[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P14] Wedge deformation from multi-seamount subduction in Nankai Trough off Muroto and its implication to 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.Japan Agency for Marine-Earth Science and Technology)

Keywords:Nankai Trough, slow earthquake, high pore pressure, seamount subduction

Numerical simulations have suggested that seamount subduction leads to high pore fluid pressures resulting in slow earthquakes. However, there is still a lack of comprehensive understanding on the link between seamount subduction, pore fluid pressure, and slow earthquake activity because the effect of sequential seamount subduction is rarely discussed. Four subducted seamounts have been reported off Muroto where slow earthquakes occur. We examined the seismic reflection profiles crossing these seamounts and compared them with sandbox and numerical models to understand its deformation history. Seamount A located 135.1°E, 32.1°N represents an early-stage subduction where it creates a localized uplift in the outer wedge and the basal decollement is deflected upward. Seamounts B1 and B2 uplifted the Minami-Muroto Knoll. It represents the next stage where a thick elongated unit is observed at the trailing-edge of the seamount, which is interpreted as underplated sediments or trench-fill sediments dragged by the seamount. A suture or thrust slice was also formed, marking the change in seafloor slope and formation of a new imbricate thrust. Seamount C, the largest subducted seamount, represents late-stage subduction. The underplated sediments appear as a reflective unit below the thrust slice. This repeat in structures (i.e. underplated sediments and thrust slice) is consistent with numerical models. We therefore propose that slow earthquake activity off Muroto may be related to high pore fluid pressure between the seamounts which is caused by the compression of Seamounts A, B1, and B2 with the underplated sediments of Seamount C.