Subduction Earthquake Cycle through the lens of analogue modelling: the role of the upper plate rheology

Subduction megathrusts are among the largest fault systems on Earth and are responsible for generating megaearthquakes-the most powerful earthquakes and one of the most destructive natural phenomena. However, obtaining natural data on the Subduction Earthquake Cycle (SEC) in these areas is challenging due to the long recurrence intervals of such events. To overcome this limitation, we used analogue models to reproduce in the laboratory hundreds of seismic cycles under different conditions in just a few minutes. The models feature a single velocity weakening asperity (i.e., rice) surrounded by a velocity-neutral material (i.e., sand). Using a parametric approach, we systematically varied two key parameters of our single asperity model: (1) the rheology of the upper plate, which affects its stiffness and (2) the normal load (σ_n) applied on the asperity. We performed four distinct models, each with a different upper plate stiffness. For each upper plate stiffness we implemented four σ_n (i.e., 16 models in total). High-resolution monitoring of our models, combined with Particle Image Velocimetry, allowed for a detailed analysis of the analog earthquakes. The variation in upper plate rheology enabled the models to simulate the transition from stick-slip behavior to stable sliding, governed by the ratio k/kc, the stability parameter within the rate-and-state framework. Moreover, the models demonstrate that this variation is a controlling factor of magnitude and recurrence time of the analogue events. Comparing the results with natural data, we found that all the models exhibit moment magnitudes (Mw) comparable to those of natural megaearthquakes. The possibility of crossing the k/kc=1 threshold allows us to explore the stick-slip behavior in a regime that includes period doubling linked to the coexistence of faster and slower slip rates. The findings in our experimental models demonstrate the influence of the upper plate rheology in the spectrum of megathrust slip behaviors, providing constraints that could potentially be applied to natural subduction zones.