Earthquake occurrence in the stress shadow provides a unique opportunity for extracting the information about the physical processes behind earthquakes because it highlights processes other than the ambient stress change in earthquake generation. In this study, we examined the fault structure and the spatiotemporal distribution of the aftershocks of the 2019 M6.7 Yamagata-Oki earthquake, which occurred in the stress shadow of the 2011 M9.0 Tohoku-Oki earthquake, to better understand the earthquake generation mechanism. Moreover, we investigated the temporal evolution of the surface strain rate distribution in the source region by using GNSS data. The earthquake detection and hypocenter relocation succeeded in delineating three planar structures of earthquakes. The results suggest that individual aftershocks were caused by a slip on the macroscopic planar structures. Aftershock hypocenters rapidly migrated upward from the deeper part of the major plane (fault) similar to the recent earthquake swarm sequences triggered by the 2011 Tohoku-Oki earthquake in the stress shadow in the upper plate. East–west contraction strain rate in the source region of the Yamagata-Oki earthquake with E–W compressional reverse fault mechanism changed to the E–W extension as a result of Tohoku-Oki earthquake, and it continued until the occurrence of the Yamagata-Oki earthquake. The upward hypocenter migrations, together with the earthquake occurrence in the stress shadow and in the E–W extension strain rate field, suggest that the reduction in the fault strength due to the uprising fluids contributed to the occurrence of this earthquake sequence. Localized aseismic deformations, such as aseismic creeps, beneath the fault zone may also have contributed to the earthquake occurrence. The results support the hypothesis that aseismic processes in the deeper part of the fault play crucial roles in the occurrence of shallow intraplate earthquakes.

The development of seafloor seismic observations facilitates reliable estimation of the rupture directivities of offshore earthquakes. We used seismic waveforms obtained by a new seafloor seismic network (S-net) and onland stations to systematically examine the rupture directivities of interplate earthquakes along the Japan trench. We estimated the rupture directions of 206 (M w 3.5-5) events, most of which occurred near the base of the seismogenic zone. We found that most earthquake ruptures (>~80 %) were directional, primarily propagating in the updip direction. This tendency cannot be explained by the effect of the bimaterial interface. The prevalence of updip rupture in the data suggests that deep, steady creep and upward fluid migration along the plate interface affected earthquake ruptures in the subduction zone. The updip ruptures redistributed the accumulated shear stress from the base of the seismogenic zone to the shallow large seismic patches. Furthermore, the updip ruptures may open up ways for deeper fluids to migrate further upward along the plate interface. Both the stress redistribution and the upward fluid migration facilitate the occurrence of shallow megathrust earthquakes.

The 2011 Mw 9.0 Tohoku-oki earthquake is one of the world’s best-recorded ruptures. In the aftermath of this devastating event, it is important to learn from the complete record. We describe the state of knowledge of the megathrust earthquake generation process before the earthquake, and what has been learned in the decade since the historic event. Prior to 2011, there were a number of studies suggesting the potential of a great megathrust earthquake in NE Japan from geodesy, geology, seismology, geomorphology, and paleoseismology, but results from each field were not enough to enable a consensus assessment of the hazard. A transient unfastening of interplate coupling and foreshock activity were recognized before the earthquake, but did not lead to alerts. Since the mainshock, follow-up studies have (1) documented that the rupture occurred in an area with a large interplate slip deficit, (2) established large near-trench coseismic slip, (3) examined structural anomalies and fault-zone materials correlated with the coseismic slip, (4) clarified the historical and paleoseismic recurrence of M~9 earthquakes, and (5) identified various kinds of possible precursors. The studies have also illuminated the heterogeneous distribution of coseismic rupture, aftershocks, slow earthquakes and aseismic afterslip, and the enduring viscoelastic response, which together make up the complex megathrust earthquake cycle. Given these scientific advances, the enhanced seismic hazard of an impending great earthquake can now be more accurately established, although we do not believe such an event could be predicted with confidence.

Following the 2011 M9 Tohoku-Oki earthquake, the interplate seismicity drastically increased in the downdip extension; however, it disappeared within the rupture area. An Mw7.0 earthquake occurred in the downdip extension off Miyagi in March 2021, followed by an Mw6.7 earthquake in May 2021. To examine the initial evolution of the next M9 earthquake cycle, we examined the regional seismicity and source processes of the two M~7 earthquakes. We found that the March Mw7.0 earthquake was nucleated at a conditionally stable patch where repeating earthquakes emerged after the Tohoku-Oki earthquake. The earthquake initiation from a conditionally stable patch at the deep plate boundary is probably a transient feature in the postseismic period of the previous M9 earthquake. The stress enhancement caused by the Mw7.0 event facilitated the subsequent May Mw6.7 earthquake. These two M~7 earthquakes ruptured the western seismic patches of the 1978 Mw7.5 Miyagi-Oki earthquake, which is the most recent typical earthquake in an ~40-year interval of M~7.5 earthquake sequence, and loaded the eastern shallow seismic patches for the sequence. Interplate seismicity in the updip area disappeared after the 2011 Tohoku-Oki earthquake. Assuming that the spatial pattern of interplate earthquakes will be restored to a situation similar to that before the Tohoku-Oki earthquake, the seismically active area should gradually expand to the updip area. Continued monitoring of interplate seismicity is essential to examine how plate-locking evolves during the M9 earthquake cycle.