[1] The 7.5 ka Socompa sector collapse emplaced 25 km 3 of fragmented rock as a thin, but widespread (500 km 2 ), avalanche deposit, followed by late stage sliding of 11 km 3 as Toreva blocks. Most of the avalanche mass was emplaced dry, although saturation of a basal shear layer cannot be excluded. Modeling was carried out using the depth-averaged granular flow equations in order to provide information on the flow behavior of this well-preserved, long run-out avalanche. Results were constrained using structures preserved on the surface of the deposit, as well as by deposit outline and run-up (a proxy for velocity). Models assuming constant dynamic friction fail to produce realistic results because the low basal friction angles (1 to 3.5°) necessary to generate observed run-out permit neither adequate deposition on slopes nor preservation of significant morphology on the deposit surface. A reasonable fit is obtained, however, if the avalanche is assumed simply to experience a constant retarding stress of 50-100 kPa during flow. This permits long run-out as well as deposition on slopes and preservation of realistic depositional morphology. In particular the model explains a prominent topographic escarpment on the deposit surface as the frozen front of a huge wave of debris reflected off surrounding hills. The result that Socompa avalanche experienced a small, approximately constant retarding stress during emplacement is consistent with a previously published analysis of avalanche data.Citation: Kelfoun, K., and T. H. Druitt (2005), Numerical modeling of the emplacement of Socompa rock avalanche, Chile,

International audienc

Numerical modelling of a rapid, partial destabilization of Anak Krakatau Volcano (Indonesia) was performed in order to investigate the tsunami triggered by this event. Anak Krakatau, which is largely built on the steep NE wall of the 1883 Krakatau eruption caldera, is active on its SW side (towards the 1883 caldera), which makes the edifice quite unstable. A hypothetical 0.280 km3 flank collapse directed southwestwards would trigger an initial wave 43 m in height that would reach the islands of Sertung, Panjang and Rakata in less than 1 min, with amplitudes from 15 to 30 m. These waves would be potentially dangerous for the many small tourist boats circulating in, and around, the Krakatau Archipelago. The waves would then propagate in a radial manner from the impact region and across the Sunda Strait, at an average speed of 80–110 km h−1. The tsunami would reach the cities located on the western coast of Java (e.g. Merak, Anyer and Carita.) 35–45 min after the onset of collapse, with a maximum amplitude from 1.5 (Merak and Panimbang) to 3.4 m (Labuhan). As many industrial and tourist infrastructures are located close to the sea and at altitudes of less than 10 m, these waves present a non-negligible risk. Owing to numerous reflections inside the Krakatau Archipelago, the waves would even affect Bandar Lampung (Sumatra, c. 900 000 inhabitants) after more than 1 h, with a maximum amplitude of 0.3 m. The waves produced would be far smaller than those occurring during the 1883 Krakatau eruption (c. 15 m) and a rapid detection of the collapse by the volcano observatory, together with an efficient alert system on the coast, would possibly prevent this hypothetical event from being deadly.

[1] Geophysical granular flows display complex nonlinear, nonuniform, and unsteady rheologies, depending on the volumetric grain concentration within the flow: kinetic, kinetic-collisional, and frictional. To account for the whole spectrum of granular rheologies (and hence concentrations), we have used and further developed for geophysical-atmospheric applications a multiphase computer model initially developed by U.S. Department of Energy laboratories: (Geophysical) Multiphase Flow with Interphase Exchange. As demonstrated in this manuscript, (G)MFIX can successfully simulate a large span of pyroclastic phenomena and related processes: plinian clouds, pyroclastic flows and surges, flow transformations, and depositional processes. Plinian cloud simulations agree well with the classical plume theory and historical eruptions in the upper altitude of the cloud (H T ) versus mass flux diagram. At high mass flux (>10 7 kg/s), plinian clouds pulsate periodically with time because of the vertical propagations of acoustic-gravity waves within the clouds. The lowest undercooled temperature anomalies measured within the upper part of the column can be as low as À18 K, which agrees well with El Chichón and Mt. St. Helens eruptions. Vertical and horizontal speed profiles within the plinian cloud compare well with those inferred from simple plume models and from umbrella experiments. Pyroclastic flow and surge simulations show that both end-members are closely tight together; e.g., an initially diluted flow may generate a denser basal underflow, which will eventually outrun the expanded head of the flow. We further illustrate evidence of vertical and lateral flow transformation processes between diluted and concentrated flows, particularly laterally from a turbulent ''maintained over time fluidized zone'' near source. Our comprehensive granular rheological model and our simulations demonstrate that the main depositional process is mainly a progressive vertical aggradation.

[1] Landslides that occur on oceanic volcanoes can reach the sea and trigger catastrophic tsunamis. Réunion Island has been the location of numerous huge landslides involving tens to hundreds of cubic kilometers of material. We use a new two-fluid (seawater and landslide) numerical model to estimate the wave amplitudes and the propagation of tsunamis associated with landslide events on Réunion Island. A 10 km 3 landslide from the eastern flank of Piton de la Fournaise volcano would lift the water surface by about 150 m where it entered the sea. The wave thus generated would reach Saint-Denis, the capital of Réunion Island (population of about 150,000 people), in only 12 min, with an amplitude of more than 10 m, and would reach Mauritius Island in 18 min. Although Mauritius is located about 175 km from the impact, waves reaching its coast would be greater than those for Réunion Island. This is due to the initial shape of the wave, and its propagation normal to the coast at Mauritius but generally coast-parallel at Réunion Island. A submarine landslide of the coastal shelf of 2 km 3 , would trigger a ∼40 m high wave that would severely affect the proximal coast in the western part of Réunion Island. For a landslide of the shelf of only 0.5 km 3 , waves of about 2 m in amplitude would affect the proximal coast.

International audienc

Giant mass failures of oceanic shield volcanoes that generate tsunamis potentially represent a high-magnitude but low-frequency hazard, and it is actually difficult to infer the mechanisms and dynamics controlling them. Here we document tsunami deposits at high elevation (up to 132 m) on the north-western slopes of Tenerife, Canary Islands, as a new evidence of megatsunami generated by volcano flank failure. Analyses of the tsunami deposits demonstrate that two main tsunamis impacted the coasts of Tenerife 170 kyr ago. The first tsunami was generated during the submarine stage of a retrogressive failure of the northern flank of the island, whereas the second one followed the debris avalanche of the subaerial edifice and incorporated pumices from an on-going ignimbrite-forming eruption. Coupling between a massive retrogressive flank failure and a large explosive eruption represents a new type of volcano-tectonic event on oceanic shield volcanoes and a new hazard scenario.

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