南海トラフ付加体の地震発生帯-日米3D調査概要- Seismogenic Zone in the Nankai Accretionary Wedge:General Summary of Japan-U.S.Collaborative 3-D Seismic Investigation
A three-dimensional seismic survey was carried out at the western Nankai Trough accretionary wedge from June 18 to August 18, 1999. This experiment was Japan-U.S. collaborative investigation on seismogenic zones. The cruise imaged an 8 X 80 km area with 81, 80km-long, high quality, seismic reflection lines, all of which have nearly continuous coverage.<BR>The main objective of our experiment was to image the plate boundary fault at which major earthquakes and tsunamis are generated. Our primary goal is to image the thrust and identify the boundary between aseismic and seismic zones for a large inter-plate earthquake.<BR>On-board two-dimensional data processing has been carried out. This gives us a significantly clear image of the inner structure of the accretionary wedge at our survey area. One impressive image of the profile shows the large thrust slice zone that is classified by our interpretation. A number of out-of-sequence thrusts have developed and are concentrated in the area where the water depth is approximately 4, 000 to 3, 000m. A decollement plane touches the oceanic plate (layer II) down at there in first. We identified that the boundary between the stable sliding zone (ocean-ward) and the unstable stick slip zone (landward) is located there. We propose that the boundary is the up-dip limit of the seismogenic zone. Further data processing is ongoing. A true three-dimensional structure will reveal much more details and a clear image of seismogenic zone at the Nankai subduction margin.
地學雜誌 109(4), 531-539, 2000-08-25
Tokyo Geographical Society