Monitoring stress recovery and accumulation associated with megaquakes helps to assess their recurrence. Previous studies proposed a high likelihood of imminent recurrence for the 2011 Tohoku and the 17th-century Hokkaido megaquakes belonging to the magnitude-9 class, although their current stress state remains uncertain. Here we compare the occurrence of small earthquakes relative to larger ones, using b-values, showing high b-values in the source area of the Tohoku earthquake, indicating low stresses. In contrast, low b-values occurred in the source area of the 17th-century earthquake, indicating high stresses as seen before the Tohoku event. Around the low-b-value zone, phenomena that provide insight into subsequent large earthquakes are observed, and which were reported for worldwide earthquakes: seismic quiescence, a seismic gap, strong plate coupling, and slow earthquake activity avoiding megaquakes’ rupture zones. Results imply that the Hokkaido and Tohoku megaquakes occur nonrandomly in time, rather their recurrence intervals are more characteristic. Megaquakes off Hokkaido and the region of the 2011 Tohoku earthquake occur at characteristic intervals between periods of gradual stress recovery in their source zones, according to an analysis of b-values and seismic data from 2000 to 2025.
Nanjo et al. (Thu,) studied this question.