Waveform Tomography Imaging of a Megasplay Fault System in the Seismogenic Nankai Subduction Zone

Abstract

We apply frequency-domain acoustic waveform tomography to form quantitative, high-resolution P-wave velocity images of a megasplay fault system within the central Nankai subduction zone offshore of southwest Japan, using controlled source Ocean Bottom Seismometer (OBS) data originally acquired in 2004. The waveform inversion was conducted in two stages: 1) using the phase-only logarithmic objective function, and 2) followed by the phase-and-amplitude logarithmic objective function. The two-stage approach minimizes artifacts due to elastic effects, and enables illumination of deeper parts of the model. The Laplace-Fourier domain approach was also adopted to mitigate non-linearity of waveform tomography by emphasizing the contribution from early parts of waveforms. We handled the difficulties arising from the data acquisition such as sparse OBS intervals and bubble oscillation by an appropriate data preprocessing, source estimation, and wavenumber filtering. The waveform tomography successfully delineated major geological features including the megasplay fault, and associated overpressured low velocity zones. The results yield significant improvements in the data fit to the OBS data, in comparison with those obtained from traveltime tomography. A comparison with the previous migration images not only confirmed the validity of the results, but also highlighted the ability of waveform tomography to fill in the part not imaged by the migration images.