Slope Failure Induced by the December 2002 Eruption at Stromboli Volcano

We reconstruct the sequence of landslides that occurred soon after the beginning of the December 2002 eruption on the NW Hank of Stromboli volcano. Landslides involved the northeastem part ofthe Seiara del Fuoco (SdF) slope, an old collapse scar filled by products of volcanic activity, producing tsunami waves that severely damaged the coast of the island of Stromboli. Volumes of the mass detached from the subaerial and submarine slope were quantified by comparing preslide and postslide slope surfaces obtained by aerophotogrammetric and bathymetric data, which also allowed, in conjunction with field observations and helicopter surveys, the reconstruction of geometry and kinematics of landslides. According to the reconstructed sequence, 2 days after the beginning of the eruption, the upper part ofthe NE sector ofthe SdF slope experienced major displacements (few tens of meters). Movements propagated downslope and affected the nearshore portion ofthe submerged slope without a rapid sliding ofthe displaced mass into the sea. The following hours were characterized by a progressive increase ofdeformations, localized along shear zones extending over two thirds of the subaerial slope. This phase proceeded until a submarine slide about 6 millions of cubic mters in volume occurred, causing a first tsunami wave. The subaerial mass delimited by the shear zones and unbuttressed at its foot, then slipped into the sea producing a second tsunami wave. The main landslide event (and the minor slumps which followed) removed a volume of about l0 millions of cubic mters of the infilling deposit, to a thickness of at least 65 m. Hypotheses were fomulated on the mechanisms that controlled the different phases of the instability sequence. Since hydraulic and stress/strain conditions progressively changed during the slope evolution, the formulated mechanisms are also based on geotechnical analyses and considerations on the mechanical behavior of volcaniclastic materials. The process that led to the landslide events was initiated by forces exerted by magma intruded into the slope, while further steps ofthe evolution of slope stability conditions (especially the submarine failure) were controlled by the particular shear behavior of the volcaniclastic material, mainly iniiuenced by grain crushability. ln fact, strength progressively decreased as shear strains proceeded, and the intensely sheared saturated material forming the submarine slope may have become susceptible of failure when sudden strain/stress increments occurred.