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Remobilization of surficial slope sediment triggered by the A.D. 2011 Mw 9 Tohoku-Oki earthquake and tsunami along the Japan Trench

The A.D. 2011 Tohoku-Oki M w 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) xs 210 Pb, requiring only centimeters-deep
sediment remobilization over large areas of the seafloor. Some contain
 134 Cs and 137 Cs 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 km 2 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.