Unexpected megathrust slip evolution revealed by the 2024 Mw 7.1 and the 2025 Mw 6.8 Hyuga-nada earthquakes in southwest Japan

• Postseismic data of the 2024 event prefers most afterslip in the main source region. • Afterslip migrates downdip into short- and long-term slow slip event regions. • An additional slip and aftershocks are excited one month later at 60–80 km depths. • Afterslip equals 63 % of coseismic moment and may help trigger the 2025 event. • Overlapping megathrust slip suggests transitions between fast and slow slip modes. Subduction faults were thought to slip either seismically as earthquakes or aseismically through creep, representing two end-member frictional behaviors. A dynamic transition from interseismic creep to coseismic failure during rupture had also been proposed. However, the potential for a subduction fault segment to transition between seismic and aseismic mode in a time scale of days to months remains poorly understood, constrained by the remote offshore nature of these zones and limited instrumentation in most subduction regions. Here, we investigate the coseismic and time-dependent postseismic slip associated with the 2024 and 2025 megathrust events in Hyuga-nada, Japan, using dense inland GNSS data, aftershocks and a finite-element model. Our preferred source model of the 2024 event identifies a quasi-circular, thrust-dominated rupture with a geodetic moment magnitude (Mw) of 7.1 and a maximum slip of ∼1.5 m. This event occurred at relatively greater depths (15–30 km), compared to previous M7 events in the region, along the downdip edge of the inferred subducting Kyushu-Palau Ridge. Kinematic analysis of postseismic displacements indicates that the initial 50 days of afterslip were excited concurrently with ample plate-interface aftershocks in the earthquake source region and migrated downdip to the northwest into an area previously characterized by long-term and short-term slow slip events. These overlaps between coseismic and postseismic slip, as well as between postseismic slip and slow slip events, align with earlier observations from the 1996 doublet events in the vicinity of the 2024 rupture, albeit with less dense data. Approximately one month after the mainshock, an additional aseismic slip episode initiated at depths of 60–80 km, coinciding with a burst of M0–2 aftershocks in the same region. Approximately five months later, an Mw 6.8 event occurred within the main afterslip area of the 2024 event, forming a doublet with the 2024 event and largely overlapping with the rupture zone of the December 1996 event. Our findings highlight the complex, depth-dependent interplay between subducting geometrical irregularities and local megathrust rheology, which governs the partitioning of seismic and aseismic slip to accommodate the oceanic plate subduction. This interplay also appears to allow the subduction fault to transition mechanically between fast and slow slip modes on a large scale and over an unexpectedly short timescale. The unexpected dynamic slip behavior underscores the need for a systematic, global reassessment of the effects of subducting topography on megathrust slip behavior and earthquake hazards.