Description: Highlights • Magnetic fabric in the area of large slip during the 2011 Tohoku-Oki EQ are presented. • Magnetic fabric in the frontal prism reflects the WNW-directed plate convergence. • Those in underthrust sediments records a uniaxial strain state and underconsolidation. • The contrast indicates strain decoupling across the plate-boundary décollement. • Large co-seismic slip could occur in shallow décollement where plates are decoupled. Abstract The Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) drilled and cored across the plate-boundary décollement at Site C0019 near the Japan Trench, where large slip occurred during the 2011 Tohoku-Oki earthquake (MwMw 9.0). Anisotropy of magnetic susceptibility (AMS) data obtained from core samples show a striking change in magnetic fabric across the décollement. In the frontal prism above the décollement, the maximum AMS axes display a strong preferred orientation in the northeast–southwest direction, with the intermediate and minimum AMS axes distributed in the vertical plane. In the underthrust sediments below the décollement, the maximum and intermediate AMS axes are subhorizontal but variable in direction, and the minimum AMS axes display an approximately vertical preferred orientation. The AMS ellipsoids for all samples have an oblate component, but the AMS ellipsoids of the underthrust sediments are generally less oblate than those of the frontal prism. The magnetic fabric of the sediments in the prism are consistent with horizontal tectonic shortening nearly parallel to the plate convergence direction in the Japan Trench, while that in the underthrust sediments represents a vertical, uniaxial strain. The magnetic fabrics of the frontal prism and underthrust sediments indicate an abrupt change in strain across the décollement at shallow depths, and imply that large co-seismic slip occurred along a weak plate-boundary décollement that is mainly decoupled over the long-term.

Description: The mechanics of great subduction earthquakes are influenced by the frictional properties, structure, and composition of the plate-boundary fault. We present observations of the structure and composition of the shallow source fault of the 2011 Tohoku-Oki earthquake and tsunami from boreholes drilled by the Integrated Ocean Drilling Program Expedition 343 and 343T. Logging-while-drilling and core-sample observations show a single major plate-boundary fault accommodated the large slip of the Tohoku-Oki earthquake rupture, as well as nearly all the cumulative interplate motion at the drill site. The localization of deformation onto a limited thickness (less than 5 meters) of pelagic clay is the defining characteristic of the shallow earthquake fault, suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes.