The mechanism of a catastrophic loess flowslide induced by the Ms6.2 earthquake in Jishishan County, Gansu Province, China

A catastrophic loess flowslide was triggered by the Ms6.2 earthquake occurred in Jishishan County, Gansu Province, China, in 2023. However, there is still a lack of insight into the mechanism of the sudden initiation, high mobility, and long run-out of this earthquake-induced flowslide. In the presented work, based on field investigation, a series of cyclic undrained shear tests were conducted on both saturated intact and reconstituted loess specimens, considering the real shear force during earthquake and the seasonal thermal conditions. It is found that saturated loess has a low liquefaction resistance with a failure mode of cyclic mobility, and the soil dynamic shear strength is much lower than the shear stress induced by earthquake. Additional seasonal freeze–thaw cycles make the soil properties even worse, resulting in an immediate failure with zero apparent viscosity when earthquake occurs. The particle size distribution and micro-structural characteristics of both laboratory tested and in situ collected specimens were further analyzed. It reveals that the soil structure suffers significant aggregate fragmentation during cyclic shearing, and the newly created finer particles are easier to be transported to form the flowslide with long distance ran-out. It can be concluded that the high soil water content resulted from long-term and large-scale irrigation, and the low soil cyclic shear resistance resulted from weak soil structure and seasonal freeze–thaw cycles are the main reasons for this loess flowslide. The results of this study not only help to further understand the mechanism of earthquake-induced loess landslide, but also provide a useful reference for the evaluation and prevention of possible disasters in loess areas.