Internal structure, active tectonics and dynamic topography of the eastern Nankai accretionary prism toe, Japan, and its tsunamigenic potential

Abstract

AbstractThe eastern Nankai accretionary prism toe was surveyed to evaluate the nature and deformation of its frontal thrust. According to the determined porosities and yield strengths, turbidites were successively buried down to depths of 250–300 m before accretion, and were then exposed at the prism toe by uplift along the Tenryu frontal thrust during 3.4–1.98 Ma. Consolidation tests provided reasonable estimates of burial depth and, when combined with exposed sediment dates, yield prism toe uplift rates of 0.74–2.27 m ka–1. The displacement along the frontal thrust is estimated to be 500–900 m and the slip rates are 1.47–4.55 m ka–1, corresponding to the highest class of active faults on land in Japan. During the surveys of the Tenryu frontal thrust zone, we discovered a new active fault scarp that was several tens of centimetres high, interpreted to be a protothrust located c. 100 m south of the frontal thrust. This scarp is associated with chemosynthetic biocommunities. The thrust might potentially be the result of displacement during the East Nankai (To-Nankai) earthquake (Mw 8.1) in 1944. These lines of evidence indicate that the Tenryu frontal thrust is still active and that displacement along the thrust might induce a tsunami during future Tokai or To-Nankai earthquakes.