Periodic slow slip and megathrust zone earthquakes in northeastern Japan

Both aseismic and seismic slip accommodate relative motion across partially coupled plate boundary faults. In the northeastern Japan subduction zone, aseismic slip occurs in the form of decelerating afterslip following large earthquakes and as relatively steady slip on uncoupled areas of the subduction thrust between great ruptures. Here we report on a new quasi-periodic slow-slip behavior that is widespread in the megathrust zone inferred from offshore small repeating earthquakes and on-land GPS data. The two independent data sets show that much of the subduction zone experiences periodic slip accelerations at periods ranging from 1 to 6 years, which often coincide with or precede clusters of large (M ≥ 5) earthquakes. The examination of the spatio-temporal distribution of small repeating earthquakes with respect to the M ≥ 5 earthquakes suggests that the slow-slip pulses trigger the M ≥ 5 seismicity. We also show that the clustering of M ≥ 5 earthquakes with a ~3-year period in the off-Sanriku area has persisted since 1930 (81 years). These results suggest that inherently periodic slow-slip events along the megathrust 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.