What Control Segmentations of Mega-Thrust Earthquakes in the Nankai Seismogenic Zone : a Review of High-Resolution Wide-Angle Seismic Surveys

A series of wide-angle seismic surveys has been carried out in recent years of the entire Nankai seismogenic zone under the working hypothesis, "structure of subducted plate might control segmentation of rupture zones". In this paper we summarize the key results of these surveys. Along a profile crossing the rupture zone of the 1946 Nankaido earthquake we successfully imaged a large-scale subducted seamount, which is 50km wide and 3km high. The seamount is interpreted to be one member of the Kinan seamount chain, which is recognized on the incoming plate. Comparing the location of the subducted seamount and a detailed rupture distribution of the 1946 event shows that the rupture starting off Cape Shiono turned around the landward foot of the seamount. Another structure preventing rupture propagation was also found along a profile looking at the eastern edge of the 1944 Tonankai earthquake. High-resolution wide-angle seismic data clearly imaged two lines of subducted ridges, which are interpreted to be a subducted ridge parallel to the present day ridge system on the incoming plate. One of the subducted ridges is situated immediately outside the rupture zone where GPS data indicate strong coupling at present. We propose, from these results, that a subducted convex, which underlays a back stop structure, played the role of a barrier preventing earthquake rupture propagation