## 5 Discussions  
This study evaluates the dynamic response of a typical type of landslides, that is, high-steep rock slide, under different types of seismic waves. Part of the analysis of the numerical results have been discussed in previous section, and they will be described as conclusions in next section as well. In addition, there are still some discussions worth of attention.  
(1) Selection of the input seismic waves:  
At time of the Jiuzhaigou earthquake, China Earthquake Networks Center collected the seismic waves of 66 stations. The seismic waves selection criteria in this study are as follows: the station as close to the study area as possible. However, the recorded seismic waves are quite different in the total 66 stations, e.g., the amplitude. Therefore, it is worth discussing whether it is available to select seismic waves only by the value of distance.  
(2) The validity of the typical high-steep rock slide numerical modeling:  
This study is to explore the regularity of the dynamic response of the typical high-steep rock slide under different seismic waves. Therefore, the numerical model is simplified based on the actual landslide. However, the conclusions investigated in this study are  
Figure 21: Failure behavior of the rock slide with joints dip angle 0\({}^{\circ}\) (**a**) and 60\({}^{\circ}\) (**b**) under three-directional seismic wave.  
Figure 20: Numerical results of local damping 0.001 (**a**) and 0.01 (**b**) under three-directional seismic wave.  
consistent with those obtained in other studies. For example, seismic waves have edge and altitude amplification effects [36], and the Up-Down seismic waves have little impact on the dynamic response of the landslide [29]. The available of the numerical results can be verified. The follow-up work will study a specific landslide to explore how to select seismic wave loading in 3 DEC numerical modeling.  
(3) Effects of the orientation of the slope:  
Firstly, it should be noted that the free-field boundary in 3 DEC requires that the all five surfaces (except the top surface) of the numerical model should be parallel to the coordinate axis. Moreover, the seismic wave loading is limited to the coordinate axis; that is, it can only be loaded along \(X\), \(Y\), and \(Z\) directions. Owing to the left-handed coordinate system adopted in the numerical modeling, the Up-Down seismic wave is determined along the \(Y\) direction. The horizontal seismic waves (East-West and North-South seismic waves) are corresponding to the \(X\) or \(Z\) directions. However, the actual orientation of the slope is not exactly towards north, south, east, or west. It may lead to the inconsistency of the seismic wave loading in numerical modeling with the actual vibration of the slope. For example, this study assumed that the orientation of the slope is towards east, corresponding to the \(X\) direction. Therefore, the input X-directional seismic wave loading can be regarded as the East-West seismic wave. Once the actual slope is not towards east, the \(X\) direction is not actually the East-West seismic wave.  
(4) The reason for the numerical simulation time longer than that of the actual situation:  
In general, the failure process of the rock slide occurs very fast and lasts for a few seconds. However, the convergence time of the numerical simulation is much longer than that of actual rock slide. The reason for the numerical simulation time longer than that of the actual situation might be the 3 DEC calculation principle. The calculating unit of 3 DEC is called \"block\". The overall system can be influenced by each single \"block\". Therefore, it may take a relatively longer time to ensure each \"block\" is in a stable state. However, in the general failure process of the landslide, the time of the failure process of the rock slide can be estimated roughly by observing the rock slide stops.