Insights into flank instability from geomechanical assessment of fresh volcanic products at the Chillán Viejo and Antuco volcanoes (Southern Andes of Chile)

Steep slopes, excessive volcanic edifice volume, weathering and/or alteration generating weak substratum, and magmatic-volcanic or seismic activity are known factors that all control volcano flank instability. These factors are critical in generating volcanic debris avalanches, which are rapid-onset catastrophic events that can cause substantial damage to infrastructure and loss of life within minutes to hours. Despite this, volcano hazard maps of Andean volcanoes do not specifically incorporate analysis of zones or areas prone to flank collapse. In a new project (Fondecyt 11241126), we aim to determine the distribution and geometry of potential sites of flank instability, and their controlling factors, that may produce potential future volcanic debris avalanches at the Chillán Viejo (3,195 m asl) and Antuco (2,979 m asl) volcanoes. In 1883, the upper south-southeast flank of Chillan Viejo volcano collapsed after an eruptive cycle, producing a 600 m-length scar. Comparatively, at ca. 7 ka BP, the western flank of Antuco collapsed catastrophically, producing a 6.4 km³ debris avalanche deposit [1]. Rapid edifice growth and regeneration at both volcanoes demands the assessment of future flank collapse scenarios. In the field, we sampled representative lithological units of both fresh and hydrothermally altered rocks for petrological and geochemical characterisation. Simultaneously, we performed rebound tests in substratum units, lava flows, and intrusive bodies using N-type Schmidt test hammers. These measurements were complimented by the construction of three-dimensional outcrop models from Unmanned Aerial Vehicle surveys for structural analysis. The samples collected were analysed in the laboratory in order to constrain the physical and mechanical rock properties of both intact and hydrothermally altered blocks. Preliminary results are compared with textural and compositional features of the effusive products to better understand the mechanical behaviour of both volcanic edifices, identify potential sites of future collapse, and ascertain potential collapse drivers.

This is a contribution to Fondecyt iniciación 11241126 and the European Research Council (ERC) SYNERGY grant 101118491 "ROTTnROCK". 

References:
[1] Romero, J. E., Moreno, H., Polacci, M., Burton, M., & Guzmán, D. (2022). Mid-Holocene lateral collapse of Antuco volcano (Chile): debris avalanche deposit features, emplacement dynamics, and impacts. Landslides, 19(6): 1321-1338.