## 1 Introduction  
Tibet Region is the main part of the Qinghai-Tibet plateau and located at the first rung of terrain in China. In addition, Tibet area is located at the boundary of Indus-Asia plate with frequent tectonic activities, which lead to the extremely high frequency of geological disasters such as rockfalls and landslides. The volumes of failure rock masses in this area are often very large, such as the Yigong landslide on 9 April 2000, whose volume was \(2.8\times 10^{8}\) to \(3.0\times 10^{8}\) m\({}^{3}\)[1] and the Motuo County Gande Township avalanche on 22 December 2008, whose volume was about \(6.128\times 10^{8}\) m\({}^{3}\)[2]. Comparing with other geohazards, rockfall occurs more frequently and widely. With the development of the National Western Development Plan, the Belt and Road, Global Economic Interconnection Strategy, the number of highway and railway projects in Tibet has been gradually increasing. The impacts of the rockfall disasters on these projects have received considerable attention. For example, [PERSON] [3], [PERSON] et al. [4; 5], and [PERSON] et al. [6] made deep analysis and risk assessment on the occurrence mechanisms and movement characteristics of the rockfall disasters along the Sichuan-Tibet railway and highway, respectively. In recent years, with the launching of ultra-high voltage transmission projects such as Central Tibet Grid Interconnection Project and Strategic Electricity Transmission Passage of Sichuan-Tibet Plateau, the threats of rockfall and landslide have become the key points and difficulties in design, construction and operation periods of power grid engineering. Rockfall is one of the main geohazards that deeply constrain the development of mountain area. In order to manage the prevention work and reduce the loss of rockfall disaster, many researchers have applied different methods to carry out rockfall investigation and hazard/risk analysis. [PERSON] et al. applied event tree analysis to assess the rockfall risk along a transportation corridor [7]. [PERSON] et al. made use of laser scanning data to identify rockfall sources, and three-dimensional (3D) modeling in Geographic Information System (GIS) to carry out the hazard zonation [8]. [PERSON] et al. added normalized cumulative distribution function of cliff unit to the Slope Angle Distribution procedure to quantify rock-mass-failure susceptibilities [9]. Rockfall hazard assessment is mainly about evaluating the occurrence probability of an event of certain magnitude and kinematic characteristics (velocity, energy, etc.) over the period from blocks leaving the parent rock to reaching the impacted body [10; 11]. In order to satisfy more detailed engineering design requirements, researchers gradually change the method of rockfall hazard assessment from qualitative analysis to quantitative analysis. This is largely due to the increasingly mature three-dimensional (3D) numerical simulation technology of rockfall. Researchers can obtain the trajectory, velocity, energy, and other information of falling blocks and conduct the quantitative analysis of rockfall hazard in the 3D space by 3D numerical simulation technology. There are already a lot of software available for 3D numerical simulations of rockfall, such as STONE/HY-STONE [12], RAMMS: Rockfall [13], Rockyfor3D [14] et al. Nowadays, the methods for quantitative analysis of rockfall hazard with trajectory data are mainly divided into the following two kinds. The first kind of methods combined the frequency of rockfall. For example, the Cadanave method proposed by [PERSON] et al. [15] considered the probability of the rock block breaking away from the parent rock, the number of falling blocks and the probability of the blocks reaching the target. Improved Cadanave method developed by [PERSON] et al. [10] improved the shortcomings of data classification and approximate processing in original method. The Matterock method combined the occurrence probability, arrival probability, energy and other parameters to analyze the hazard of rockfall [16].