Persistent asperities at the Kermadec subduction zone controlled by changes in forearc structure: 1976 and 2021 doublet earthquakes

To what degree large earthquakes are ‘characteristic’ is a critical question in understanding the fundamentals of earthquake physics and the seismic cycle. Here we study the ruptures of the 2021 Mw7.4 and Mw8.1 doublet earthquake sequence in the Kermadec subduction zone and compare them with the 1976 doublet that occurred at the same location. We find that although the 2021 mainshock likely re-ruptured the same asperity as the 1976 Mw7.9 event, the detailed slip distribution is different. Other ruptures in the doublets also differ in character and location. Our observations indicate the variability between large earthquakes on the same segment of the plate boundary in each earthquake cycle. All earthquakes occur in an isolated area of the megathrust, which is bounded by changes in the lithospheric structure of the overriding plate as indicated by bathymetric and gravity data. This high-seismicity region is coincident with an isolated forearc sedimentary basin, possibly formed by basal erosion related to seismogenesis, suggesting that seismic slip has persisted here for several million years. Refined up-dip aftershock and background seismicity focal mechanisms have a steeper dip angle than the slab interface, suggesting these events are located within the subducting oceanic slab, possibly forming a rougher plate interface that facilitates basal erosion. We conclude that the stress heterogeneity within this bounded seismogenic zone is long-lived and has produced a rich spectrum of earthquake ruptures.