Morpho-structural criteria for the identification of spreading-induced deformation processes potentially compromising stratovolcano stability

Characterisation of surface deformation at stratovolcanoes is essential for a better understanding of the processes that can compromise edifice structural stability and potential for flank collapse. Spreading produced by the presence of a hydrothermal system or intrusion of a viscous magma body can produce similar deformation signatures, and both processes have implications for flank instability. In this work, we perform analogue models and consider examples from real volcanoes (Damavand, Ubinas, Semeru and Casita) so as to characterise and recognise surface deformation patterns produced by spreading due to the presence of a hydrothermal system and in response to magma intrusion. The experiments show that there are differences in the resulting surface deformation associated with each process. Magma intrusion results in a sharp transition between areas of subsidence and uplift, and is associated with faults with oblique strikes in the upper part of the edifice. Instead, asymmetric flank spreading is associated with hydrothermal system and results in flank bulging close to the base of the edifice. Although laboratory analogue models show different deformation responses that could be diagnostic of the associated processes, application in the field is difficult as often these diagnostic features are not preserved during evolution. However, basal bulging represents a potential diagnostic for the identification of asymmetric volcano flank spreading associated with hydrothermal activity, and the potential for instability. Remote sensing techniques can allow identification of such surface deformation features, providing a useful tool for hazard assessment and design of monitoring strategies at potentially unstable volcanoes.