## 1 Introduction  
Large scale occurrence of slope failures can be verified either when the equilibrium between forces acting on the slopes and their own resistances are imminently unbalanced through the action of short term forcings (dynamic triggers) or progressively through the cumulative and repetitive action of inner or external forces [1, 2]. Periodic fluctuations of the hydraulic pore pressures can drive slope deformations toward failures. These processes are responsible for medium- to long-term variation in strength and can be considered preparatory factors for slope instability. Weathering, damaging or progressive erosion are all considered slow and effective preparatory actions [3] for landsl in natural slopes. At the same time, changes in the water content and soil saturation degree in the shallow and unsaturated portion of the slopes can impose suction stresses and control the position of capillary fringes, which provide transitory stabilization [4]. Changes in hydraulic and physical conditions can instead statically predispose the action of short-term forcings since they control the mechanical strength and the bulk density of the geological layering. In this regard, the role of covers, which derive from vertical weathering profiles of the soil, in the predisposition to shallow landslides when triggering events, such as intense rainfall and earthquakes, are concurrent is not fully constrained. Such uncertainty arises from the objective difficulties in spatializing the thickness that the alteration covers have on the slopes as well as in the reconstruction of the geometries through which they are connected to the alluvial or debris at the valley floor. The presence over large areas of pre-existing landslides can, in turn, play an effective role in facilitating new instability since residual strengths act within landslide mass. In this context, when a dynamic trigger acts, quantifying the concurrent presence/absence of specific predisposition is important to adequately assess landslide scenarios if associated with the spatial distribution of the intensity of the seismic shaking. While conditioning of different sets of parameters (e.g., topographic) on seismic slope stability has been parametrically analyzed [5; 6; 7] and the sensitivity of specific landslides often approached, the predisposing contributions of rainfall prior to or directly after an earthquake were rarely analyzed in literature [8; 9; 10]; few are the complete and detailed landslide inventories referred to landslides induced by an earthquake under wet-to-saturated soil conditions. Rare are also deterministic studies dealing with the predisposing role of rainfall prior to an earthquake in controlling the spatial trend and distribution of coherent failures [11; 12]. Among these studies, [PERSON] [13] and [PERSON] [14] reported cases of coherent slides occurring on a clayey slope, saturated by above-ordinary pre-seismic precipitations. The evaluation of the importance of causative factors among rainfall-induced landslides (RfIL) and earthquake trigger landslides (EqTLs) should account for the hydraulic response of slope material, whose behavior relies on the local combination of lithology, nature of weathering, unsaturated permeability, rainfall intensity, local topography, and vegetation cover [15; 16]. Because of the large requirements of input data and the sharp variability of pore pressure distribution, such analyses usually refer to specific landslides and are approached by numerical analyses and physically-based models [17]. The reliable zonation of thickness and hydraulic properties of soil cover that can be mobilized under seismic shaking is also tricky. These uncertainties require the application of empirical techniques validated by ground evidence, aimed at reconstructing the distribution of the alteration covers [18; 19], as well as reconstruction of physical initial soil conditions and mechanical parameters represents a challenging and fundamental goal for the analysis of landslide scenarios in a multi-hazard perspective.