Field investigation and numerical analysis on the flow slide of loess slope at Xinzhuang village, Haiyuan, China

Abstract Background The identification and supplementation of historical seismic flow-slide events, combined with systematic investigations, are critical for advancing the comprehensive understanding of flow-slide mechanisms. During the systematic investigation of geological disasters induced by the 1920 Haiyuan mega-earthquake, a seismically-induced low-angle loess slope flow slide phenomenon was newly discovered in the area of Xinzhuang Village, Jiucai Township, Haiyuan County, which is in the intensity XI degree area. Methods Detailed field investigations, geotechnical explorations, and numerical simulations were conducted to characterize the morphology, kinematic features, and depositional patterns of this flow-slide. The geological conditions and seismic loading effects were analyzed to elucidate its triggering mechanisms. Results The results indicate that liquefaction of the L2 loess layer under seismic shaking initiated the flow-slide. Heterogeneous liquefaction and uneven stress distribution led to a multi-phase kinematic pattern: tensile failure at the rear slope, shear deformation in the upper-middle section, deposition in the mid-lower section, and traction-driven movement at the front. Consequently, the displaced soil mass formed undulating terrain features, with troughs in tensile zones and crests in depositional areas. Conclusions The phenomenon of seismic flow-slide is found in the loess hilly area for the first time, which expands the case base of seismic flow-slide and provides a site for the systematic study of the mechanism of seismic flow-slide; It is confirmed that the special loess-paleosol sedimentary structure of loess is the key to cause seismic flow slide, which will cause water accumulation, inhibit the dissipation of pore water pressure during the earthquake, produce seismic liquefaction, and induce the occurrence of flow slip; The mechanism of flow slide in Xinzhuang is summarized, and the reason why the accumulation forms a unique high and low undulating landform after flow slide is explained in the manuscript.