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We successfully apply the acoustic Laplace–Fourier waveform tomography method to delineate P-wave velocity structures of the mega-splay fault system in the central part of the seismogenic Nankai subduction zone offshore Japan, using densely sampled wide-angle ocean bottom seismograph (OBS) data originally acquired in 2004. Our success is due to new and carefully designed data preconditioning and inversion strategies to mitigate (i) the well-known non-linearity of waveform inversion, (ii) the challenges arising from crustal-scale survey designs (e.g. undersampling of the OBSs), and (iii) modelling errors due to the use of the acoustic assumption.

We identify a sixfold set of key components that together lead to the success of the high-resolution waveform tomography image: (i) Availability of low-frequency components (starting at 2.25 Hz) reducing the non-linearity, and access to large offset data …