[S-CG61] Quest for the state & mechanism of subduction-zone earthquake through experiments across-scales

[SCG61-P04] Mapping prism taper angle and roughness of the subducted oceanic crust off Muroto using seismic reflection data and its implication for slow earthquake activity

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

Keywords:slow earthquake, seismic survey, Nankai Trough, seismicity gap, low friction, seamount subduction

We examined the possible causes of the variable slow earthquake activity along the Nankai Trough using a dense network of multi-channel seismic profiles between 133.4°-136.2°E. Slow earthquakes are active between 134.2-135.4°E (active zone, AZ-1), and eastward of 135.9°E (AZ-2). There is a lack of slow earthquakes west of AZ-1, and between AZ-1 and AZ-2. The oceanic crust and decollement horizons were tracked. As one of the parameters to evaluate roughness of the oceanic crust and the decollement, we measured the root mean square (RMS) of the height from regression lines of the oceanic crust and the decollement. The seafloor (α) and basal (β) slopes were also calculated from the slope of the regression lines. The decollement was only identifiable until the trench-slope break then becomes untraceable because of unclear reflection, step-down, merging with the crust, or complex structural features associated with duplexing. Decollement RMS is relatively uniform throughout the survey area. The crust shows varying RMS within AZ-1 (σ=211m) due to the numerous subducted seamounts. Along the seismicity gap between AZ-1 and AZ-2, the crust RMS is significantly lower (σ=85m) and shows a relatively steady change in roughness, which combined with the smooth decollement may have caused the lack of slow earthquakes. The average taper angle (α+β) in AZ-1 is also lower (3.7±0.4) compared to inactive areas to its west (7.0±0.5) and east (8.6±0.5), which is indicative of low friction. Thus, we propose that the slow earthquake activity in Muroto is likely promoted by seamount subduction and low friction.