Study on Development Stage and Evolution of Landslide in Wide Fracture Zone of Anninghe Active Fault

Anninghe fault zone is one of the famous regional active faults in Southwest China. The occurrence of the east branch fault in the Mianning-Xichang section is 265°–290° ∠60°–80°, and the width of fracture zone ranges from 400 to 700 m. The upper Triassic Baiguowan formation (T3bg) strata are mainly exposed along the Anninghe fault zone, and the lithology is mainly sandstone, carbonaceous mudstone and shale. Field research shows that the fault fracture zone is generally composed of gray white and gray black fault gouge which are 50-100 m thick in the center, and broken rock masses which are 200–300 m wide on both sides. The Anning River fault zone has a steep angle in this section, and its strike is consistent with the direction of the Anning River. Therefore, the wide fault fracture zone is linearly exposed to the surface in the middle and lower parts of the left bank slope of the Anning River. Under the downwash of surface water, gullies are easily formed in the fault fracture zone. With the continuous downward cutting and headward erosion of the gullies, when the trench is cut to a certain depth, the fracture zone rock masses on the slopes on both sides of the bank begin to form small landslides. The deeper the trench is cut, the larger the scale of the landslides, and larger-scale landslide-debris flow chain disasters are gradually formed. From then on, the landslides expand backwards year by year, and there are multiple sliding-debris flow disasters every year in the rainy season. When the crown of the landslide expands to the boundary of the fault fracture zone, the complete and hard bedrock which is not affected by the fault is exposed, the crown of the landslide tends to be stable. The slopes on both sides gradually slow down as the landslides expand year by year, and also tend to be stable. The landslides in the fault fracture zone gradually stopped developing too. Based on the investigation and study of the landslide in the wide fracture zone of the east branch of the Anninghe fault, this paper expounds the formation mechanism of the landslide in the wide fracture zone, systematically summarizes the development-evolution process of the landslides in the broad fault fracture zone. This paper divides it into 4 evolutionary stages: “formation period”, “development period”, “climax period” and “rest period”. At the same time, according to the characteristics of landslides at different stages of evolution, corresponding prevention methods and suggestions are put forward, which provides a new method for the investigation and research of similar landslides from another perspective.