Seismic imaging of subsurface structural variations along the Japan trench south of the 2011 Tohoku earthquake rupture zone

Abstract

The coseismic slip during the 2011 Tohoku earthquake has been revealed to reach the Japan trench axis, but the detailed slip behaviors in different zones along the trench remain an unsolved problem. To investigate the along-trench structures that directly affect the coseismic activities, we collected densely distributed seismic reflection data south of the major coseismic slip zone in the region of 36–37.5°N. Our seismic data document numerous local structural variations along different segments of the study zone: varying thicknesses of incoming sediments; chaotic structures in the north and fold-and-thrust belts in the south at the wedge front; different morphologies of the subducting plate, correlating with the physical properties above the plate interface and influencing, in turn, shallow megathrust slip and tsunami genesis. The southern limit of the 2011 event large coseismic rupture zone at ∼37°N works as a transition zone, which appears to correlate with the landward extension of a subducting channel in the deep places, and the shallower detachment fault structures in the graben beneath the trench axis that are less developed than those in the large slip zone further north. It also appears to correlate with the different elastic properties along the basal layer of the overriding plate caused by regionally various thickness and lithologies.