[S07] PM-1

[S07-06] Dense reflection survey shows link between geometrical and physical property changes in the Nankai Trough with slow earthquake activity

*Paul Caesar Mason Flores^1,2, Shuichi Kodaira^2,1, Gaku Kimura², Kazuya Shiraishi², Yasuyuki Nakamura², Gou Fujie², Tetsuo No², Yuka Kaiho² (1. Graduate School of Environment and Information Sciences, Yokohama National University, 2. Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology)

Factors possibly affecting the clustered distribution of very low frequency earthquakes and tremors in the Nankai Trough were examined using prestack depth migrated profiles with line intervals of 4-8 km from off Cape Shionomisaki to off Tosa Bay. Decollement roughness was measured by the root mean square (RMS) of its height from a reference line. Prism taper angle was measured 30 km from the frontal thrust by linear fitting of the seafloor and decollement, and was used to estimate the effective basal friction (μ_b) and pore pressure ratio (λ*_α+β) by applying the critical taper theory. The p-wave velocity from the underthrust section within 10 km from the frontal thrust was also used to calculate the pore pressure ratio (λ*_Vp). Results showed that a rough decollement off Muroto, indicated by the standard deviation of the RMS (σ_RMS=104-147 m), and variable underthrust thickness is associated with high slow earthquake activity. In contrast, a smooth decollement (σ_RMS=74-82 m) is associated with a seismicity gap. Decollement roughness is associated with multiple subducted seamounts. Slow earthquakes also occur in a wide zone of low taper angle (3.8°), indicating low μ_b (0.1-0.4), and high λ*_α+β (0.6-0.8). The λ*_Vp also indicate elevated pressures off Muroto (0.6-0.7) than the surrounding areas and shows high correlation (0.80) with λ*_α+β. This indicates that the high λ* in Muroto is a reliable feature. We infer that this is caused by an interplay of sediment underplating and fluid expulsion from the underplated sediments due to multiple seamount subduction.