# Geophysical Study of a Large Landslide Affecting the Urban Area of Albuuuelas (S Spain)
## 6 Discussion  
The proposed model obtained after the integration of all available data show a rotational slide (sensu [1]) organized into several slide blocks (Figures 8 and 9). The movement (rotation) of blocks is greater towards the landslide toe, which may be interpreted as due to a retrogressive landslide. Initially, instability affects the lower parts of the slope, although as movement progressed, the upper parts of the slope became also unstable, increasing the size of the whole landslide. Upper blocks exhibited a lower displacement, and their effect on topography was reduced.  
The results show that the thicknesses of both sandstone (reef sandstone and calcarenite) and red silt units are highly variable in the landslide area, with successive reductions and increases in thickness. As previously explained, this is interpreted as the effect of multiple failure surfaces cutting these geological units, causing the estimated changes in thickness. Because red silt units are also affected, it was deduced that the basal failure surface of the whole landslide should be found at the base of these units, and that intermediate failures root into this surface (Figures 8 and 9). This agrees with the contrast in shear strength of the involved materials, given the higher shear strength of basement materials compared with red silt unit (base of the Neogene sedimentary fill). The failure surface may hardly penetrate into the basement, and it more likely affects the sedimentary units or runs along the contact basin basement--red silt. In some cases, where the basement is deeper, basal failure occurs within the red silt unit (sections A-A' and C-C'). This surface is found more than 60 m deep in the upper part of slide and about 50 m in its middle section.  
The obtained results and the modeled cross-sections show the importance of two factors in the occurrence of this landslide. First, the valley suffered an intense process of erosion in the past, where river incision led to the erosion of most of the sedimentary fill (Figure 10). This undercut of valley slopes contributed to an initial destabilization of the slope. Additionally, a general dip of layers towards the free face of the slope also contributed to the general failure of the valley slope. From the proposed sections, the erosion has removed the sedimentary fill in section B-B' (Figure 8), but there are still some remains of this fill in section A-A' (and C-C', Figures 8-10). It is reasonable to expect that river continues its incision (<1 mm/yr; [28]), but that it would be more effective on red silt (section A-A') than in the marble and schist found in section B-B' (Figure 8), so it is likely that the future evolution of this landslide would be more pronounced in its western flank. This is in agreement with displacement observed from interferometric techniques [30], which show that maximum displacements within the urban area of Albufuuelas occur just in the western part of the town (displacements above 2 mm/yr, with a maxima >12 mm/yr along the line of sight).  
The framework of the landslide mechanism, which can be reconstructed following the geophysical investigations and the consequent refinement of the engineering-geological model of the slope on which the town of Albufuuelas stands, leads to the recognition of retrogressive dynamics of the landslide that, however, appears to concentrate its mobility in the portions of the slope that are already involved. The distribution of activities does not therefore appear to be widening. On the other hand, the progradation of the landslide mass seems to be disadvantaged by the intense erosion at its foot, caused by the Saleres River and evidenced by the deep incision of the deposits connected to the landslide mass itself. From a kinematic point of view, the landslide reveals the presence of numerous blocks, which are dislodged by secondary scarps and whose mobility can be distinguished with exposure to the possible reactivation of parts of the slope when trigger events, such as earthquakes, should occur.