# Geophysical Study of a Large Landslide Affecting the Urban Area of Albuuuelas (S Spain)
## 1 Introduction  
Managing landslide hazards requires the knowledge of both the hosting slope and the unstable mass. In general, the morphology and location of failure surfaces are fundamental for evaluating how the stability of a given landslide may evolve due to changes of different triggering factors [1, 2]. These data are usually collected by drilling expensive boreholes. Although this procedure is very effective and allows for the retrieval of undisturbed samples that may be tested in laboratory to estimate geotechnical properties of the materials affectedby instability, it may become prohibitive when landslide mass is very large because the number of boreholes needed is too high. For these cases, geophysical methods represent an interesting solution because they allow for the fast coverage of wide areas at a reasonable cost, and they do not disturb the area under study [3; 4; 5]; this last characteristic is of special interest when the landslide affects urban areas. At present, electromagnetic [6; 7], electrical [8; 9] and seismic methods [10; 11; 12] are among the most used geophysical techniques applied to landslide studies. The usefulness of these techniques is based in the consideration that there exists a contrast in the physical properties of the different materials involved (landslide mass versus stable slope). Limitations occur when such a contrast is due to presence of boundaries without interest in the slope stability problem. For instance, results of electrical methods are highly dependent of the presence of water in the medium, but it may not necessarily be related to the location of landslide boundaries [5]. Another drawback may also be associated with the characteristics of the terrain under study. For example, the particular case of our study, the Albufuuelas landslide, is characterized by an irregular relief, with difficulties in access and the availability of space. In these conditions, the application of some geophysical techniques, such as electrical techniques, would be quite complicated due to the extension of wiring that would be required to be able to investigate the expected depths. In the case of seismic methods, those based in the analysis of ambient noise (passive methods) have gained popularity in recent times [5; 13; 14; 15]. The basis of these techniques is to find the contrast of mechanical properties between a deep and rigid basement, and a layer (or multiple layers) of softer material resting on this basement. The study may be performed through single stations equipped with triaxial sensors (HVSR technique) [16] or by arrays of vertical sensors analyzed using the spatial autocorrelation (SPAC) method [17], the extended spatial autocorrelation (ESAC) analysis [18; 19] or the frequency-wavenumber (_f-k_) transform [20; 21], among other possible methods. The HVSR method provides the fundamental resonance frequency of SH waves (e.g., [22]), while surface waves dispersion curves result from array data (e.g., [19; 23; 24]). Both type of results can be inverted, providing a soil column depicting the local stratigraphy, where each layer is characterized by its thickness and shear wave velocity [25; 26]. The town of Albufuuelas (Granada province) is located close to the southern border of the Granada basin (Figure 1) and was built on a slide (Figure 2), which is characterized by a very low rate of movement [27]. This town was destroyed during the Andalusia earthquake (24/12/1884), Mw 6.5 [28] (Figure 1), with the oldest houses in the town being those rebuilt after the earthquake. Currently, these older houses are rotated at angles of up to 6 degrees (Figure 2). This was initially interpreted as due to the accumulated landslide displacement after the reconstruction of the town [27]. However, data from the systematic measurement of the tilting of houses [29] and from SAR interferometry [30] seem to point out that a complex pattern of progressive ground settlement may also explain the observed tilting.