Isotopic and sedimentary signature of megathrust ruptures along the Japan subduction margin

The M9.3 2004 Sumatra and M9.0 2011 Tohoku earthquakes and tsunamis had devastating consequences for the adjacent heavily populated coasts. They had large shallow coseismic slip that reached the trench, which is thought to be a key factor in generating the huge tsunamis. The sedimentary record contains unique spatial and temporal information on the effects of megathrust earthquakes. It can provide a better understanding of earthquake-related tectono-sedimentary processes and of the earthquakes themselves. We present new results obtained from high-resolution multibeam bathymetry, subbottom profiles acquired with Parasound and ten, 10 m long piston cores recently collected from the Japan Trench margin during R/V Sonne SO251 expedition. We identify the 2011 Tohoku event-deposit extending for ~250 km along the Japan Trench basins from the enrichment and concentrations of short-lived radioisotopes (excess) xs²¹⁰Pb, ¹³⁷Cs and ¹³⁴Cs in all ten cores. The high enrichment of xs²¹⁰Pb points to surficial sediment remobilization as the main source. In three cores from the northern, central and southern segments of the Japan Trench, for the first time, we identify sediment remobilization by its ^87/86Sr, εNd and Pb isotope signatures and the composition and texture of the sediment. With this new approach, we fingerprint older sedimentation events, potentially linked to earthquakes and infer processes responsible for their deposition. Thick, acoustically transparent and homogeneous units we name “homogenites”, characterize these event deposits. Physical, geochemical and acoustic properties link homogenites to the 2011 Tohoku and previous earthquakes. We propose a multi-stage entrainment and depositional process related with both the high frequency and the exceptionally large low frequency and long duration motions expected from ruptures of the shallow megathrust below the compliant outer upper-plate. These large long-period motions could propagate along the outer upper plate beyond the rupture zone and could generate widespread entrainment of surficial sediment.