Morphology and inner structure of Luanshibao rock avalanche in Litang, China and its implications for long-runout mechanisms

Large rock avalanches usually move rapidly and travel long distances, making them extremely hazardous, and often result in catastrophic damages. The currently proposed hypotheses for the mechanisms causing their high mobility remain controversial (Petley, 2013). The Luanshibao ancient granite rock avalanche, with a Be-10 exposure age of 3510 +/- 346 years B.P. and a deposit volume of 67 Mm(3), straddles on the Litang active fault and was deposited on an open-flat river terrace without confinement. It travelled about 4000 m, with an apparent coefficient of friction of 0.22. Field investigation, interpretation of satellite images, and study of the morphology and inner structure of its deposit were used to analyze its kinetics and dynamics, and its long runout mechanisms. The study showed that there was probably a two-stage movement transition from rock-slope collapsing to debris spreading, based on the remarkable high secondary scarps and a long belt of bare blocks. The runout is closely related to the volume detached from the secondary scarp. Three different kinds of deposit morphologies were recognized, i.e., parallel transverse ridge-grooves, X grooves and rhombic platforms, and round-elliptical hummocks, from the proximal to the distal end, indicating the strain response to stresses in the debris in motion due to compression, shear and tension, respectively. These deposit morphologies as well as the inner structures such as homogeneous layers of blocks, inner shear belts and jigsaw fractured blocks, suggested that the extra runout debris moved as a pattern of laminar-flow or as a flow-sheet. However, the strong mixing of fluvial pebbles and avalanche debris in the distal hummocky zone suggested a pattern of turbulent flow. The possible liquefaction of saturated fluvial sediments by a triggering strong earthquake, the extra pore pressure generated by the high-speed undrained shearing of debris in the substrate and the entrainment of a large amount of Quaternary sediment, could have contributed to the great reduction of friction of the basal layer. The particle size decreasing with distance away from the secondary scarps and the disintegration of jigsaw structures, suggest that rock fragmentation occurred during motion. This implies that the long runout of the rock avalanche resulted from many factors.