ALBERT

Beschreibung: We explored the submarine portions of the Enriquillo-Plantain-Garden Fault zone (EPGFZ) and the Septentrional-Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high-resolution multi-beam echo-sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate-boundary structures are a series of strike-slip fault segments associated with pressure ridges, restraining bends, step-overs and dogleg offsets indicating very active tectonics. Several distinct segments 50 to 100 km long cut across pre-existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike-slip regime. Along the most recent trace of the SOFZ, we measured a strike-slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS-derived motion of 9.8 ± 2 mm/yr has remained stable during the entire Quaternary. This article is protected by copyright. All rights reserved.

Beschreibung: 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.