Enhanced Subsurface Imaging of Western Nankai Trough Using Full Waveform Inversion

The Nankai Trough, where the Philippine Sea Plate converges beneath the Eurasian Plate, is situated off SW Japan and is one of the most seismically active subduction zones in the world, producing earthquakes such as the 1944 Tonankai and 1946 Nankai events. Within the Nankai Trough, the region of Shikoku is a geologically distinct segment located within the Tonankai and Nankai rupture zones. Shikoku is distinct due to the presence of a geological backstop, where the rigid forearc crust of the Eurasian Plate resists deformation, causing the compression and thickening of the accretionary prism. This backstop effect creates a structural boundary that influences sediment accretion, tectonic stress distribution, and seismic rupture behavior. The subducting Shikoku Basin crust is thinner and has a shallower dip angle in comparison to the other parts of the trench. The shallow subduction angle beneath Shikoku leads to highly heterogeneous stress and deformation patterns in the accretionary prism compared to steeper-dipping segments. Previous studies employing wide-angle seismic reflection and refraction surveys and tomographic methods have provided valuable insights into the broad-scale structure of the region. These efforts have delineated the geometry of the subducting Philippine Sea Plate and regional velocity structures within the accretionary prism and forearc. However, conventional methods often fail to resolve heterogeneities and variations in the deep subduction interface that critically influence seismic coupling. Furthermore, the role of Shikoku’s unique backstop configuration and the characteristics of the subduction interface at depths exceeding 40 km remain poorly constrained due to the limited resolution of traditional imaging techniques.

To overcome this, we take advantage of wide-angle data from ocean-bottom seismometers (OBS) spaced at 5 km along seismic profiles SK01, SK02, and SK03, acquired between 2009–2010 by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). We carry out Full Waveform Inversion (FWI) of the OBS data, a cutting-edge seismic imaging method leveraging the complete seismic waveform data to produce high-resolution velocity models of the subsurface. FWI iteratively refines the velocity model by minimizing discrepancies between observed and simulated seismic waveforms, enabling higher resolution at greater depths with unprecedented accuracy.

The obtained high-resolution velocity models provide a clear representation of previously under-resolved features in the off-Shikoku region, particularly the crustal structure and geometry of the Moho. These models overcome the limitations of traditional methods in imaging the subsurface at greater depths, addressing critical gaps in geological interpretation and advancing our understanding of tectonic processes in the region. By revealing fine-scale details of the subducting crust and Moho, this study further aids in developing effective planning for megaquakes and tsunami risk strategies and provides insights that could be applied to other regions with similar tectonic characteristics.