Formation and evolution mechanism of a catastrophic mudflow in a complex disaster-prone environment in a strong earthquake-disturbance region

Earthquake-triggered mudflows are typical in scale and hazard, and their formation mechanism is extremely complex. In this study, the liquefaction and mobility mechanisms of a catastrophic mudflow, namely, the Yongguangcun (YGC) mudflow, in Minxian, Gansu Province, China, under the coupled action of historical earthquakes, active faults, groundwater, long-term rainfall before an earthquake, and the 2013 Mw6.6 Minxian–Zhangxian earthquake were systematically analyzed. Through a detailed field investigation and laboratory testing, the stratigraphic structure of the YGC mudflow was revealed, a geomechanical model was established, and the complex chain process leading to the formation of the YGC mudflow was elucidated. This process includes sliding along the contact zone between the loess and strongly weathered mudstone, liquefaction of the saturated loess under the groundwater table, and liquefaction and collapse of the unsaturated loess above the groundwater table. The slightly low terrain provides the topographic conditions required for groundwater convergence, and sets the conditions for the deformation and further liquefaction of saturated loess during earthquakes. The undulating terrain in the meizoseismal area enhances the complexity of the earthquake waves. In summary, the YGC mudflow was caused by long-term geological evolution and the synergistic effects of other factors; and the site conditions, such as the local topography and groundwater, are the fundamental reasons for the failure and mobility differences between the YGC mudflow and the eastern landslide. The results of the investigation of this mudflow would enrich our understanding of mudflows, promote research on the formation mechanism of geological disasters under complex conditions on the Loess Plateau, and provide important information for improving the scientific prevention and control of landslides of the same type.