Structural factors controlling the rupture process of a megathrust earthquake at the Nankai trough seismogenic zone

The Nankai Trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100-200 yr. The 1946 Nankaido earthquake is well known as an unusual event among these earthquakes, because the rupture zone estimated from long-period geodetic data is more than twice as large as that derived from seismic wave data. In the summer of 1999, an onshore-offshore deep seismic survey was performed along a 355 km long profile in the western Nankai Trough seismogenic zone. Seismic signals both from an airgun array (207 l) and land explosions (maximum of 500 kg) were recorded simultaneously by 98 ocean-bottom seismographs and 93 land seismic stations. Conventional 2-D seismic reflection data were also acquired along part of the offshore profile. From the wide-angle seismic data, we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth. The seismic velocity image also shows that the seamount is now colliding with the Japanese island arc crust. From this significant structure, this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake, i.e. the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount, and then the rupture was deflected around the subducted seamount at the down-dip end of the locked zone between Cape Muroto and Cape Ashizuri. Another significant structure, a highly reflective layer, is obtained beneath Shikoku Island. A very slow P-wave velocity (3 km s(-1)) is necessary in a thin layer at the base of the island arc crust in order to explain the observed high-amplitude reflection phases. An area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer. This suggests a possible water layer at the base of the island arc crust. The water may be generated by dehydration of the downgoing probably partially serpentinized mantle, which is implied by a low P -wave velocity (7.5 km s(-1)) beneath the subducted seamount. A locally observed non-slip region during the 1946 earthquake at the eastern part of Shikoku Island is interpreted as a result of weak coupling at the possible water layer.