Meta-Stable States in Granular Shear Zones Under Cyclic Loading: Implications for Earthquake-Triggered Landslides

Understanding the dynamic response of granular shear zones under cyclic loading is critical for elucidating the mechanisms of earthquake-induced landslides. Existing prediction methods struggle to capture the complexities of landslide dynamics, particularly the transition from slow creep to rapid runout during seismic events. Here, we present results from ring shear experiments that simulate shear zone behavior under dynamic loading. Our study reveals that granular shear zones exhibit post-seismic creep, which increases gradually with each loading cycle. Crucially, we have observed a meta-stable state characterized by a significant increase in post-seismic creep, which precedes shear zone failure. This meta-stable state occurs when the weakened shear resistance approaches the applied shear stress, indicating a phase transition from a solid to a fluid-like state and may serve as a critical precursor for instability. These findings offer a compelling explanation for widespread post-seismic landslide movement and the dynamic triggering of landslides. By incorporating the identified co-seismic weakening and post-seismic healing mechanisms into existing methodologies, such as modified Newmark models, we can potentially greatly improve the accuracy of landslide displacement predictions and advance our understanding of dynamic triggering. This work provides a framework for better assessing seismic landslide hazards.