Presentation Information
[T6-O-5](Invited) Basement topography and accretionary prism structure along the entire Nankai Trough revealed by dense seismic surveys
*Kazuya Shiraishi1, Ryuta Arai1, Yasuyuki Nakamura1, Gou Fujie1 (1. JAMSTEC)
【ハイライト講演】南海トラフ全域で実施された世界でも類を見ない稠密な反射法地震探査により，沈み込む海洋プレート基盤の詳細な三次元形状と付加体構造が明らかになりました．本講演では，海山や古海嶺などの基盤起伏が巨大地震セグメントの形成に与える影響と，東海沖で見出された上盤構造の側方変化がスロー地震活動と密接に対応することを紹介します．地震発生場を支配する「沈み込む基盤」と「上盤構造」の新たな関係に迫る，最新成果をぜひご覧ください． ※ハイライト講演とは．．．
Keywords:
subduction zone,Nankai Trough,seismic survey
Recurring megathrust earthquakes accompanied by tsunamis pose a major hazard along the Nankai Trough, where spatially localized slow earthquakes have also been repeatedly observed. To better understand the seismogenic zone, numerous seismic surveys and scientific drilling projects have investigated the relationship between geological structure and seismic activity. Between 2018 and 2025, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) conducted dense two-dimensional multichannel seismic reflection surveys across the entire Nankai Trough using both newly acquired and legacy datasets. Survey lines were spaced 4–8 km perpendicular and 10–25 km parallel to the trench axis.
The new surveys substantially improved crustal imaging compared with the regional three-dimensional model of Nakanishi et al. (2018). In particular, the geometry of the subducting basement—the boundary between the igneous oceanic basement and overlying sediments—was resolved to depths of 15–20 km with horizontal resolutions of several kilometers (Shiraishi et al., 2025a). Three distinct basement domains were identified. The western Hyuga-nada region is characterized by the northern extension of the Kyushu–Palau Ridge and a steeply dipping plate interface; the central Nankai region by north–south-trending depressions and arcuate seamounts and ridges; and the eastern Tonankai–Tokai region by prominent relief associated with the subducting Paleo-Zenisu Ridge. These features originated during Shikoku Basin spreading and subsequent magmatism before being transported into the subduction zone.
These three domains closely coincide with the segmentation of expected megathrust rupture areas, suggesting that inherited basement topography strongly influences forearc structure and coseismic slip distribution. In contrast, slow earthquakes do not systematically coincide with basement relief, indicating that basement topography alone is insufficient to explain their occurrence. Instead, slow earthquake activity is likely controlled by a combination of sediment properties, geological structure, fault geometry, and local mechanical and hydrological conditions. Integrating seismic reflection images with physical property models derived from OBS refraction and wide-angle reflection data is therefore essential (Arai et al., 2023; Shiraishi et al., 2025b), while three-dimensional structural variations unresolved by two-dimensional surveys may also contribute to slow earthquake localization (Shiraishi et al., 2020).
Offshore Tokai, where slow earthquakes are relatively sparse despite pronounced basement relief, the dense seismic data reveal distinctive structural variations within the overriding accretionary prism. The Paleo-Zenisu Ridge exhibits marked along-strike variations, including a prominent basement high south of Ise Bay. Across this high, significant contrasts occur in fault architecture, décollement geometry, underthrust sediments, and sediment thickness above the basement. These lateral structural changes closely correspond to the distribution of slow earthquakes. We propose that oblique subduction of the laterally variable Paleo-Zenisu Ridge modified the overriding plate, producing lateral variations in plate-boundary physical and chemical conditions that ultimately formed the boundary between the Tonankai and Tokai earthquake segments.
References
Arai, Miura, Nakamura, et al. (2023) Nat. Commun., 14:5101. https://doi.org/10.1038/s41467-023-40762-4
Nakanishi, Takahashi, Yamamoto, et al. (2018) Geol. Soc. Am. Spec. Pap. 534:69–86. https://doi.org/10.1130/2018.2534(04).
Shiraishi, Yamada, Nakano, et al. (2020) Earth Planets Space 72:72. https://doi.org/10.1186/s40623-020-01204-3
Shiraishi, Nakamura, Arai, et al. (2025a) Sci. Rep. 15:25530. https://doi.org/10.1038/s41598-025-08846-x
Shiraishi, Fujie, Nakamura, Arai (2025b) Mar. Pet. Geol. 173:107258. https://doi.org/10.1016/j.marpetgeo.2024.107258
The new surveys substantially improved crustal imaging compared with the regional three-dimensional model of Nakanishi et al. (2018). In particular, the geometry of the subducting basement—the boundary between the igneous oceanic basement and overlying sediments—was resolved to depths of 15–20 km with horizontal resolutions of several kilometers (Shiraishi et al., 2025a). Three distinct basement domains were identified. The western Hyuga-nada region is characterized by the northern extension of the Kyushu–Palau Ridge and a steeply dipping plate interface; the central Nankai region by north–south-trending depressions and arcuate seamounts and ridges; and the eastern Tonankai–Tokai region by prominent relief associated with the subducting Paleo-Zenisu Ridge. These features originated during Shikoku Basin spreading and subsequent magmatism before being transported into the subduction zone.
These three domains closely coincide with the segmentation of expected megathrust rupture areas, suggesting that inherited basement topography strongly influences forearc structure and coseismic slip distribution. In contrast, slow earthquakes do not systematically coincide with basement relief, indicating that basement topography alone is insufficient to explain their occurrence. Instead, slow earthquake activity is likely controlled by a combination of sediment properties, geological structure, fault geometry, and local mechanical and hydrological conditions. Integrating seismic reflection images with physical property models derived from OBS refraction and wide-angle reflection data is therefore essential (Arai et al., 2023; Shiraishi et al., 2025b), while three-dimensional structural variations unresolved by two-dimensional surveys may also contribute to slow earthquake localization (Shiraishi et al., 2020).
Offshore Tokai, where slow earthquakes are relatively sparse despite pronounced basement relief, the dense seismic data reveal distinctive structural variations within the overriding accretionary prism. The Paleo-Zenisu Ridge exhibits marked along-strike variations, including a prominent basement high south of Ise Bay. Across this high, significant contrasts occur in fault architecture, décollement geometry, underthrust sediments, and sediment thickness above the basement. These lateral structural changes closely correspond to the distribution of slow earthquakes. We propose that oblique subduction of the laterally variable Paleo-Zenisu Ridge modified the overriding plate, producing lateral variations in plate-boundary physical and chemical conditions that ultimately formed the boundary between the Tonankai and Tokai earthquake segments.
References
Arai, Miura, Nakamura, et al. (2023) Nat. Commun., 14:5101. https://doi.org/10.1038/s41467-023-40762-4
Nakanishi, Takahashi, Yamamoto, et al. (2018) Geol. Soc. Am. Spec. Pap. 534:69–86. https://doi.org/10.1130/2018.2534(04).
Shiraishi, Yamada, Nakano, et al. (2020) Earth Planets Space 72:72. https://doi.org/10.1186/s40623-020-01204-3
Shiraishi, Nakamura, Arai, et al. (2025a) Sci. Rep. 15:25530. https://doi.org/10.1038/s41598-025-08846-x
Shiraishi, Fujie, Nakamura, Arai (2025b) Mar. Pet. Geol. 173:107258. https://doi.org/10.1016/j.marpetgeo.2024.107258
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