## 4 Distribution Characteristics and Mechanism of Rockfall Source Areas  
The identification of rockfall source areas was mainly based on the DEM of 10 m resolution, assisted by aerial photos and remote sensing images. In total, 41 source areas of typical rockfalls were identified according to aerial photos, remote sensing interpretation, and field verification in study area. Based on the geological map and the DEM, the principal distribution characters of 41 source areas of typical rockfalls were obtained. The slope angles were mainly between 40\({}^{\circ}\) to 90\({}^{\circ}\), slope aspects were mainly between 160\({}^{\circ}\) to 260\({}^{\circ}\), and elevations were mainly between 3600 m to 4900 m. By inputting these distributing thresholds into ArcGIS, 90 source areas of rockfalls in study area were identified in detail (Figure 2a) [31]. Based on the DEM with 10 m resolution, the identification of rockfall sources was limited to rockfalls greater than a scale of 10 m. Furthermore, it was not possible to locate and map each rockfall source area, especially on sub-vertical slopes. Final identification results show that the sources of rockfalls are mainly distributed in the altitude between 3600 and 4800 m (as shown in Figure 4), and mainly on the adrets (Figure 2a.). The elevation distributions of each lithology in the typical profile are shown in Figure 2b,c. Conglomerate is mainly distributed between 3400-4200 m and granite is above 3700 m. Lang county ophiolite melange and phyllite are distributed below elevation of 3600 m. Comparing the elevation data above, we can obtain that rockfall source areas are mainly distributed in conglomerate and granite areas. We have confirmed the correctness of this conclusion by field investigation works. Figure 4: The elevation distributions of rockfall source areas in the study area. Figure 3: The methodology flow chart of this study. The study area located at the YTSZ. Profiles in Figure 2b,c show that the fault facets are mainly distributed in the granite area within the elevation range of 4800-5100 m, with a dip direction of southwest and dip angle of 40\({}^{\circ}\)-55\({}^{\circ}\). We conducted detailed field investigation at two sites in study area (locations of site A and B are shown in Figure 1b), where the lithologies of the both outcrops are conglomerate rocks. The orientation of slope is 210\({}^{\circ}\)\(\angle\) 45\({}^{\circ}\) on both site of A and B. We used GPS real-time kinematic (GPS-RTK) technology and DJI Mavic2 Pro unmanned aerial vehicle (UAV) to carry out aerial photographic survey, Agisoft Photoscan to build 3D point cloud model, and COLTOP software to extract structural feature of rock mass. The slope was too steep to climb and the limiting flight altitude of DJI Mavic2 Pro UAV was 500 m, thus, we could only get the aerial photographs of conglomerate area and phyllite area which were distributed in the lower part of the slope. As mentioned above, phyllite area was not the typical area of rockfall. Therefore, we only analyzed the rock mass structure of the conglomerate area. Figure 5 shows the data of field investigation. The black frames in Figure 5 are the statistical window of joints, with the length of 50 m. According to the results of field measurement and COLTOP analysis, there are four sets of joints for rock masses in study area. The attitudes of J1, J2, J3, and J4 are about 193\({}^{\circ}\)\(\angle\) 43\({}^{\circ}\), 311\({}^{\circ}\)\(\angle\) 81\({}^{\circ}\), 116\({}^{\circ}\)\(\angle\) 67\({}^{\circ}\), and 232\({}^{\circ}\)\(\angle\) 80\({}^{\circ}\), respectively. Details for the four sets of joints are shown in Figure 5. J1 is almost dipping towards the valley whose orientation is in accordance with Yarlung Tsangpo Fault. The combination of J1, J2, and J4 cuts the rock mass into wedges. When there are heavy rainfalls, earthquakes, or other trigger factors, it is extremely possible for these unstable blocks to move downward in the form of rockfalls.