Crustal seismic imaging from multifold ocean bottom seismometer data by frequency domain full waveform tomography: Application to the eastern Nankai trough

International audienceIn 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−1), 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