The 2021 Mw7.0 and Mw6.7 Miyagi‐Oki earthquakes nucleated in a deep seismic/aseismic transition zone Possible effects of transient instability due to the 2011 Tohoku earthquake

In March and May 2021, Mw7.0 and Mw6.7, respectively, deep interplate earthquakes occurred off Miyagi, Japan, near the downdip edge of the rupture area of the 2011 M9 Tohoku earthquake. We study the two Mw similar to 7 earthquakes to investigate how a great (Mw similar to 9) earthquake cycle impacts the generation of smaller (Mw7-8) earthquakes in the deep seismic/aseismic transition zone. Our hypocenter relocation results show that many repeating earthquakes have emerged in the downdip area after the Tohoku earthquake. The March Mw7.0 event was initiated in the slip area of an emerging interplate repeating earthquake sequence, suggesting that the nucleation of the Mw7.0 event occurred in a conditionally stable area where only aseismic slip occurred before the Tohoku earthquake. Downdip high-rate creep in the postseismic period is a probable reason for the temporal aseismic-to-seismic transition that allowed the occurrence of repeating earthquakes and the rupture initiation of the Mw7.0 earthquake. The May Mw6.7 earthquake originated in an area where earthquake activity was high locally, both before and after the Tohoku earthquake, and shear stress increased after the March event. This suggests that both frictional properties and shear stress change contributed to the rupture initiation. The slip inversion results indicate that these two Mw similar to 7 earthquakes ruptured the western part of the 1978 Mw7.6 Miyagi-Oki earthquake source area and loaded the eastern part. Continued monitoring of seismicity patterns, including possible updip expansion of the interplate seismicity, is essential to examine how strain energy, plate-locking state, and creep evolve during the Mw9.0 earthquake cycle.