The December 2002 landslides at Stromboli volcano (Italy) and their mechanisms interpreted by large scale ring shear tests

The island of Stromboli (Southern Italy) is the subaerial part of a large volcanic edifice 4000 m high that rises for about 900 m above the sea level. Most of the NW flank of Stromboli volcano is formed by a wide depression, called Sciara del Fuoco, which collects the products of the persistent volcanic activity and drive them to sea. The Sciara is filled by a deposit more than 200 m thick, consisting of irregular alternations of layers of volcanoclastic materials, which represent the most abundant component of the deposit, and sheets of primary products (i.e., pyroclastites and lava flows). Between December 29th and 30th 2002, the thrust exerted by the magma intruded during a major eruption, triggered a sequence of large-scale, subaerial and submarine instability phenomena on the NW flank of Stromboli culminating in a submarine and two subaerial destructive landslides. Landslides involved about 20 millions of m3 of slope materials producing two series of tsunami waves that seriously damaged the coastal areas of the Stromboli island, with a maximum run-up of 10 m, and the facing Panarea Island (20 km far). After the first emergency phase, the Italian Department of Civil Defence promoted a research programme on the eruption and its consequences. The work presented here summarizes a joint research activity between the National Research Council (Italy) and the Disaster Prevention Research Institute (DPRI) of Kyoto University (Japan) aimed at: 1) characterizing the mechanical behaviour of the volcanoclastic materials in the conditions that possibly developed during the landslide processes; 2) refining and supporting with an experimental evidence the mechanisms of landslide initiation and propagation suggested on the basis of in situ observations and post-failure data. Different tests were performed in the large-scale ring shear apparatus in DPRI to simulate: a) the deformation phase that involved the slope without provoking disrupting failure and that preceded the landslide initiation; b) the triggering of the following rapid submarine landslide that provoked the first tsunami. In the latter case, different drainage conditions were adopted in order to investigate the influence of displacement rate on pore pressure increment and strength reduction during the shearing process. Test results indicate that liquefaction phenomena can be invoked to explain the mechanism of the submarine landslide and the subsequent long run-out of the failed materials (greater than 1000 m).