Search

Both aseismic and seismic slip accommodate relative motion across partially coupled plate-boundary faults. In northeastern Japan, aseismic slip occurs in the form of decelerating afterslip after large interplate earthquakes and as relatively steady slip on uncoupled areas of the subduction thrust. Here we report on a previously unrecognized quasi-periodic slow-slip behavior that is widespread in the megathrust zone. The repeat intervals of the slow slip range from 1 to 6 years and often coincide with or precede clusters of large [magnitude (M) ≥ 5] earthquakes, including the 2011 M 9 Tohoku-oki earthquake. These results suggest that inherently periodic slow-slip events result in periodic stress perturbations and modulate the occurrence time of larger earthquakes. The periodicity in the slow-slip rate has the potential to help refine time-dependent earthquake forecasts.

An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms. Plain Language Summary: Growing evidence suggests that fluid movements at depth can cause earthquake swarms in the overriding plates of subduction zones. However, there is less evidence of fluid involvement in the occurrence of large (M > 6) earthquakes, and it has been reported that the generation environments of large earthquakes and swarm earthquakes tend to be different. During intense swarm activity in the northeastern Noto Peninsula, Japan, starting at the end of 2020, an M6.2 earthquake occurred on 5 May 2023, followed by many aftershocks. To clarify the occurrence mechanism of the M6.2 mainshock, we relocated >20,000 small earthquakes and estimated the rupture process. The results showed a systematic upward migration of earthquakes in the preceding swarm period on the deep side of the mainshock fault (depth from 20 to 12 km). The mainshock rupture initiated near the shallow end of the swarm activity and propagated updip (depth from 12 to 9 km). Many aftershocks followed in shallower part of the same fault (depth from 10 to 5 km). Upward fluid migration likely caused the observed systematic upward earthquake migration throughout the sequence. The results indicate that earthquakes as large as M > 6 can be triggered by fluid migration during enhanced seismicity in swarms. Key Points: An Mw 6.2 event initiated near the shallow end of a fault where the preceding earthquake swarm showed systematic upward migrationThe Mw 6.2 rupture propagated farther updip, followed by many shallow aftershocks on the faultUpward fluid migration can trigger earthquakes as large as Mw > 6 [ABSTRACT FROM AUTHOR]

The 2011 Tohoku-Oki great earthquake increased the difficulty of evaluating the long-term probability of seismic activity along the Japan Trench because of the unknown impact of the unprecedentedly large slip. In this study, the long-term activity of "Miyagi-ken-Oki earthquakes", an M > 7 earthquake sequence off Miyagi Prefecture, located at the edge of the source area of the Tohoku-Oki earthquake was simulated. We conducted numerical simulations of earthquake generation cycles based on the rate- and state-dependent friction law representing the stress accumulation and release process on the plate interface. We also validated the results based on analyses of observational data, including time intervals and afterslip distributions for the previous Miyagi-ken-Oki earthquakes. The simulation results were then compared with repeating-earthquake-derived interplate slip observations over 30 years. The results showed that the spatial and temporal characteristics of aseismic slips on the plate interface near the M > 7 Miyagi-ken-Oki source were qualitatively reproduced. The time interval between the M ~ 9 earthquake and the first M > 7 earthquake is shorter than the average recurrence interval of M > 7 earthquakes during the latter stage of each M ~ 9 earthquake cycle. In contrast, the interval between the first and the second M > 7 earthquakes is the longest in each M ~ 9 earthquake cycle. The time intervals of the M > 7 earthquakes fluctuated in the early stage compared to those in the latter stage of the M ~ 9 earthquake cycle. These characteristics were associated with the extent of the locked and afterslip areas in and around the source. Hence, monitoring the spatio-temporal distribution of the slip rate in and around the source area during the preparation process of earthquakes occurring in the downdip area off Miyagi Prefecture is very important to assess whether the next M > 7 earthquake is approaching. Furthermore, earthquake cycle simulations combined with earthquake and slow slip monitoring can provide meaningful insights for long-term assessments of great interplate earthquakes. [ABSTRACT FROM AUTHOR]