Initiation mechanism of an earthquake-triggered loess-mudstone interface landslide: insights from DEM numerical simulation

The discrete element method (DEM) was employed to investigate the initiation mechanism of an earthquake-triggered loess-mudstone landslide that occurred in 1920 in Gansu Province, China. The slope macroscopic damage phenomena, the microscopic crack evolution characteristics of the sliding zone, and the stress and velocity characteristics during the landslide initiation were analyzed. First, our findings show that the slope failure begins with tension damage at the upper slope, and the sliding zone is formed after the damaged crack group in the loess–mudstone interface propagates to the slope toe. Furthermore, the micro-crack revolution results reveal that the expansion rate of the sliding zone begins to decrease in the middle section of the slope. Combined with the rapid growth of the micro-crack after this section was damaged, the middle section of the slope is identified as the locked segment. During the failure of the locked segment, the stress of this section exhibits a sharp drop, suggesting a brittle damage form of the locked segment. In addition, velocity measurement results indicate that the sliding mass has an initial speed of about 4.75 m/s, which is rare in the low-angle loess-mudstone interface landslides. We conclude that the strain energy is converted into kinetic energy during the locked segment failure process, producing a start-up acceleration effect. Combined with seismic peak ground acceleration (PGA) analysis results, we proposed that the sequential effects of the seismic amplification, failure of the locked segment, and start-up acceleration caused the unique failure mode of the Sunjiagou landslide.