## 2 The Stage-Wise Methodology for the GHM Characterization of the Landslide Mechanisms
Therefore, the landslide event is the final paroxysmal episode of a deformation process across the slope, being defined as a progressive failure (Figure 5; [29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]) which takes place before the first activation of the landslide body. When dealing with reactivation mechanisms, before the mobilization of the landslide body, the processes are similar to those of 'first failure' landslides, but the displacement field is strongly controlled by pre-existing shear bands (eventually of the material discontinuities). The modelling of the landslide 'propagation' [42; 43; 44; 45], instead, requires different analytical tools which depend on the type of landslide. In particular, for the six landslide classes discussed in this paper (Figure 2), different modelling strategies must be used to predict the propagation, whether they are: (i) slow landslides, whose displacement is limited [16]; or (ii) large propagation landslides, often fast [16; 42], with propagation distances from hundreds of meters to kilometers (earthflows, mudflows, debris-flows and flows; [15; 16]). The prediction of the landslide mass propagation can  
Figure 5: The stage-wise methodology for the GHM characterization of the landslide mechanisms [10]. (**a**) Phenomenological diagnosis, I level of analysis; (**b**) limit equilibrium method, II level of analysis; (**c**) numerical modelling, III level of analysis. still make use of continuum mechanics if the propagation ranges from decimeters to tens of meters in years (category (i)), according to analytical strategies which are similar to those used to predict ground settlements determined by tunnel excavations [46], eventually adopting computational methods accounting for large straining. For the landslides in category (ii), instead, the propagation modeling should involve the use of either discrete element codes [24; 47; 48; 49; 50] or numerical codes that consider the soil mass as a viscous fluid [51; 52; 53; 54; 55] of high granular content [56; 57; 58]. Despite improvements in slope numerical modelling, it should be pointed out that the coupled modelling of all the processes active in slopes is still a scientific challenge. Therefore, numerical modelling should account for the slope factors and physical laws which are active in the slope (Table 1) according to their level of influence on the landslide mechanism. To this end, numerical modelling should be always preceded by experimental analyses of the slope processes performed in the field [14; 15], supported by advanced monitoring devices. It follows that neither phenomenological analyses nor numerical modelling alone can provide reliable assessments of landslide mechanisms; rather, a stage-wise methodology which starts with phenomenological studies in the fields of geology, geomorphology, and geotechnics (in the field and the laboratory) should be routinely implemented to deduce the most important aspects of reliable quantitative modelling of landslide mechanisms. In particular, as illustrated in Figure 5, the methodology should be composed of three stages: the first (I), aimed at the identification of the slope factors (Table 2) and the phenomenological interpretation of the landslide mechanism; the second (II), when a simplified quantitative analysis of the slope stability is carried out through the limit equilibrium method, for a first check of the phenomenological interpretation obtained in the first stage; and the third (III), representing the numerical analysis, in which the boundary value problem is solved by means of numerical modelling, in the context of a geotechnical model constructed on the basis of the results of the previous stages. An exhaustive discussion of the stage-wise methodology is reported in [10; 11; 59].