The 2021 and 2022 Fukushima-Oki earthquake doublet: Reactivations of the bending-related faults inside the Japan Trench subducting slab

The Pacific and Okhotsk plates are converging at a rate of about 9 cm/a, thereby forming the Japan Trench subduction zone. Two large earthquakes occurred in the offshore region of Fukushima on 13 February 2021 (Mw 7.1) and 16 March 2022 (Mw 7.3). The occurrences of the two earthquakes are close in space and time, thus constituting the Fukushima-Oki earthquake doublet. The joint inversions of the onshore and offshore strong-motion data and the teleseismic P waves reveal that both earthquakes belong to intraslab events, and the unilateral ruptures of the 2021 and 2022 events propagate southwestward and northeastward, respectively. Both events are dominated by thrust motions, and major fault slips are mainly localized away from the rupture initiating area. The geometries and locations of the two seismogenic faults suggest that the Fukushima-Oki doublet arises from the reactivations of two preexisting bending-related faults inside the subducting Pacific plate, which is driven by the downdip compressional stress regime caused by plate unbending. Furthermore, the regional tectonic settings indicate a complicated seismogenic environment for the Fukushima-Oki doublet, in which the dehydration embrittlement and seamount subduction are likely to play significant roles. Additionally, the distinctive rupture patterns of both events are also associated with the variation of fluid content and the small-scale structure of a slab slice enclosed by the two seismogenic faults within the subducting plate, which may control the heterogenous distributions of fault slips and aftershocks. The analysis of Coulomb failure stress changes suggests that the 2021 event and the 2022 foreshock collectively increase the stress loading, which may facilitate the occurrence of the 2022 mainshock, and the seismicity of the seismic belt consisting of the three intraslab sequences following the great 2011 Tohoku-Oki earthquake is also likely to remain active in the future.