Slope stability models for rainfall-induced lahars during long-lasting eruptions

In this study we analyse the spatial distribution of potential lahar sources during long-lasting eruptions using two slope stability models (SHALSTAB and TRIGRS). The analysis is based on observed deposit grain-size and thickness, rainfall data and slope angle of the lahar events, in the area surrounding the Cordon Caulle volcano, Chile, that occurred during the 2011 eruption. The main phase of the eruption (4-7 June) was characterized by eruptive plumes from 7 to 11 km high which dispersed most of the tephra eastward toward Argentina, with a total estimated volume of about 1 km(3). Tephra-fallout blanketed the region from ESE to ENE of the volcano with the thickness of the tephra-fallout layers between approximately 1 m (15 km from vent) and 0.06 m (240 km from the vent). On 10 June 2011, a major lahar occurred close to the Argentina-Chile border that reached the National road 231 (28 km from the vent). Three field campaigns were undertaken to collect samples and data from tephra-fallout deposits and triggering mechanisms in the lahar source area. Model input parameters were obtained from geotechnical tests and field measurements. Several metrics were used for model evaluation and best fit to the data were obtained for simulations considering non-cohesive ash layers for SHALSTAB and a cohesion of 0.5 kPa for TRIGRS. Both models are sensitive to the physical properties of the tephra-fallout deposit and the hydraulic and materiel strength properties of the study area. They both also show good agreement with field data but provide different information: TRIGRS provides an estimate of the timing (based on a storm event) and location of a potentially unstable area, while SHALSTAB simulations result in landslide susceptibility classes based on critical rainfall value. These outcomes provide fundamental insights into lahar triggering during long-lasting volcanic eruptions and are crucial to the compilation of lahar hazard maps and emergency management plans in the South Andes volcanic region. (C) 2018 Elsevier B.V. All rights reserved.