Morphological and geothermal features around subducted seamount in Hyuga-Nada, western Nankai Trough

The interaction between subducting seamounts and overriding sediments perturbs the stress field and effective strength, affecting the conditions for megathrust earthquake generation and likely weakens interplate coupling. In the westernmost Nankai Trough around Hyuga-nada, M8-class earthquakes have not been reported yet. The Kyushu–Palau Ridge (KPR), marking the boundary between the Shikoku Basin and the West Philippine Basin (WPB), is subducting beneath Hyuga-nada. Slow earthquakes are frequently observed around the subducted KPR (sKPR). Key controlling factors for earthquake generation include seamount geometry, stress perturbations induced by subduction, and weakening plus permeability enhancement due to fracturing of the overriding strata.

In addition to estimating the BSR-derived heat flow, we conducted seafloor heat flow measurements, combined with interpretation of reflection seismic data, to delineate the morphology of the overriding plate and near-surface deformation structures. The sKPR lies beneath the Toi Seamount (Tsmt, exposed above the seafloor). Its eastern and western edges coincide with magnetic anomaly boundaries, while its northern edge corresponds to the northern slope of Tsmt. The coincidence between steep basement slopes and areas of frequent low-frequency tremors (LFTs) suggests that LFT activity is controlled by the “edges” of sKPR.

The influence of KPR subduction is evident in seafloor morphology and deformation structures. Numerous faults and lineaments are identified beneath the seafloor, with compressional structures dominant to the N–NW and extensional structures to SE. In the N–NW, multiple NE–SW trending ridges are present, and thrusts formed during accretionary prism development may have been exhumed by seamount collision. In contrast, the SE side is characterized by abundant collapse and landslide deposits.

Heat flow estimated from BSR depths around sKPR is ~40 mW/m² or lower, consistent with surface heat flow measurements, reflecting the cold (old) nature of the subducted sKPR and WPB. On the northern (leading) side, BSR-derived heat flow is lower (~25 mW/m²) above SW–NE trending thrust faults. This is likely due to seamount-driven compression and thickening of sediments, and reducing the thermal gradient. Blockage of sediment transport by Tsmt, also promotes thickening and cooling. Conversely, surface heat flows exceeding 300 mW/m² were observed near thrusts in front of Tsmt. While water temperature fluctuations, deep-sea turbidites, or slope erosion may contribute, the proximity to the base of a thrust-fault scarp, the identification of a low-velocity zone near the LFT cluster from OBS data, and chemical anomalies in pore waters suggest fluid expulsion along fault conduits under frontal compression. Poroealstic modeling supports this interpretation, showing pore fluid circulation induced by seamount loading if high permeability around the KPR is assumed. The fluid discharge is driven by the horizontal compression leading to overpressure and the fault pathway formation. However, the number of data points remains limited, alternative explanations cannot be excluded. Direct evidence of fluid discharge (e.g., biological communities) is lacking. Verification must therefore await future investigations.