Ancient landslides with platform geomorphology occasionally reactivate, posing serious geohazards. On September 9, 2021, persistent heavy rainfall triggered the reactivation of the Dahekou ancient landslide within a gently sloping geomorph0logy at the core of Zhangjiantan syncline in China's western Qinling-Daba Mountains. This event caused one death, damaged 80 houses, and blocked the Yushui River. This study reconstructs the sliding process of the Dahekou landslide and deciphers the complex landslide initiation mechanisms through field surveys, unmanned aerial vehicle (UAV) imagery analysis, drilling, electrical resistivity tomography (ERT) and small baseline subset-interferometric synthetic aperture radar (SBAS-InSAR) monitoring. We divide the sliding process of the Dahekou landslide into three stages. Two new landslides (#1 and #2) occurred at 18:30 on September 9, 2021. Subsequently, the ancient landslide (#3) slid in the 230 degrees direction at approximately 20:30 on September 9, 2021, then changed the direction to 170 degrees-240 degrees at 22:30 on the same day, and moved in the direction of 300 degrees at 10:00 the next day. Finally, the reactivated ancient landslide (#3) formed two partially sliding masses, with volumes of approximately 158x10(4) m(3) and 160x10(4) m(3), along the directions of 170 degrees-240 degrees and 300 degrees, respectively, damaging 80 houses and blocking the Yushui River. Field surveys suggest that new landslides #1 and #2 are rock landslides and soil landslides, respectively, with volumes of approximately 230x10(4) m(3) and 7.49x10(4) m(3). Compared with the InSAR data, the new landslide #1 thrust the ancient landslide #3, with an uplift velocity rate of 22.68 mm/a at the rear edge, from September 2020-September 2021. An analysis of drill hole data reveals that the bedding in the landslide area has complex geological conditions, comprising mudstone prone to slipping with different degrees of weathering. Notably, the core of the Zhangjiatan syncline sits on the sliding bedding of the ancient landslide, contributing to a change in the sliding direction. This comprehensive study reveals that the landslide #1 loading and thrusting, the persistent and heavy rainfall, and the complex geological conditions influenced the reactivated ancient landslide. Considering the intricacies of landslide failure mechanisms, we advocate for giving more attention in the future to the zone of potentially slip-prone strata located at the edge of ancient landslides.

Due to the significant decrease in strength of loess after encountering water, loess landslides induced by rainfall are very catastrophic and widely distributed in the Chinese Loess Plateau. On September 17, 2011, a catastrophic loess landslide induced by rainfall occurred in Baqiao district, Xi'an, Shaanxi Province, China, resulting in 32 casualties and bringing great fear to the local residents. This landslide event was characterized by three individual landslides. Field investigations, geological exploration and model experiments were conducted to reveal its initiation and movement mechanisms. The results show that 1) Multiple groups of fissures in the ring-cut adits were found at a location 3 m inward from the slope surface. The minimum opening width of these fissures is 0.5 cm, and the maximum is 4 cm. The fissures develop nearly vertically and have good extensibility and connectivity. 2) the whole process of rainfall-induced landslides can be divided into 3 stages: rainfall infiltration and weight increase; crack expansion and slope deformation; slope collapse and creep deformation. 3) The volumetric water content, pore water pressure and vertical stress variation of the soil in our model all increase first and then decrease. Specifically, these three parameters increase slowly during the pretest and stabilization periods and increase fast shortly before the landslide occurrence. The volumetric water content of the soil on the side containing joints increases faster, verifying that the joints act as preferential channels that accelerate rainwater infiltration. The results of the study provide an important scientific foundation for future research on rainfall-induced loess landslides and their deep-seated mechanisms, and fill the gaps in research related to large-scale physical modeling experiments.

The Jiaju landslide is a large soil-rock palaeolandslide in the Danba of the Upper Dadu River in southwestern Sichuan Province, China. In this work, geological investigations, long-term comprehensive monitoring, experiments, and numerical simulations were used to determine the formation mechanism and evolution of this landslide. A complex armchair-shaped terrain with a substantial height difference between the rear and the foot of the slope, multiple structural defects within the landslide-accumulation region, and a coupling effect among multiple factors controlling river erosion and human activity were identified. The dynamic landslide deformation process was recorded by the integration of global positioning system (GPS) and inclinometer data, and the kinematic behaviour of the landslide was simulated using a Fast Lagrangian Analysis of Continua in 3 Dimensions (Flac3D) model in three stages. (1) During the last rapid uplift of the Tibetan Plateau and strong river erosion, the Jiaju bank slope was deformed and failed, forming a massive, thick palaeolandslide with a volume of approximately 2.76 x 107 m3. (2) The structure of the landslide materials deteriorated, and the weak structural plane was softened by groundwater and surface loading. The landslide was reactivated, with deformation and failure occurring through sliding and fracturing. (3) The landslide exhibited creep deformation with multistage and multilayer sliding surfaces via front-slope toe excavation and river-level fluctuations.