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Subduction off Miyagi and Fukushima prefectures, northern Honshu, has been recognized as having a triple-planed structure of seismicity at the deep thrust zone in the 40-60km depth range. This triple seismic zone is composed of thrust-type earthquakes, down-dip compressional and down-dip tensional earthquakes from top to bottom. At the time of the 2011 Tohoku-oki earthquake, peculiar phenomena such as radiation of short-period seismic waves and pre- and after- slow slips within the asperities of M7 class earthquakes were observed over this thrust zone west of the 2011 main rupture zone. Further to the south, where the Philippine Sea plate is subducting beneath Kanto, a triple seismic zone has also been recognized particularly under southwest Ibaraki prefecture. The thrust-type earthquakes at the top of the triple seismic zone off northern Honshu and beneath Kanto have been believed to be interplate events representing the relative motion between the overriding and subducting plates. I conclude that the thrust-type earthquakes beneath southwest Ibaraki prefecture are in fact within the crust of the subducting Philippine Sea slab, not at the surface because their slip vectors are different from the relative motion between the subducting and overriding plates. Therefore, there would be an aseismic plate boundary above the seismicity. I also show that the dip angles of the westerly dipping fault planes of the thrust-type earthquakes off Miyagi prefecture are smaller by 6°in average than the dip of the slab surface in this region, except for the six years prior to the Tohoku-oki earthquake, i.e. prior to 2006. Furthermore, the slip vectors coincide with the relative motion between the overriding and subducting plates only during this period. I infer that the topmost earthquakes of the triple seismic zone off Miyagi prefecture prior to 2006 are thus likely to be within the crust of the subducting plate. The slow slips before and after the Tohoku-oki earthquake would have occurred not within the asperities but along the aseismic plate interface, and the short-period seismic waves would have been radiated due to fractures within the crust associated with the overshooting rupture at the time of the Tohoku-oki earthquake. Many of the so-called repeating earthquakes at the topmost surface of the subducting plate would be in fact intra-crustal events within the slab. M9 earthquakes would interact with the triple seismic zone, not only mechanically, but also through fluid migration, because earthquakes in the triple seismic zone involve dehydration reaction. The irregularity of the occurrence of M9 earthquakes might be due to the inhomogeneous distribution of hydrated minerals in the incoming plate. The subduction zones having M9 earthquakes or under Kanto have a collisional character. I propose to term subduction having both a collisional character and a triple seismic zone as "super-subduction". The relative motion between the plates is accommodated by the deformations of the crust of the subducting slab as a "plate boundary zone". The viewpoint of "super-subduction" is necessary to understand earthquakes in subduction zones with a collisional character and dehydration reactions in the slab. [ABSTRACT FROM AUTHOR]

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 (M w 7.1) and 16 March 2022 (M w 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. • Source rupture processes of two Fukushima-Oki earthquakes in northeastern Japan. • Joint inversions of onshore and offshore strong-motion and teleseismic waveforms. • Two unilateral ruptures separately propagating southwestward and northeastward. • Both intraslab events caused by reactivations of preexisting outer-trench faults. • Seismogenesis attributed to heterogenous fluid content and regional tectonics. [ABSTRACT FROM AUTHOR]