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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 1) 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 …