Application of large scale ring shear tests to the analysis of tsunamigenic landslides at the Stromboli volcano, Italy

The island of Stromboli (Southern Italy) is a 4,000-m-high volcanic edifice about 900 m above sea level. Most of the NW flank is formed by a wide scar (Sciara del Fuoco) filled by irregular alternations of volcaniclastic layers and thin lava flows. Between 29 and 30 December 2002, a submarine and a subaerial landslide involved the northernmost part of the Sciara del Fuoco slope and caused two tsunami waves with a maximum run-up of 10 m. Mechanisms of the rapid submarine landslide and the preceding deformation of the subaerial and submarine slope were investigated using large-scale ring shear tests on the saturated and dry volcaniclastic material. The shear behaviour of the material under different drainage conditions was analysed during tests conducted at DPRI, Kyoto University. Pore pressure generation, mobilised shear strength and grain crushing, within a range of displacements encompassing the different stages of evolution of the slope, were considered. Experimental results suggest that even at larger displacements, shear strength of the dry material explains the virtual stability of the slope. Conversely, full or partial liquefaction can be invoked to explain the submarine failure and the subsequent long runout (more than 1,000 m) of the failed materials.