Likelihood and unlikelihood of megaquake recurrence implied by frequency-magnitude distribution of Japanese seismicity

Abstract Constraints on the recurrence of subduction-zone earthquakes are important for seismic hazard assessment. Previous studies proposed a high-likelihood of imminent recurrence for the 2011 magnitude-9 Tohoku megaquake and the 17th-century Hokkaido megaquake of comparative magnitude off the Japanese Pacific coast, although the implication of future seismicity remains uncertain. Here, the occurrence of small earthquakes was compared to larger ones, using b-values, to show that persistently high b-values occurred in the source area of the Tohoku earthquake, indicative of no imminent recurrence. In contrast, low b-values occurred in the source area of the 17th-century earthquake, indicative of a sign seen before the Tohoku event. Around the zone of low b-value, we observed phenomena providing insight into subsequent large earthquakes, which were often reported for past large earthquakes: seismic quiescence, a seismic gap, strong plate coupling, and the absence of slow earthquake activity. Monitoring b-values, together with geophysical observations, plays a role in hazard assessment off the coast of Japan.