Full-Waveform Inversion for Imaging Faulted Structures: A Case Study from the Japan Trench Forearc Slope

Abstract

Full-waveform inversion (FWI) of limited-offset marine seismic data is a challenging task due to the lack of refracted energy and diving waves from the shallow sediments, which are fundamentally required to update the long-wavelength background velocity model in a tomographic fashion. When these events are absent, a reliable initial velocity model is necessary to ensure that the observed and simulated waveforms kinematically fit within an error of less than half a wavelength to protect the FWI iterative local optimization scheme from cycle skipping. We use a migration-based velocity analysis (MVA) method, including a combination of the layer-stripping approach and iterations of Kirchhoff prestack depth migration (KPSDM), to build an accurate initial velocity model for the FWI application on 2D seismic data with a maximum offset of 5.8 km. The data are acquired in the Japan Trench subduction zone, and we focus on the area where the shallow sediments overlying a highly reflective basement on top of the Cretaceous erosional unconformity are severely faulted and deformed. Despite the limited offsets available in the seismic data, our carefully designed workflow for data preconditioning, initial model building, and waveform inversion provides a velocity model that could improve the depth images down to almost 3.5 km. We present several quality control measures to assess the reliability of the resulting FWI model, including ray path illuminations, sensitivity kernels, reverse time migration (RTM) images, and KPSDM common image gathers. A direct comparison between the FWI and MVA velocity profiles reveals a sharp boundary at the Cretaceous basement interface, a feature that could not be observed in the MVA velocity model. The normal faults caused by the basal erosion of the upper plate in the study area reach the seafloor with evident subsidence of the shallow strata, implying that the faults are active.

Connection between high pore-fluid pressure and frictional instability at tsunamigenic plate boundary fault of 2011 Tohoku-Oki earthquake

“…The spatial location of this line coincides with 2D seismic line MY101, which was acquired in 1999, and several studies 6 , 15 – 17 , 30 have been conducted to estimate the coseismic slip of 2011 Tohoku-Oki earthquake around this line. Previously, we used line D13 in a different study 42 to apply seismic full waveform inversion on the shallow sedimentary strata in the forearc upper slope.…”

“…We processed the seismic data to prepare it for depth imaging by applying bubble suppression, deghosting, swell noise removal, surface related multiple elimination (SRME), residual multiple suppression using parabolic radon transform (PRT), time-variant band-pass filtering, and FX deconvolution. We developed an initial P-wave interval velocity model by using the layer stripping approach 42 and further updated the velocity model using several iterations of grid-based traveltime tomography 43 . For the sake of computational efficiency, we applied Kirchhoff prestack depth migration 44 (KPSDM) during the velocity model building stage but after obtaining the final velocity model we applied reverse time migration 45 – 47 (RTM) to produce the seismic depth sections with the highest quality from the target area within 40–0 km distance toward the trench.…”

Seismic Reflection Images of Possible Mantle-Fluid Conduits and Basal Erosion in the 2011 Tohoku Earthquake Rupture Area

“…However, fluid ovepressure in basal shear zones of the overlying plate may have favored the rupture of the 2011 Tohoku event. This hypothesis can be tested by a further investigation such as full-waveform inversion (e.g., Jamali Hondori et al, 2021) of longer-offset (>10 km) seismic reflection data, which would provide high-resolution V p data and thus accurate pore-fluid pressures along the basal shear zone.…”