Coseismic slip and early afterslip of the 2024 Hyuganada earthquake modulated by a subducted seamount

Subducted rough topography, such as seamounts, complicates seismic and aseismic slip behavior along megathrusts. The 2024 M 7.1 Hyuganada earthquake occurred along the megathrust with the subduction of Kyushu-Palau Ridge (KPR) offshore Kyushu, southwestern Japan. Therefore, this earthquake provides a valuable opportunity to observationally illustrate the role of subducted seamounts in modulating seismic and aseismic slip processes.  We inferred coseismic slip and 1-week afterslip using GNSS coordinates. The inferred mainshock slip was located in the down-dip of the seamount, suggesting that this earthquake was initiated under enhanced compression due to the subducted seamount. Furthermore, considering that subducted seamounts might act as a soft barrier, the mainshock rupture was probably arrested by this seamount. The inferred 1-week afterslip peaked at the up-dip of the mainshock peak and overlapped with the seamount. This up-dip afterslip is accompanied by four aftershock clusters. Assuming that the activation timing of aftershocks marks the arrival of the afterslip front, various onset timings of these clusters suggest different migration rates of afterslip in different directions. In particular, the activation of a cluster up-dip of the seamount is delayed, suggesting that the migration rate of the afterslip front is slowed down along the path across the seamount. Little afterslip is inferred in a segment south of the mainshock, where the interseismic slip deficit rate is low. We interpret these observations that the megathrust there is somehow insusceptible to stress perturbation and seems to creep steadily across the mainshock occurrence. Our results geodetically highlight that the subducted KPR introduced mechanical heterogeneity of megathrust at an order of 10 km.