A >100-million-year record of soft-sediment deformation structures in incoming-plate sediments of the Japan Trench Subduction Zone

Soft-sediment deformation structures (SSDS) are commonly used to infer past geological events, yet their long-term variability in marine sediment archives remains poorly constrained. We investigate the types, abundance, and temporal variations of SSDS throughout a 286.89-m sediment succession spanning >100 million years, recovered during the International Ocean Drilling Program (IODP) Expedition 405 at Site C0026 on the incoming Pacific Plate prior to the Japan Trench subduction. We identified 309 SSDS within 193 deformed horizons, comprising nine types of liquefaction–fluidization-related and shear-related structures. Liquefaction–fluidization-related structures preferentially occur within intervals with high lithological contrasts, particularly volcanic ash beds and color-banded clay intervals. However, lithological differences alone cannot explain the stratigraphic trends in SSDS abundance and diversity. Instead, SSDS abundance and diversity broadly correspond to tectonic evolution, increasing during ridge-proximal and outer-rise stages and decreasing during the abyssal-plain stage. While some SSDS may have formed at or near the sediment–water interface contemporaneously with the triggering earthquake, stratigraphic patterns and cross-cutting relationships indicate that others developed only after burial and changes in consolidation state. These observations imply that host-sediment age does not necessarily correspond directly to the timing of the earthquake recorded by SSDS, as commonly assumed in many paleoseismological studies. Together with different estimates in deformation thresholds and inferred shaking intensities, our results suggest that individual SSDS types may serve different roles in paleoseismic reconstruction. At the population scale, variations in SSDS abundance, diversity, and deformation style provide a framework for evaluating long-term changes in seismic activity and tectonic evolution.