Coupled Pore Water Pressure Generation and Shear Thinning Rheology Control the Hypermobility of Co-Seismic Loess Landslides

Seismic liquefaction in loess deposits can trigger catastrophic long-runout landslides, yet existing studies typically treat liquefaction triggering and post-failure flow mobility as separate processes, leaving the physical linkage between initiation and motion poorly constrained. This study integrates undrained ring shear test and rheological test to investigate how the coupled evolution of pore water pressure, shear rate, and rheology governs both the onset of seismic liquefaction and the subsequent hypermobile flow behavior of loess. Test results show that the liquefaction of the loess sample under low shear stress occurs only as initial pore water pressure generation to a certain threshold. Dynamic perturbation of low-frequency and high-amplitude cyclic loading promotes the onset of liquefaction failure. Following liquefaction, the loess exhibits pronounced shear thinning behavior, characterized by a rapid viscosity reduction with increasing shear rate. This rate-dependent weakening establishes a positive feedback between acceleration and viscosity loss, enabling sustained high mobility during fast shearing. By explicitly linking seismic liquefaction triggering with post-failure flow rheology, this study identifies critical pore water pressure, shear rate, and viscosity thresholds as key precursors, providing a framework for improving hazard prediction and risk mitigation of co-seismic loess landslides worldwide.