Large rock avalanche is the most devastating landslide type in the world. Southeastern Qinghai-Tibet plateau is one of the areas the most susceptible to large rock avalanche in the world. To explore initial failure conditions of rock avalanches in the area, source slopes’ characteristics of 15 cases were analyzed with focusing on topography, geological structure and trigger type based on site investigation, satellite image analysis and unmanned aerial vehicle survey. It is found that: 1) runouts of the rock avalanches depended strongly on heights of source slopes and their drop heights with minimum slope height of 150m, drop height of 100m and slope angle of 30? ; 2) rock mass prone to rock avalanches were massive granite, basalt, and bedded limestone, sandy slate, slate and gneiss, all of which are either very competent or competent rocks with brittle failure natures, and of which occurrence of bedded rocks were either steeply dip slope, obliquely dip slope or reversely dip slope; 3) all of the source slopes’ failure were controlled strongly by discontinuities, including dip-slope, reverse dip-slope and obliquely joints as well as bedding planes. Field observation and joint survey revealed that five kinds of initial failure modes commonly occurred in those rock avalanches’ source slopes, specifically compound wedge slide, compound planar slide, flexural slide, toppling fall and buckling slide. Although it is not uncommon that some rock avalanches occurred with no specific trigger, earthquake was primary trigger of most rock avalanches, followed by glacier melting, both of which agitated those rock avalanches’ extremely rapid motion and exceptional long runout. Combining failure modes, rock mass type, slope structure, controlling discontinuities and driving forces, initial failure of rock avalanches’ source slopes in southeastern Qinghai-Tibet plateau could be categorized into different groups in terms of geomechanics, including compound wedge slide or planar slide of massive rock slopes driven by gravity or plus earthquake, or pore water pressure, compound wedge slide or planar slide of obliquely dip bedded rock slope driven by gravity or plus earthquake, toppling fall or flexural slide of reversely dip bedded rock driven by gravity or plus earthquake, and buckling slide of steeply dip slope bedding rock driven by gravity or plus earthquake.
