Geostatistical characterisation of internal deformation of the Japan Trench frontal prism using seismic and logging-while-drilling data (Site C0019)

Understanding how deformation is distributed within accretionary prisms is essential to constrain their structural evolution, internal organisation and seismo-tsunamigenic potential. Researchers would typically use seismic images to characterise large-scale subsurface structure, but accretionary prisms often appear internally chaotic-to-transparent in such data, lacking coherent reflectors. This is likely due to strong lateral heterogeneity and stratal disruption around the scale of the seismic resolution, resulting from intense deformation of accreted sediments and remobilised mass transport deposits.IODP Expedition 405 “JTRACK” (October-December 2024) drilled the frontal prism of the Japan Trench subduction zone at Site C0019, targeting the decollement that hosted extremely large slip (>50 m) to the trench during the 2011 Mw 9.1 Tohoku-Oki earthquake. The expedition involved continuous coring and logging-while-drilling (LWD) through the prism, resulting in an improved time-depth tie with regional 2-D seismic profiles. At this site the seismic images show a highly chaotic internal prism, which introduces large uncertainties in structural interpretation and inhibits the upscaling and lateral extrapolation of borehole-scale observations from Site C0019.Here we present an approach to characterise this heterogeneous internal structure by inverting for scale-independent geostatistical parameters (vertical and lateral correlation lengths, dip) from the combined seismic image and LWD sonic velocity data. We use a Bayesian Markov Chain Monte Carlo approach that gives spatially varying, probabilistic estimates of the geostatistical parameters. The lateral correlation length, specifically, can be used as a proxy for the degree of stratal disruption/deformation, as shorter correlation lengths reflect more intense deformation. While the aspect ratio of the correlation lengths is generally well-constrained for seismic data, the estimate of vertical correlation length from the borehole logs is necessary to resolve the other individual geostatistical parameters away from the borehole.Initial results reveal that the degree of deformation varies significantly within the frontal prism, with the lateral correlation length varying between around 10-50 m. The degree of deformation appears to be compartmentalised by steeply dipping structures that could correspond to fault zones also observed in cores from C0019. These distinct changes in lateral correlation length correspond to lithological units interpreted from core observations. They also coincide with significant changes in vertical correlation length from the sonic log, anisotropy of seismic velocity from core samples and bedding orientation from borehole images. The results demonstrate that seismically-derived geostatistical parameters can delineate internal compartmentalisation of the frontal prism, providing a framework for tectonic and mass transport deposit interpretation and for the extrapolation of core-scale observations. Future work will extend this analysis using parallel profiles along the Japan Trench axis, allowing for mapping of frontal prism internal deformation in three dimensions away from existing drill sites.