Toward an Updated Earthquake-Cycle Framework at Subduction Zones: Evidence, Processes, and Implications

A viscoelastic deformation cycle at subduction zones has been revealed following the surge of great megathrust earthquakes in the early 21^st century. This cycle is broadly divided into inter-, co-, and post-seismic phases, constrained by deformation data collected before, during, and after these earthquakes. However, the framework for understanding the longer term earthquake-cycle process remains unclear, particularly from the early postseismic to the late interseismic phases, primarily due to the lack of observations covering these century-long periods.

Building on previous work, we have demonstrated that landward viscoelastic relaxation driven by megathrust locking is necessary to produce the long-wavelength late interseismic deformation patterns commonly observed at global subduction zones. Using the unique century-long leveling data combined with contemporary GNSS observations in southwest Japan, we further propose that a short-wavelength deformation emerges during the early interseismic phase, eventually evolving into a long-wavelength pattern.

Incorporating early postseismic offshore observations, we synthesize an updated earthquake-cycle framework featuring four detailed phases following a megathrust earthquake. This refined framework supports a general model capable of reproducing deformation patterns across all phases. The model underscores two fundamental processes common to different subduction zones and phases of the earthquake cycle: cyclical stick-slip behavior along the megathrust and associated landward-seaward viscous mantle flow.

As a further advancement, this model simulates continuous horizontal and vertical deformation in space and time, revealing three critical spatiotemporal data gaps at global subduction zones. By predicting deformation patterns at various subduction zones, including those vulnerable to global sea-level rise, the model provides valuable guidance for future instrumentation planning to fill the data gaps and offers insights into potential breakthroughs in addressing key challenges in earthquake-cycle research.