Rupture Kinematics of the 2021 Miyagi-Oki Earthquakes: Repeated Seismic Events in High-Velocity Bodies of the Japan Trench Subduction Zone

The 2021 Miyagi-Oki earthquakes (M_w 7.1 and M_w 6.8) occurred in rapid succession within the Japan Trench subduction zone, one of the most seismically active regions globally. To investigate the rupture processes and interactions between these events, we performed the joint inversions of strong-motion and teleseismic waveforms. The inversion results for the M_w 7.1 Miyagi-Oki earthquake reveal a bilateral rupture along a megathrust fault. A large asperity in the southern section of the seismogenic fault extends from deep portions to the top edge, with a maximum slip of 1.4 m. In contrast, the M_w 6.8 Miyagi-Oki earthquake exhibits a unilateral rupture with a peak slip of about 1.1 m. Aftershocks following both earthquakes generally occurred at shallower depths than the initial rupture, likely influenced by focal depth, fault geometry, and fluid migration. The recurrence of earthquakes in the Miyagi-Oki region reflects the periodic release of accumulated strain on high-velocity bodies due to interseismic locking and stress loading from nearby large earthquakes. Coulomb stress analysis suggests that the 2021 M_w 7.1 Fukushima-Oki earthquake may have triggered the subsequent 2021 M_w 7.1 Miyagi-Oki earthquake, with both events contributing to the M_w 6.8 Miyagi-Oki earthquake. These findings enhance our understanding of earthquake triggering mechanisms and provide key insights into future seismic hazards in the Japan Trench subduction zone.