Deep seismic imaging of the eastern Nankai trough, Japan, from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration

[1] The easternmost segment of the Nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench. It is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the Izu collision zone. We investigate this area with densely sampled active seismic data. One hundred ocean bottom seismometers, spaced only 1 km apart, were deployed along a line perpendicular to the trough. A two-step tomographic approach, designed for this type of acquisition, is applied: first, a long-wavelength velocity model is built by a linearized inversion of first arrival travel times, its resolution is assessed, and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested; second, this tomographic model is used to exploit late reflected arrivals with a prestack depth migration, the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model. Thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge. Our results bring new insights on the deep crustal zones of the margin and its evolution. The existence of presently inactivated major thrusts is very clearly evidenced in the backstop. A general timing for its evolution and deformation is proposed. Farther seaward, the presence of a subducted ridge beneath the accretionary wedge is confirmed. Its compressive structure and origin are demonstrated. It exhibits a thickened lower crust which nature is discussed. Besides, strong analogies between this "Paleo-Zenisu'' subducting ridge and the Zenisu ridge, observed farther south, are also emphasized, bringing evidences for a steady state crustal deformation pattern.