohoku-Oki Mw 9 earthquake ruptured the megathrust up to the Japan Trench with a large displacement and caused a catastrophic tsunami. This study is the first to use short-lived radioisotopes, including those emitted by the damaged nuclear reactors at the Fukushima Daiichi nuclear power plant (Japan), to document the remobilization of the upper few centimeters of sediment as a highly significant process triggered by the earthquake and its aftershocks. Targeting the post-earthquake environment allowed characterization of the sedimentary signature of this event for a better understanding of paleoearthquakes in Japan and other tectonically active boundary areas. The results stem from 23 piston cores recovered by the 2013 expedition NT13-19 of the Japan Agency for Marine-Earth Science and Technology. We document submarine homogeneous muddy flow deposits that were triggered by ground motion in 2011. They are highly enriched with excess (xs) xs210Pb, requiring only centimeters-deep sediment remobilization over large areas of the seafloor. Some contain 134Cs and 137Cs radioisotopes derived from the Fukushima nuclear reactors, indicating that sedimentation persisted for at least 30 days after the main shock. We found these deposits at all sampling sites in an ∼5000 km2 area of the seafloor in 4000–6000 m of water depth. The study area extends for ∼260 km parallel to the strike of the trench. The thickness of this “Tohoku layer” (3–200 cm) increases toward the zone of maximum megathrust slip, where deposits are thickest. These results demonstrate that the shaking of the seafloor above large megathrust ruptures near the trench remobilized surficial unconsolidated sediment for hundreds of kilometers. The characteristics of these deposits may typify deposits resulting from large fault slips like that of the Tohoku-Oki earthquake, but also other earthquake deposits, contributing to their identification in the sedimentary record globally.

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