Tectonic features of the Japan Trench convergent margin off Sanriku, northeastern Japan, revealed by multichannel seismic reflection data

Near the Japan Trench convergent plate margin the seaward edge of the conti nental plate is deformed by subduction of the oceanic plate. We report the results of a multichannel seismic survey in the northern Japan Trench in wh ich this deformed zone is demarcated from the rigid continental framework b y a pronounced landward dipping reflector. The oceanic plate also undergoes deformation as the two prates interact in the subduction processes, result ing in a progressive deformation or destruction of a horst structure along the top of the subducting oceanic crust. This may eventually lead to the fo rmation of a smooth plate boundary at the greater depth. More than 45 km la ndward from the trench axis, a smooth reflector suggesting a stable slip pl ane is visible along and above the oceanic crust. Our data indicate that th e deformed zone pinches out landward similar to 60 km from the axis at 13 k m depth and the slip plane becomes less obvious there. Seismicity of interp late earthquakes rapidly increases landward from this location, leading us to speculate that this is where coupling at the plate boundary becomes stro ng enough for earthquakes to occur. We conclude that the updip limit of the seismogenic zone of interplate earthquakes in the study area is characteri zed by the tectonic feature of a pinch-out of the deformed sediments which overlie the subducting oceanic crust.