BackgroundThe 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.MethodsDetailed 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.ResultsThe 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.ConclusionsThe 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.

On December 18, 2023, the M S 6.2 Jishishan earthquake triggered a large-scale liquefaction disaster of loess sites in Jintian and Caotan villages, Zhongchuan town, Minhe County, Haidong City, Qinghai Province. To clarify the micro-mechanism of the liquefaction disaster, the Q3 Malan loess layer of the disaster site and its overlying red silty clay layer samples were selected and quantitatively analyzed for the differences in physical properties, structure, microstructural parameters, and mineral compositions. Based on the discrepancy results, the micro-mechanisms between loess microstructure and macro-mechanical properties of soil and liquefaction disaster were investigated. The research shows that compared with red silty clay, the dynamic index of loess corresponding to the physical indices of Zhongchuan loess obviously exceeds the critical threshold of liquefaction under actual seismic intensity. Additionally, its pore structure is dominated by point contact and weakly cemented overhead macropore structure, and its quantitative pore microstructure parameters and mineral composition show significant liquefaction potential. The comprehensive analysis of the liquefaction mechanism shows that the rapid deformation of the soil skeleton and the destruction of the cementation and contact system of the water-sensitive minerals under seismic loading and hydraulic force lead to the collapse of the overhead macropores, the damage of structural strength, the increase of the complex pore channels, the rapid accumulation of pore water pressure, and the reduction of the effective stress, which leads to the liquefaction of the loess.

Water content is one important factor on which velocities, heights, and runout of debris flows and similar phenomena depend. To this purpose, we need two ingredients (i) a mathematical model describing the incorporation of water into the moving soil, which results in a change of water content, and (ii) a rheological model with properties depending on the water content. Modeling of such problems can be done either by using either a: (i) two phases approach, where velocities of solid and water may be different, using two sets of nodes, or (ii) two phases approach where velocities of both are assumed to be the same, using a single set of nodes. In both cases, the models have to implement a mechanism for the water inflow-or outflow-. We will modify both types of two phases models (one or two sets of nodes for solid and water) to include the change of water content due to water inflow. Implementation in the SPH requires extending the algorithm and updating the smoothing length because it is based on the mass of particles and their relative position. Updating the smoothing length when only changes mass is to be avoided. Regarding the rheological model, we will introduce a new model for frictional debris flows implementing a Voellmy coefficient which depends on water content. Alternatively, we will propose a more consistent model based on Bagnold's idea of introducing a 1D concentration parameter (lambda) . Finally, we will illustrate the proposed model capability with two examples, a dam break problem, and a real case in El Salvador where the water content played an important role in the propagation properties of a debris flow.