Numerical modeling of the emplacement of Socompa rock avalanche, Chile

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.