Investigating megathrust slip during and following the 2024 Mw 7.1 Hyuga-nada earthquake in southwest Japan

We use a 3-D finite-element model to invert the dense GNSS observations for coseismic and postseismic megathrust slip of the 2024 Mw 7.1 Hyuga-nada earthquake in westernmost Nankai subduction zone. The results reveal a quasi-circular, thrust-dominated rupture with a maximum slip of ~ 1.5 m, spanning depths of 15–30 km along the downdip edge of the subducting Kyushu-Palau ridge. Following the mainshock, a complex evolution of postseismic slip is observed. Initially, afterslip is concentrated predominantly within the coseismic rupture area. Over subsequent weeks, afterslip steadily migrates downdip to depths of 30–50 km, overlapping with regions historically associated with short- and long-term slow slip events (SSEs). Approximately one month after the earthquake, an additional episode of accelerated aseismic slip is detected at depths of 60–80 km, accompanied by a burst of M1–2 microseismicity. This distinct spatiotemporal evolution of megathrust slip suggests a complex interplay among the temperature and stress states of the subduction system, the megathrust rheology and the subducting Kyushu-Palau ridge. Through systematic resolution tests, we demonstrate that this event may represent the first well-constrained “seamount earthquake” captured by modern inland geodesy.