Modelling the instability phenomena on the NW flank of Stromboli Volcano (Italy) due to lateral dyke intrusion

The aim of the paper is to provide quantitative elements on the mechanisms that controlled the slope instabilities occurred after two major recent eruptions at Stromboli volcano (i.e. 2002–2003 and 2007). After a brief description of the geological setting and of the largest well documented instability phenomena on volcano flanks, the main objective is pursued using three-dimensional stress-strain analyses for modelling the effects of the magma intrusion on the stability of the volcano flank. Modelling is based on the results of an extensive geotechnical characterization of the volcaniclastic and lava materials. The numerical analyses investigate the influence of different paths and geometry of magma intrusion as well as the spatial variation of mechanical properties. As a result, favourable conditions for specific failure modes are identified. The stress-strain analyses show that magma intrusion can cause both a local failure of a wedge immediately downslope from the dyke or deep-seated movement involving large part of the slope; these two mechanisms were consistent with the deformative patterns observed during the 2007 and 2002–2003 eruptions, respectively. The magma thrust induces shear strains up to levels associated to appreciable grain crushing even below the sea level, where flow liquefaction can be invoked to explain the occurrence of past submarine slides. The submarine landslide is analysed by a combined finite element – limit equilibrium approach that demonstrates that instability conditions can develop if the loading exerted by the upper portion of the slope is sufficiently fast to produce undrained conditions. The analyses also support the hypothesis that at the 2002 subaerial failure occurred, due to combined effects of a magma mild thrust and the removal of the toe support caused by the preceding submarine slide.