Seismic reflection imaging of seismogenic zone structures at Japan Trench convergent margin

The Pacific Plate is subducting beneath the northeastern Japan arc at the Japan Trench, where many large interplate earthquakes of greater than M7.5 frequently occur. They have caused extensive damage in the area. Although previous authors clarified sedimentary structures at the forearc region of the margin, deep structures relevant with generation of the earthquakes were not imaged because the seismic source energy was insufficient. In order to figure out tectonic structures at the seismogenic zone, we have been conducting seismic imaging study since 1996. Prestack depth migration successfully delineated deep structures: a deformation feature of subducting horst-graben structures, a wedge-shaped sedimentary unit at the tip of the island arc crust and a channel-like sedimentary unit near the plate boundary. Interestingly, velocity of the sedimentary units is lower than that of their upper layers. If the low velocity of these units results from existence of fluid as many authors assume, the units may play an important role in coupling at the plate boundary. Actually the units are distributed at low-seismicity area in the margin, which suggests the harmonious relationship between the units and lower coupling. We report here the results of our seismic surveys and discuss on the relationship between structures and earthquake generation in the study area.