## 3 Overview of the Da'anshan Landslide Disaster
Meteorological data from the observation point in Da'anshan Village showed that the cumulative daily rainfall from 7 July to 10 August (the day just before the landslide) had reached 364 mm (Figure 7). The daily rainfall exceeded 70 mm and 60 mm on August 6 and 9, respectively. The infiltration of rainfall will lead to an increase in pore pressure, and the long-term immersion of water will lead to the decline in rock strength, especially in the shatter zone of the slope. The landslide occurred after two heavy rainfall events in August, so the recent heavy rainfall was one of the main factors triggering the landslide. Figure 6: Geological features near the slope toe. (**a**) The soft–hard interlayer structure (SZ means “Shatter Zone”); (**b**) strong weathered and shattered zone; (**c**) fissure water from rock mass. Screenshots in Figure 8 are from the video recorded by the witness. The numbers marked in the upper-right corner of each image represent the time in \"seconds: milliseconds\" format. For the convenience of comparison, we regard the time when the slope toe showed visible deformation in the video as the initial time (marked as 0:000 in the upper-right corner of Figure 8a). The instability of the slope first occurred at the foot of the slope, near the pavement of the road (marked with a yellow dotted line). Figure 8: Video screenshots of the Da’anshan landslide. (**a**) The triggering moment of landslide, there was no visible detachment on the slope. (**b**) After 1 s sliding, a small amount of detachment occurred on the slope. (**c**) After 1 s 476 ms sliding, a small amount of detachment occurred. (**d**) After 1 s 952 ms sliding, the amount of detachment gradually increased. (**e**) After 2 s 952 ms sliding, the front of the slide hit the road below it. (**f**) After 3 s 952 ms sliding, frontal impact range further expanded. (**g**) After 4 s 476 ms sliding, the front of the landslide slid off the road. (**h**) After 5 s 476 ms sliding, the main body of the landslide fully fragmented and slid off the road surface. Figure 7: The daily and cumulative precipitation in Da’anshan village. At 1 s (Figure 8b), the top of the unstable slope was partially detached from the surrounding stable slope. After that, the range of detachment expanded laterally, but the downward displacement at the top developed slowly. At 1 s 952 ms (Figure 8d), the displacement has become obvious, but the displacement at the lower part of the slope (near the yellow dotted line) is still small, and the soil protruding downward from this part has not reached the pavement. The sliding body maintained good integrity during this process. At 2 s 952 ms (Figure 8e), the downward displacement of the top of the landslide is further increased, and a small number of broken stones near the slope foot fall onto the pavement and stir up dust. At 3 s 952 ms (Figure 8f), the sliding body decomposes significantly during the impaction process with the road surface. With the development of displacement, the integrity of the landslide declined, and the main part of the landslide hit the pavement and aroused a lot of dust (Figure 8g,h). Thereafter, both the impact area and avalanche velocity of the landslide increased significantly. Then, the rockslide gradually evolved into a rock avalanche.