Topography of the subducting basement in the Nankai Trough seismogenic zone

1. Key Points

• Detailed topography of the top surface of the subducting basement (i.e., the boundary between the top of the oceanic crust and the overlying sedimentary layers or accretionary prism) has been revealed via seismic reflection profiles throughout the entire Nankai Trough.
• Three domains, divided based on topographical features, correspond to the major segments of the megathrust seismogenic zones. Topographical reliefs formed by past tectonic and igneous activities before subduction may have affected the development of present geological structures and the slip behaviors during megathrust earthquakes.
• Prominent basement undulations do not necessarily correspond to the localized distribution of slow earthquakes, suggesting the need for additional factors that control spatiotemporal variations in the seismicity.
• Further investigation of geophysical conditions in the overlying layers, combined with monitoring earthquake activity and modeling geological evolution, will contribute to elucidating the causes of the variety of earthquake activity.

2. Overview

Dr. Kazuya Shiraishi and colleagues at the Research Institute for Marine Geodynamics, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), have revealed the detailed topography on the subducting basement along the entire Nankai Trough via seismic reflection profiles collected from 1997 to 2024. They generated a new surface model that captured the topographical features on a scale of several kilometers, over 730 km long and 150 km wide (Fig.1). The topographical reliefs formed by past tectonic and igneous activities before subduction have affected the present heterogeneity in geological structures, physical properties, and various seismic activities along the Nankai Trough. They found that the three domains, divided based on topographical features, correspond to the segments of megathrust seismogenic zones. This result suggests that the large-scale topographical features of the basement may influence the activity of megathrust earthquakes and associated fault slip distribution. On the other hand, the distribution of prominent basement undulations does not necessarily correspond to the localized distribution of slow earthquakes, including tremors and very low-frequency earthquakes. These findings suggest that basement undulations alone do not control the occurrence of slow earthquakes, and that additional local factors should be taken into account to understand the spatiotemporal variations in the seismicity. The basement topography and geological structures clarified in this study will be useful for more detailed investigations into the physical properties and deformation structures caused by the basement undulation. Further studies of the geophysical conditions in the overlying layers, combined with continuous monitoring of earthquake activity and numerical modeling of geological evolution, are expected to contribute to a better elucidation of the causes controlling various seismic activities.

The findings of this study were published in Scientific Reports on 30 July 2025 (Japan Time). This study was supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JP16H06475、JP21H05202, JP22K03789, JP24K07180), and the funding for the Research Project for Disaster Prevention on the Great Earthquakes along the Nankai Trough from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

Fig.1. The topographical features of the subducting basement along the entire Nankai Trough. It can be divided into three domains in an east-west direction (blue arrows), which generally correspond to the megathrust seismogenic zones (red arrows). The black broken lines indicate the north-south trending depressions related to the past seafloor spreading in the Shikoku Basin.