[1] In offshore surveys, the deep crust is generally investigated by traveltime tomography applied to sparse ocean bottom seismometer data. The inferred velocity models are of limited resolution precluding a quantitative analysis of deep tectonic discontinuities. If dense arrays of ocean bottom seismometers can be deployed, the resulting data sets become amenable to full waveform processing, which should lead to a significant improvement in the resolution of structures. Such acquisition and processing were achieved on the eastern Nankai subduction zone. Full waveform inversion is entirely implemented in the frequency domain, enabling efficient finite difference wave modeling and allowing to limit the inversion to a few discrete frequencies of increasing value. Such a hierarchical procedure defines a multiresolution approach to seismic imaging. The data set was recorded by 93 instruments deployed along a 105-km-long profile. Thirteen frequencies ranging from 3 to 15 Hz were inverted sequentially. Wavelengths imaged by the full waveform inversion typically range between 0.5 and 8 km. Full waveform tomography reveals an intense compression in the upper prism and underlying backstop, as evidenced by several thrusts, the frontal ones still active, corresponding to negative velocity anomalies. The subducting Paleo-Zenisu ridge is also reconstructed, above which stands the décollement. Velocities in the upper mantle beneath the ridge are rather low (7.5 km s À1 ), suggesting partially serpentinized mantle beneath the ridge. This paper demonstrates the feasibility of full waveform tomography based on dense ocean bottom seismometer data sets and its ability to quantitatively image the entire crust with a significantly improved resolution compared to what is usually achieved through traveltime tomography.Citation: Operto, S., J. Virieux, J.-X. Dessa, and G. Pascal (2006), Crustal seismic imaging from multifold ocean bottom seismometer data by frequency domain full waveform tomography: Application to the eastern Nankai trough,

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S U M M A R Y Two-ship multichannel seismic profiles, deep penetration (ECORS) and conventional seismic lines (LIGO surveys) are used to study the crustal structure of the Gulf of Lion (Western Mediterranean). 11 full ESPs (Expanded Spread Profiles) with total shot-receiver ranges up to 60 km were shot in 1981 perpendicular to the margin of the Gulf of Lion and in 1988 a deep MCS seismic profile (ECORS-CROP program) was performed parallel to the ESPs. These ESPs were analysed by matching traveltime and amplitude variations in both the x -f and z-p domains. The resulting P-wave velocity/depth model has the following features, (a) beneath the continental slope of the Provenqal margin a rapid rise of the Moho from 20 to 14 km and the existence of an anomalous 7.2-7.4 km s-' velocity layer, (b) from the base of the slope to the extensive salt-domes domain a 5-6 km thin crust which does not appear typically oceanic in nature, (c) quite typical oceanic crust up to the Sardinian margin. Gravity modelling is consistent with the seismic results. The OCB (ocean-continent boundary) could be placed north of that postulated by previous authors, where the data indicate a remarkably narrow transition between continental and 'oceanic' crust, or south where a typical oceanic crust, which correlates well with the domain of the salt domes and of large magnetic anomalies, has been determined.A very prominent reflector is clearly seen, at the base of the continental slope, on the seismic reflection profiles and corresponds to the top of an 7.2-7.4kms-' velocity layer. The high-velocity layer is 2-3 km thick where the crust is thinnest and has a limited lateral extent seawards. This anomalous crustal structure could be the result of extremely thinned and possibly broken up, continental crust underplated and intruded by partial melt, or could represent serpentinized peridotite material. Important questions about the evolution of the Gulf of Lion cannot be addressed using these new results alone without addition of other constraints. Nevertheless a two-stage mechanism of drifting and rifting of this part of the Western Mediterranean Sea is proposed.