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 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. An along-strike neighbor segment of the mainshock peak was left unruptured during the afterslip, but whether this segment causes another earthquake remains elusive only with the available observations. Our results geodetically highlight the mechanical heterogeneity of megathrust with ridge subduction at an order of 10 km.