When subduction changes its grip: cycle-to-cycle variability in interseismic coupling and coseismic slip

Identifying frictionally locked regions of subduction megathrusts from geodetic observations remains a challenging task in tectonic geodesy. Natural geodetic records typically capture only a fraction of seismic cycles, restricting our ability to assess temporal variations in interseismic coupling and their relationship to frictionally locked regions on subduction interfaces, commonly referred to as asperities. Clarifying this relationship is important, because interseismic coupling is widely used as an indicator of seismic potential, but coupled regions may include both mechanically locked asperities and surrounding unlocked regions. Scaled seismotectonic models provide an effective framework to investigate these processes, by simulating hundreds of seismic cycles within a short time interval under controlled laboratory conditions, with predefined asperity distributions and high-resolution deformation monitoring. Here, we explore the spatiotemporal variability of interseismic coupling, coseismic slip and their connection to predefined asperities using Foamquake, a well-established 3D seismotectonic model, which simulates megathrust seismic cycles.Through kinematic inversions of surface deformation, we derive cycle-by-cycle maps of interseismic coupling and coseismic slip and analyse their statistical behavior across models with different asperity configurations and applied normal stress. Our results show pronounced cycle-to-cycle variability in interseismic coupling, even within asperity regions, with highly coupled areas systematically extending beyond the asperity boundaries. Coseismic slip shows a positive but highly scattered correlation with preceding interseismic coupling, suggesting that while coupling is a necessary condition for large slip, it alone does not determine rupture magnitude.