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In recent years, numerous ancient landslides initially triggered by historic earthquakes on the eastern Tibetan Plateau have been reactivated by fault activity and heavy rainfall, causing severe human and economic losses. Previous studies have indicated that short-term heavy rainfall plays a crucial role in the reactivation of ancient landslides. However, the deformation behavior and reactivation mechanisms of seasonal rainfall-induced ancient landslides remain poorly understood. In this paper, taking the Dandu ancient landslide as an example, field investigations, ring shear experiments, and interferometric synthetic aperture radar (InSAR) deformation monitoring were performed. The cracks in the landslide, formed by fault creeping and seismic activity, provide pathways for rainwater infiltration, ultimately reducing the shear resistance of the slip zone and causing reactivation and deformation of the Dandu landslide. The deformation behavior of landslides is very responsive to seasonal rainfall, with sliding movements beginning to accelerate sharply during the rainy season and decelerating during the dry season. However, this response generally lags by several weeks, indicating that rainfall takes time to infiltrate into the slip zone. These research results could help us better understand the reactivation mechanism of ancient landslides triggered by seasonal rainfall. Furthermore, these findings explain why many slope failures take place in the dry season, which typically occurs approximately a month after the rainy season, rather than in the rainy season itself. [ABSTRACT FROM AUTHOR]

Most of the major rivers in the eastern margin of the Tibetan Plateau are distributed along the tectonic suture zone, where many giant landslides are present. The catastrophic Baige landslide that occurred in 2018 developed in the Jinsha River tectonic suture zone. What is the geo-structure prone to sliding in the Jinsha River tectonic suture zone? How does it control landslide evolution? A special soft rock/soil, altered clay, is found in the Jinsha River tectonic suture zone, and this altered clay may play an important role in giant landslide initiation. To understand the mechanism of the strength weakening of the altered clay and its effect on the formation of giant landslides, taking the Baige landslide as a typical case study, detailed field surveys and laboratory experiments were performed, and the process and mechanism of the transformation of altered clay into sliding zone soil were analyzed. The results indicate that (1) the altered clay is rich in platy layer clay minerals, so it has strong water sensitivity and expansibility. Furthermore, it has significant strain-softening behavior and extremely low residual strength due to the directional arrangement of clay minerals. (2) The evolution process of the Baige landslide was controlled by altered clay and rock mass discontinuities. Altered clay developed along ophiolite discontinuities and formed the weak interlayer of the slope. Due to long-term gravity creep, intermittent rainfall, and earthquakes, the strength of the altered clay was continuously weakened, and the sliding zone gradually penetrated, which finally led to the failure of the Baige landslide. The results of this manuscript are helpful for understanding the evolution mechanism of giant landslides in tectonic suture zones around the world. [ABSTRACT FROM AUTHOR]