Yuji Itoh 

Subducted rough topography complicates seismic and aseismic slip behavior. The 2024 M 7.1 Hyuganada earthquake occurred along the megathrust with ridge subduction. We inferred coseismic slip and afterslip using geodetic displacements to observationally illustrate the role of subducted seamounts in modulating seismic and aseismic slip processes. The inferred mainshock slip was confined in the down-dip of the seamount, suggesting that the seamount impeded the mainshock rupture initiated under enhanced compression. The inferred afterslip peaked at the up-dip of the mainshock peak with four aftershock clusters. Various onset timings of these clusters suggest the afterslip front migration slowed down when passing through the seamount. Little afterslip is inferred in a segment south of the mainshock, where the megathrust is somehow insusceptible to stress perturbation and seems to creep steadily across the mainshock occurrence. Our results geodetically highlight the mechanical heterogeneity of megathrust with ridge subduction at an order of 10 km.