Evolution of Coupled Seismic‐Aseismic Slip During the 2025 Mw 6.8 Sanriku‐Oki, Japan, Megathrust Sequence

Abstract On 9 November 2025, a Mw 6.8 Sanriku‐Oki earthquake struck a Japan Trench segment with inferred recurrent aseismic slip and Mw ∼7 earthquakes. Using S‐net seafloor data, we examine the preparatory and post‐mainshock evolution of this foreshock–mainshock–aftershock sequence. Seismicity began ∼10 days before the mainshock and remained clustered around the eventual rupture area. During the final ∼2 days, the foreshock region expanded by ∼20 km, reminiscent of nucleation behavior near the creep–lock transition; M ≥ 5 earthquakes were unusually frequent. The mainshock and the second‐largest event (Mw 6.4) initiated near opposite ends of the foreshock area and ruptured in opposite directions. After the mainshock, aftershocks rapidly expanded to ∼75 km within ∼0.5 days, with reactivation of repeating earthquakes, suggesting long‐range afterslip. This sequence likely reflects the evolving interplay between seismic rupture and diverse aseismic‐slip processes within a heterogeneous, coupled slip system before and after rupture of the locked region.